Vox Charta » Earth and Planetary

Jovian Early Bombardment: planetesimal erosion in the inner asteroid belt

astro-ph — Thu, 23 Feb 2012 01:49:34 +0000

The asteroid belt is an open window on the history of the Solar System, as it preserves records of both its formation process and its secular evolution. The progenitors of the present-day asteroids formed in the Solar Nebula almost contemporary to the giant planets. The actual process producing the first generation of asteroids is uncertain, strongly depending on the physical characteristics of the Solar Nebula, and the different scenarios produce very diverse initial size-frequency distributions. In this work we investigate the implications of the formation of Jupiter, plausibly the first giant planet to form, on the evolution of the primordial asteroid belt. The formation of Jupiter triggered a short but intense period of primordial bombardment, previously unaccounted for, which caused an early phase of enhanced collisional evolution in the asteroid belt. Our results indicate that this Jovian Early Bombardment caused the erosion or the disruption of bodies smaller than a threshold size, which strongly depends on the size-frequency distribution of the primordial planetesimals. If the asteroid belt was dominated by planetesimals less than 100 km in diameter, the primordial bombardment would have caused the erosion of bodies smaller than 200 km in diameter. If the asteroid belt was instead dominated by larger planetesimals, the bombardment would have resulted in the destruction of bodies as big as 500 km.

Bayesian analysis of the astrobiological implications of life’s early emergence on Earth [Replacement]

astro-ph — Thu, 23 Feb 2012 01:38:36 +0000

Life arose on Earth sometime in the first few hundred million years after the young planet had cooled to the point that it could support water-based organisms on its surface. The early emergence of life on Earth has been taken as evidence that the probability of abiogenesis is high, if starting from young-Earth-like conditions. We revisit this argument quantitatively in a Bayesian statistical framework. By constructing a simple model of the probability of abiogenesis, we calculate a Bayesian estimate of its posterior probability, given the data that life emerged fairly early in Earth’s history and that, billions of years later, curious creatures noted this fact and considered its implications. We find that, given only this very limited empirical information, the choice of Bayesian prior for the abiogenesis probability parameter has a dominant influence on the computed posterior probability. Although terrestrial life’s early emergence provides evidence that life might be common in the Universe if early-Earth-like conditions are, the evidence is inconclusive and indeed is consistent with an arbitrarily low intrinsic probability of abiogenesis for plausible uninformative priors. Finding a single case of life arising independently of our lineage (on Earth, elsewhere in the Solar System, or on an extrasolar planet) would provide much stronger evidence that abiogenesis is not extremely rare in the Universe.

Circumstellar disks in binary star systems [Replacement]

astro-ph — Thu, 23 Feb 2012 01:36:59 +0000

In this paper we study the evolution of viscous and radiative circumstellar disks under the influence of a companion star. We focus on the eccentric {\gamma} Cephei and {\alpha} Centauri system as examples and compare the disk quantities such as disk eccentricity and precession rate to previous isothermal simulations. We perform two-dimensional hydrodynamical simulations of the binary star systems under the assumption of coplanarity of the disk, host star and binary companion. We use the grid-based, staggered mesh code FARGO with an additional energy equation to which we added radiative cooling based on opacity tables. The eccentric binary companion perturbs the disk around the primary star periodically. Upon passing periastron spirals arms are induced that wind from the outer disk towards the star. In isothermal simulations this results in disk eccentricities up to {\epsilon}_disk ~ 0.2, but in more realistic radiative models we obtain much smaller eccentricities of about {\epsilon}_disk ~ 0.04 – 0.06 with no real precession. Models with varying viscosity and disk mass indicate show that disks with less mass have lower temperatures and higher disk eccentricity. The rather large high disk eccentricities, as indicated in previous isothermal disk simulations, implied a more difficult planet formation in the {\gamma} Cephei system due to the enhanced collision velocities of planetesimals. We have shown that under more realistic conditions with radiative cooling the disk become less eccentric and thus planet formation may be made easier. However, we estimate that the viscosity in the disk has to very small, with {\alpha} \lesssim 0.001, because otherwise the disk’s lifetime will be too short to allow planet formation to occur along the core instability scenario. We estimate that the periodic heating of the disk in eccentric binaries will be observable in the mid-IR regime.

Habitability of Earth-type Planets and Moons in the Kepler-16 System [Replacement]

astro-ph — Thu, 23 Feb 2012 01:35:58 +0000

We demonstrate that habitable Earth-mass planets and moons can exist in the Kepler-16 system, known to host a Saturn-mass planet around a stellar binary, by investigating their orbital stability in the standard and extended habitable zone (HZ). We find that Earth-mass planets in satellite-like (S-type)orbits are possible within the standard HZ in direct vicinity of Kepler-16b, thus constituting habitable exomoons. However, Earth-mass planets cannot exist in planetary-like (P-type) orbits around the two stellar components within the standard HZ. Yet, P-type Earth-mass planets can exist superior to the Saturnian planet in the extended HZ pertaining to considerably enhanced back-warming in the planetary atmosphere if facilitated. We briefly discuss the potential detectability of such habitable Earth-mass moons and planets positioned in satellite and planetary orbits, respectively. The range of inferior and superior P-type orbits in the HZ is between 0.657 to 0.71 AU and 0.95 to 1.02 AU, respectively.

A lucky imaging multiplicity study of exoplanet host stars

astro-ph — Wed, 22 Feb 2012 01:50:09 +0000

To understand the influence of additional wide stellar companions on planet formation, it is necessary to determine the fraction of multiple stellar systems amongst the known extrasolar planet population. We target recently discovered radial velocity exoplanetary systems observable from the northern hemisphere and with sufficiently high proper motion to detect stellar companions via direct imaging. We utilize the Calar Alto 2.2m telescope in combination with its lucky imaging camera AstraLux. 71 planet host stars have been observed so far, yielding one new low-mass (0.239 \pm 0.022M\odot) stellar companion, 4.5 arcsec (227AU of projected separation) northeast of the planet host star HD185269, detected via astrometry with AstraLux. We also present follow-up astrometry on three previously discovered stellar companions, showing for the first time common proper motion of the 0.5 arcsec companion to HD126614. Additionally, we determined the achieved detection limits for all targets, which allows us to characterize the detection space of possible further companions of these stars.

Dipole Collapse and Dynamo Waves in Global Direct Numerical Simulations

astro-ph — Wed, 22 Feb 2012 01:47:19 +0000

Magnetic fields of low-mass stars and planets are thought to originate from self-excited dynamo action in their convective interiors. Observations reveal a variety of field topologies ranging from large-scale, axial dipole to more structured magnetic fields. In this article, we investigate more than 70 three-dimensional, self-consistent dynamo models obtained by direct numerical simulations. The control parameters, the aspect ratio and the mechanical boundary conditions have been varied to build up this sample of models. Both, strongly dipolar and multipolar models have been obtained. We show that these dynamo regimes can in general be distinguished by the ratio of a typical convective length scale to the Rossby radius. Models with a predominantly dipolar magnetic field were obtained, if the convective length scale is at least an order of magnitude larger than the Rossby radius. Moreover, we highlight the role of the strong shear associated with the geostrophic zonal flow for models with stress-free boundary conditions. In this case, the above transition disappears and is replaced by a region of bistability for which dipolar and multipolar dynamos co-exist. We interpret our results in terms of dynamo eigenmodes using the so-called test-field method. We can thus show that models in the dipolar regime are characterized by an isolated ’single mode’. Competing overtones become significant as the boundary to multipolar dynamos is approached. We discuss how these findings relate to previous models and to observations.

Temperature-Pressure Profile of the hot Jupiter HD 189733b from HST Sodium Observations: Detection of Upper Atmospheric Heating

astro-ph — Wed, 22 Feb 2012 01:42:11 +0000

We present transmission spectra of the hot Jupiter HD 189733b taken with the Space Telescope Imaging Spectrograph aboard HST. The spectra cover the wavelength range 5808-6380 Ang with a resolving power of R=5000. We detect absorption from the NaI doublet within the exoplanet’s atmosphere at the 9 sigma confidence level within a 5 Ang band (absorption depth 0.09 +/- 0.01%) and use the data to measure the doublet’s spectral absorption profile. We detect only the narrow cores of the doublet. The narrowness of the feature could be due to an obscuring high-altitude haze of an unknown composition or a significantly sub-solar NaI abundance hiding the line wings beneath a H2 Rayleigh signature. We compare the spectral absorption profile over 5.5 scale heights with model spectral absorption profiles and constrain the temperature at different atmospheric regions, allowing us to construct a vertical temperature profile. We identify two temperature regimes; a 1280 +/- 240 K region derived from the NaI doublet line wings corresponding to altitudes below ~ 500 km, and a 2800 +/- 400 K region derived from the NaI doublet line cores corresponding to altitudes from ~ 500-4000 km. The zero altitude is defined by the white-light radius of Rp/Rstar=0.15628 +/- 0.00009. The temperature rises with altitude, which is likely evidence of a thermosphere. The absolute pressure scale depends on the species responsible for the Rayleigh signature and its abundance. We discuss a plausible scenario for this species, a high-altitude silicate haze, and the atmospheric temperature-pressure profile that results. In this case, the high altitude temperature rise for HD 189733b occurs at pressures of 10^-5 to 10^-8 bar.

Orbital perturbations due to massive rings [Replacement]

astro-ph — Wed, 22 Feb 2012 01:35:43 +0000

We analytically work out the long-term orbital perturbations induced by a homogeneous circular ring of radius Rr and mass mr on the motion of a test particle in the cases (I): r > R_r and (II): r < R_r. In order to extend the validity of our analysis to the orbital con?gurations of, e.g., some proposed spacecraftbased mission for fundamental physics like LISA and ASTROD, of possible annuli around the supermassive black hole in Sgr A* coming from tidal disruptions of incoming gas clouds, and to the e?ect of arti?cial space debris belts around the Earth, we do not restrict ourselves to the case in which the ring and the orbit of the perturbed particle lie just in the same plane. From the corrections to the standard secular perihelion precessions, recently determined by a team of astronomers for some planets of the Solar System, we infer upper bounds on mr for various putative and known annular matter distributions of natural origin (close circumsolar ring with R_r = 0.02-0.13 au, dust ring with R_r = 1 au, minor asteroids, Trans-Neptunian Objects). We find m_r <= 1.4 10^-4 m_E (circumsolar ring with R_r = 0.02 au), m_r <= 2.6 10^-6 m_E (circumsolar ring with R_r = 0.13 au), m_r <= 8.8 10^-7 m_E (ring with R_r = 1 au), m_r <= 7.3 10^-12 M_S (asteroidal ring with R_r = 2.80 au), m_r <= 1.1 <= 10^-11 M_S (asteroidal ring with R_r = 3.14 au), m_r <= 2.0 10^-8 M_S (TNOs ring with R_r = 43 au). In principle, our analysis is valid both for baryonic and non-baryonic Dark Matter distributions.

Multi-moment advection scheme in three dimension for Vlasov simulations of magnetized plasma [Replacement]

astro-ph — Wed, 22 Feb 2012 01:31:49 +0000

We present an extension of the multi-moment advection scheme (Minoshima et al., 2011, J. Comput. Phys.) to the three-dimensional case, for full electromagnetic Vlasov simulations of magnetized plasma. The scheme treats not only point values of a profile but also its zeroth to second order piecewise moments as dependent variables, and advances them on the basis of their governing equations. Similar to the two-dimensional scheme, the three-dimensional scheme can accurately solve the solid body rotation problem of a gaussian profile with little numerical dispersion or diffusion. This is a very important property for Vlasov simulations of magnetized plasma. We apply the scheme to electromagnetic Vlasov simulations. Propagation of linear waves and nonlinear evolution of the electron temperature anisotropy instability are successfully simulated with a good accuracy of the energy conservation.

Brightness and Fluctuation of the Mid-Infrared Sky from AKARI Observations towards the North Ecliptic Pole

astro-ph — Tue, 21 Feb 2012 02:06:21 +0000

We present the smoothness of the mid-infrared sky from observations by the Japanese infrared astronomical satellite AKARI. AKARI monitored the north ecliptic pole (NEP) during its cold phase with nine wavebands covering from 2.4 to 24 \mu m, out of which six mid-infrared bands were used in this study. A simple sinusoidal fit to the seasonal variation of the sky brightness shows that the mid-infrared brightness towards the NEP is not affected by small-scale features of the interplanetary dust cloud. We applied power spectrum analysis to the images in order to search for the fluctuation of the sky brightness. Observed fluctuation is explained by fluctuation of photon noise, shot noise of faint sources, and Galactic cirrus. The fluctuations at a few arcminutes scales at short mid-infrared wavelengths (7, 9, and 11 \mum) are largely caused by the diffuse Galactic light of the interstellar dust cirrus. At long mid-infrared wavelengths (15, 18, and 24 \mum), photon noise is the dominant source of fluctuation over the scale from arcseconds to a few arcminutes. The residual fluctuation power at 200″ after removing these contributions is at most 1.08 \pm 0.22 nW m^-2 sr^-1 or 0.05% of the brightness at 24 \mum and at least 0.52 \pm 0.13 nW m^-2 sr^-1 or 0.02% at 18 \mum. We conclude that the upper limit of the fluctuation in the zodiacal light is 0.02% of the sky brightness.

Multi-moment advection scheme in three dimension for Vlasov simulations of magnetized plasma [Cross-Listing]

astro-ph — Tue, 21 Feb 2012 01:54:57 +0000

AVAST Survey 0.4-1.0 {\mu}m Spectroscopy of Igneous Asteroids in the Inner and Middle Main Belt

astro-ph — Tue, 21 Feb 2012 01:44:15 +0000

We present the spectra of 60 asteroids, including 47 V-types observed during the first phase of the Adler V-Type Asteroid (AVAST) Survey. SDSS photometry was used to select candidate V-type asteroids for follow up by nature of their very blue i – z color. 47 of the 61 observed candidates were positively classified as V-type asteroids, while an additional six show indications of a 0.9 {\mu}m feature consistent with V-type spectra, but not sufficient for formal classification. Four asteroids were found to be S-type, all of which had i – z values very near the adopted AVAST selection criteria of i – z \leq -0.2, including one candidate observed well outside the cut (at a mean i – z of -0.11). Three A-type asteroids were also identified. Six V-type asteroids were identified beyond the 3:1 mean motion resonance with Jupiter, and six more were found with low (< 5 deg) inclination, placing these asteroids outside of the normal dynamical range of classic Vestoids, and are suggestive of a non-Vesta origin for at least some of the population.

Relativistic acceleration of Landau resonant particles as a consequence of Hopf bifurcations [Replacement]

astro-ph — Tue, 21 Feb 2012 01:41:49 +0000

Using bifurcation theory on a dynamical system simulating the interaction of a particle with an obliquely propagating wave in relativistic regimes, we demonstrate that uniform acceleration arises as a consequence of Hopf bifurcations of Landau resonant particles. The acceleration process arises as a form of surfatron established through the locking in pitch angle, gyrophase, and physical trapping along the wave-vector direction. Integrating the dynamical system for large amplitudes ($\delta B/B_0\sim0.1$) obliquely propagating waves, we find that electrons with initial energies in the keV range can be accelerated to MeV energies on timescales of the order of milliseconds. The Hopf condition of Landau resonant particles could underlie some of the most efficient energization of particles in space and astrophysical plasmas.

The Arecibo Detection of the Coolest Radio-flaring Brown Dwarf [Replacement]

astro-ph — Tue, 21 Feb 2012 01:33:23 +0000

Radio detection provides unique means to measure and study magnetic fields of the coolest brown dwarfs. Previous radio surveys have observed quiescent and flaring emission from brown dwarfs down to spectral type L3.5, but only upper limits have been established for even cooler objects. We report the detection of sporadic, circularly polarized flares from the T6.5 dwarf, 2MASS J1047+21, with the Arecibo radio telescope at 4.75 GHz. This is by far the coolest brown dwarf yet detected at radio frequencies. The fact that such an object is capable of generating observable, coherent radio emission, despite its very low, ~900 K temperature, demonstrates the feasibility of studies of brown dwarfs in the meagerly explored LTY spectral range, using radio detection as a tool.

Constraints on the Lifetimes of Disks Resulting from Tidally Destroyed Rocky Planetary Bodies

astro-ph — Mon, 20 Feb 2012 01:52:10 +0000

Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared excesses in the spectral energy distributions of HE 0110-5630, GD 61, and HE 1349-2305. All three of these stars have helium-dominated atmospheres, and their infrared emissions are consistent with warm dust produced by the tidal destruction of (minor) planetary bodies. This study brings the number of metal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRAC to 53 and 38 respectively. It also nearly doubles the number of metal-polluted helium-rich white dwarfs found to have closely orbiting dust by Spitzer. From the increased statistics for both atmospheric types with circumstellar dust, we derive a typical disk lifetime of log[t_{disk} (yr)] = 5.6+-1.1 (ranging from 3*10^4 – 5*10^6 yr). This assumes a relatively constant rate of accretion over the timescale where dust persists, which is uncertain. We find that the fraction of highly metal-polluted helium-rich white dwarfs that have an infrared excess detected by Spitzer is only 23 per cent, compared to 48 per cent for metal-polluted hydrogen-rich white dwarfs, and we conclude from this difference that the typical lifetime of dusty disks is somewhat shorter than the diffusion time scales of helium-rich white dwarf. We also find evidence for higher time-averaged accretion rates onto helium-rich stars compared to the instantaneous accretion rates onto hydrogen-rich stars; this is an indication that our picture of evolved star-planetary system interactions is incomplete. We discuss some speculative scenarios that can explain the observations.

Using Kepler transit observations to measure stellar spot belt migration rates

astro-ph — Mon, 20 Feb 2012 01:51:15 +0000

Planetary transits provide a unique opportunity to investigate the surface distributions of star spots. Our aim is to determine if, with continuous observation (such as the data that will be provided by the Kepler mission), we can in addition measure the rate of drift of the spot belts. We begin by simulating magnetic cycles suitable for the Sun and more active stars, incorporating both flux emergence and surface transport. This provides the radial magnetic field distribution on the stellar surface as a function of time. We then model the transit of a planet whose orbital axis is misaligned with the stellar rotation axis. Such a planet could occult spots at a range of latitudes. This allows us to complete the forward modelling of the shape of the transit lightcurve. We then attempt the inverse problem of recovering spot locations from the transit alone. From this we determine if transit lightcurves can be used to measure spot belt locations as a function of time. We find that for low-activity stars such as the Sun, the 3.5 year Kepler window is insufficient to determine this drift rate. For more active stars, it may be difficult to distinguish subtle differences in the nature of flux emergence, such as the degree of overlap of the “butterfly wings”. The rate and direction of drift of the spot belts can however be determined for these stars. This would provide a critical test of dynamo theory.

Secondary eclipse scanning of HD189733b: The perspectives of mapping distant worlds

astro-ph — Mon, 20 Feb 2012 01:48:16 +0000

Context. Mapping the brightness distribution of exoplanets is the next frontier for exoplanet infrared photometry studies. For tidally-locked hot Jupiters that transit and are eclipsed by their host star with non-zero impact parameter, the first steps are now possible. Aims. The aim is to use eclipse scanning from occultation ingress/egress and phase curve measurements to constrain exoplanet large-scale brightness structure. Methods. We use archived Spitzer/IRAC 8 {\mu}m data of HD189733 in a global MCMC procedure encompassing six transits, eight secondary eclipses, and a phase curve in a two-step analysis. The first step derives the planet-star system parameters. The second step investigates the structure found in eclipse scanning, using the previous planet-star system parameter derivation as Gaussian priors. Results. We find a 5-sigma deviation from the expected occultation ingress/egress shape for a uniform brightness disk, and demonstrate that this is dominated by large-scale brightness structure and not an occultation timing offset due to a non-zero eccentricity. Our analysis yields a 2D brightness temperature distribution showing a large-scale asymmetric hot spot whose finer structure is limited by the data quality and planet orbit geometry. We also present an improved upper limit for eccentricity, e<0.0081 (95% confidence). Conclusions. Reanalysis of archived HD 189733 data revealed brightness structure by using global analysis that mitigated systematics. Future eclipse scanning observations of the same exoplanet at other wavelengths will probe different atmosphere layers, ultimately generating a large-scale 3D map.

Ensemble Properties of Comets in the Sloan Digital Sky Survey

astro-ph — Mon, 20 Feb 2012 01:40:39 +0000

We present the ensemble properties of 31 comets (27 resolved and 4 unresolved) observed by the Sloan Digital Sky Survey (SDSS). This sample of comets represents about 1 comet per 10 million SDSS photometric objects. Five-band (u,g,r,i,z) photometry is used to determine the comets’ colors, sizes, surface brightness profiles, and rates of dust production in terms of the Af{\rho} formalism. We find that the cumulative luminosity function for the Jupiter Family Comets in our sample is well fit by a power law of the form N(< H) \propto 10(0.49\pm0.05)H for H < 18, with evidence of a much shallower fit N(< H) \propto 10(0.19\pm0.03)H for the faint (14.5 < H < 18) comets. The resolved comets show an extremely narrow distribution of colors (0.57 \pm 0.05 in g – r for example), which are statistically indistinguishable from that of the Jupiter Trojans. Further, there is no evidence of correlation between color and physical, dynamical, or observational parameters for the observed comets.

Theory of magnetic reconnection in solar and astrophysical plasmas

astro-ph — Mon, 20 Feb 2012 01:39:32 +0000

Magnetic reconnection is a fundamental process in a plasma that facilitates the release of energy stored in the magnetic field by permitting a change in the magnetic topology. In this article we present a review of the current state of understanding of magnetic reconnection. We discuss theoretical results regarding the formation of current sheets in complex 3D magnetic fields, and describe the fundamental differences between reconnection in two and three dimensions. We go on to outline recent developments in modelling of reconnection with kinetic theory, as well as in the MHD framework where a number of new 3D reconnection regimes have been identified. We discuss evidence from observations and simulations of solar system plasmas that support this theory, and summarise some prominent locations in which this new reconnection theory is relevant in astrophysical plasmas.

Transit-timing measurements with the model-independent barycenter method: Application to the LHS 6343 system [Replacement]

astro-ph — Mon, 20 Feb 2012 01:38:11 +0000

We present a model-independent technique for calculating the time of mid-transits. This technique, named “barycenter method”, uses the light-curve’s symmetry to determine the transit timing by calculating the transit light-curve barycenter. Unlike the other methods of calculating mid-transit timing, this technique does not depend on the parameters of the system and central star. We demonstrate the capabilities of the barycenter method by applying this technique to some known transiting systems including several \emph{Kepler} confirmed planets. Results indicate that for complete and symmetric transit lightcurves, the barycenter method achieves the same precision as other techniques, but with fewer assumptions and much faster. Among the transiting systems studied with the barycenter method, we focus in particular on LHS 6343C, a brown dwarf that transits a member of an M+M binary system, LHS 6343AB. We present the results of our analysis, which can be used to set an upper limit on the period and mass of a possible second small perturber.

Modeling the flyby anomalies with dark matter scattering: update with additional data and further predictions [Replacement]

astro-ph — Mon, 20 Feb 2012 01:34:52 +0000

We continue our exploration of whether the flyby anomalies can be explained by scattering of spacecraft nucleons from dark matter gravitationally bound to the earth, with the addition of data from five new flybys to that from the original six. We continue to use our model in which inelastic and elastic scatterers populate shells generated by the precession of circular orbits with normals tilted with respect to the earth’s axis. With 11 data points and 8 parameters in the model, a statistically meaningful fit is obtained with a chi-squared of 2.7. We give plots of the anomalous acceleration along the spacecraft trajectory, and the cumulative velocity change, for the five flybys which exhibit a significant nonzero anomaly. We also discuss implications of the fit for dark matter-nucleon cross sections, give the prediction of our fit for the anomaly to be expected from the future Juno flyby, and give predictions of our fit for flyby orbit orientation changes. In addition we give formulas for estimating the flyby temperature increase caused by dark matter inelastic scattering, and for the fraction of flyby nucleons undergoing such scatters. Finally, for circular satellite orbits, we give a table of predicted secular changes in orbit radius. These are much too large to be reasonable — comparing with data for COBE and GP-B supplied to us by Edward Wright (after the first version of this paper was posted), we find that our model predicts changes in orbit radius that are too large by many orders of magnitude. So the model studied here is ruled out. We conclude that further modeling of the flyby anomalies must simultaneously attempt to fit constraints coming from satellite orbits.

Spectral properties of oscillatory and non-oscillatory {\alpha}^2-dynamos [Replacement]

astro-ph — Mon, 20 Feb 2012 01:33:01 +0000

The eigenvalues and eigenfunctions of a linear {\alpha}^{2}-dynamo have been computed for different spatial distributions of an isotropic \alpha-effect. Oscillatory solutions are obtained when \alpha exhibits a sign change in the radial direction. The time-dependent solutions arise at so called exceptional points where two stationary modes merge and continue as an oscillatory eigenfunction with conjugate complex eigenvalues. The close proximity of oscillatory and non-oscillatory solutions may serve as the basic ingredient for reversal models that describe abrupt polarity switches of a dipole induced by noise. Whereas the presence of an inner core with different magnetic diffusivity has remarkable little impact on the character of the dominating dynamo eigenmodes, the introduction of equatorial symmetry breaking considerably changes the geometric character of the solutions. Around the dynamo threshold the leading modes correspond to hemispherical dynamos even when the symmetry breaking is small. This behavior can be explained by the approximate dipole-quadrupole degeneration for the unperturbed problem. More complicated scenarios may occur in case of more realistic anisotropies of \alpha- and \beta-effect or through non-linearities caused by the back-reaction of the magnetic field (magnetic quenching).

Running coupling in electroweak interactions of leptons from f(R)-gravity with torsion [Cross-Listing]

astro-ph — Mon, 20 Feb 2012 01:31:40 +0000

The f(R)-gravitational theory with torsion is considered for one family of leptons; it is found that the torsion tensor gives rise to interactions having the structure of the weak forces while the intrinsic non-linearity of the f(R) function provides an energy-dependent coupling: in this way, torsional f(R) gravity naturally generates both structure and strength of the electroweak interactions among leptons. This implies that the weak interactions among the lepton fields could be addressed as a geometric effect due to the interactions among spinors induced by the presence of torsion in the most general f(R) gravity. Phenomenological considerations are addressed.

Detection of Ks-band Thermal Emission from WASP-3b

astro-ph — Fri, 17 Feb 2012 01:53:20 +0000

We report the detection of thermal emission from the hot Jupiter WASP-3b in the KS band, using a newly developed guiding scheme for the WIRC instrument at the Palomar Hale 200in telescope. Our new guiding scheme has improved the telescope guiding precision by a factor of ~5-7, significantly reducing the correlated systematics in the measured light curves. This results in the detection of a secondary eclipse with depth of 0.181%\pm0.020% (9-{\sigma}) – a significant improvement in WIRC’s photometric precision and a demonstration of the capability of Palomar/WIRC to produce high quality measurements of exoplanetary atmospheres. Our measured eclipse depth cannot be explained by model atmospheres with heat redistribution but favor a pure radiative equilibrium case with no redistribution across the surface of the planet. Our measurement also gives an eclipse phase center of 0.5045\pm0.0020, corresponding to an ecos{\omega} of 0.0070\pm0.0032. This result is consistent with a circular orbit, although it also suggests the planet’s orbit might be slightly eccentric. The possible non-zero eccentricity provides insight into the tidal circularization process of the star-planet system, but also might have been caused by a second low-mass planet in the system, as suggested by a previous transit timing variation study. More secondary eclipse observations, especially at multiple wavelengths, are necessary to determine the temperature-pressure profile of the planetary atmosphere and shed light on its orbital eccentricity.

Gaps in Protoplanetary Disks as Signatures of Planets: I. Methodology and Validation

astro-ph — Fri, 17 Feb 2012 01:48:32 +0000

We examine the observational consequences of partial gaps being opened by planets in protoplanetary disks. We model the disk using a static alpha-disk model with detailed radiative transfer, parametrizing the shape and size of the partially cleared gaps based on the results of hydrodynamic simulations. Shadowing and illumination by stellar irradiation at the surface of the gap leads to increased contrast as the gap trough is deepened by shadowing and cooling and the far gap wall is puffed up by illumination and heating. In calculating observables, we find that multiple scattering is important and derive an approximation to include these effects. A gap produced by a 200 M_Earth (70 M_Earth) planet at 10 AU can lower/raise the midplane temperature of the disk by up to ~-25/+29% (~-11/+19%) by shadowing in the gap trough and illumination on the far shoulder of the gap. At the distance of Taurus, this gap would be resolvable with ~0.01″ angular resolution. The gap contrast is most significant in scattered light and at thermal continuum wavelengths characteristic of the surface temperature, reducing or raising the surface brightness by up to order of magnitude. Since gaps sizes are correlated to planet mass, this is a promising way of finding and determining the masses of planets embedded in protoplanetary disks.

Sundman Stability of Natural Planet Satellites

astro-ph — Fri, 17 Feb 2012 01:44:54 +0000

The stability of the motion of the planet satellites is considered in the model of the general three-body problem (Sun-planet-satellite). “Sundman surfaces” are constructed, by means of which the concept “Sundman stability” is formulated. The comparison of the Sundman stability with the results of Golubev’s c2h method and with the Hill’s classical stability in the restricted three-body problem is performed. The constructed Sundman stability regions in the plane of the parameters “energy – moment of momentum” coincide with the analogous regions obtained by Golubev’s method, with the value (c2h)cr. The construction of the Sundman surfaces in the three-dimensional space of the specially selected coordinates xyR is carried out by means of the exact Sundman inequality in the general three-body problem. The determination of the singular points of surfaces, the regions of the possible motion and Sundman stability analysis are implemented. It is shown that the singular points of the Sundman surfaces in the coordinate space xyR lie in different planes. Sundman stability of all known natural satellites of planets is investigated. It is shown that a number of the natural satellites, that are stable according to Hill and also some satellites that are stable according to Golubev’s method are unstable in the sense of Sundman stability.

Evidence for enhanced chromospheric Ca II H & K emission in stars with close-in extrasolar planets

astro-ph — Fri, 17 Feb 2012 01:41:46 +0000

The planet-star interaction is manifested in many ways. It was found out that a close-in exoplanet causes small but measurable variability in the cores of a few lines in the spectra of several stars which corresponds to the orbital period of the exoplanet. Stars with and without exoplanets may have different properties. The main goal of our study is to search for influence which exoplanets might have on atmospheres of their host stars. Unlike the previous studies, we do not study changes in the spectrum of a host star or differences between stars with and without exoplanets. We aim to study a large number of stars with exoplanets, current level of their chromospheric activity and look for a possible correlation with the exoplanetary properties. To analyse the chromospheric activity of stars we exploit our own (2.2m ESO/MPG telescope) and publicly available archival spectra (Keck Observatory Archive), measure the equivalent widths of the cores of Ca II H and K lines and use them as a tracer of their activity. Subsequently, we search for their dependence on the orbital parameters and mass of the exoplanet. We found a statistically significant evidence that the equivalent width of the Ca II K line emission and log R’_{HK} activity parameter of the host star varies with the semi-major axis and mass of the exoplanet. Stars with T_eff <= 5500 K having exoplanets with semi-major axis a <= 0.15 AU (P_orb <= 20 days) have a broad range of Ca II K emissions and much stronger emission in general than stars at similar temperatures but with higher values of semi-major axes. Ca II K emission of cold stars (T_eff <= 5500 K) with close-in exoplanets (a <= 0.15 AU) is also more pronounced for more massive exoplanets. The overall level of the chromospheric activity of stars may be affected by their close-in exoplanets. Stars with massive close-in exoplanets may be more active.

TNOs are Cool: A survey of the trans-Neptunian region V. Physical characterization of 18 Plutinos using Herschel PACS observations

astro-ph — Fri, 17 Feb 2012 01:39:02 +0000

We present Herschel PACS photometry of 18 Plutinos and determine sizes and albedos for these objects using thermal modeling. We analyze our results for correlations, draw conclusions on the Plutino size distribution, and compare to earlier results. Flux densities are derived from PACS mini scan-maps using specialized data reduction and photometry methods. In order to improve the quality of our results, we combine our PACS data with existing Spitzer MIPS data where possible, and refine existing absolute magnitudes for the targets. The physical characterization of our sample is done using a thermal model. Uncertainties of the physical parameters are derived using customized Monte Carlo methods. The correlation analysis is performed using a bootstrap Spearman rank analysis. We find the sizes of our Plutinos to range from 150 to 730 km and geometric albedos to vary between 0.04 and 0.28. The average albedo of the sample is 0.08 \pm 0.03, which is comparable to the mean albedo of Centaurs, Jupiter Family comets and other Trans-Neptunian Objects. We were able to calibrate the Plutino size scale for the first time and find the cumulative Plutino size distribution to be best fit using a cumulative power law with q = 2 at sizes ranging from 120-400 km and q = 3 at larger sizes. We revise the bulk density of 1999 TC36 and find a density of 0.64 (+0.15/-0.11) g cm-3. On the basis of a modified Spearman rank analysis technique our Plutino sample appears to be biased with respect to object size but unbiased with respect to albedo. Furthermore, we find biases based on geometrical aspects and color in our sample. There is qualitative evidence that icy Plutinos have higher albedos than the average of the sample.

On the connection between the magneto-elliptic and magneto-rotational instabilities [Cross-Listing]

astro-ph — Fri, 17 Feb 2012 01:37:41 +0000

It has been recently suggested that the magneto-rotational instability (MRI) is a limiting case of the magneto-elliptic instability (MEI). This limit is obtained for horizontal modes in the presence of rotation and an external vertical magnetic field, when the aspect ratio of the elliptic streamlines tends to infinite. In this paper we unveil the link between these previously unconnected mechanisms, explaining both the MEI and the MRI as different manifestations of the same Magneto-Elliptic-Rotational Instability (MERI). The growth rates are found and the influence of the magnetic and rotational effects is explained, in particular the effect of the magnetic field on the range of negative Rossby numbers at which the horizontal instability is excited. Furthermore, we show how the horizontal rotational MEI in the rotating shear flow limit links to the MRI by the use of the local shearing box model, typically used in the study of accretion discs. In such limit the growth rates of the two instability types coincide for any power-type background angular velocity radial profile with negative exponent corresponding to the value of the Rossby number of the rotating shear flow. The MRI requirement for instability is that the background angular velocity profile is a decreasing function of the distance from the centre of the disk which corresponds to the horizontal rotational MEI requirement of negative Rossby numbers. Finally a physical interpretation of the horizontal instability, based on a balance between the strain, the Lorentz force and the Coriolis force is given.

Debris disks as signposts of terrestrial planet formation. II Dependence of exoplanet architectures on giant planet and disk properties [Replacement]

astro-ph — Fri, 17 Feb 2012 01:33:49 +0000

We present models for the formation of terrestrial planets, and the collisional evolution of debris disks, in planetary systems that contain multiple unstable gas giants. We previously showed that the dynamics of the giant planets introduces a correlation between the presence of terrestrial planets and debris disks. Here we present new simulations that show that this connection is qualitatively robust to changes in: the mass distribution of the giant planets, the width and mass distribution of the outer planetesimal disk, and the presence of gas in the disk. We discuss how variations in these parameters affect the evolution. Systems with equal-mass giant planets undergo the most violent instabilities, and these destroy both terrestrial planets and the outer planetesimal disks that produce debris disks. In contrast, systems with low-mass giant planets efficiently produce both terrestrial planets and debris disks. A large fraction of systems with low-mass outermost giant planets have stable gaps between these planets that are frequently populated by planetesimals. Planetesimal belts between outer giant planets may affect debris disk SEDs. If Earth-mass seeds are present in outer planetesimal disks, the disks radially spread to colder temperatures. We argue that this may explain the very low frequency of > 1 Gyr-old solar-type stars with observed 24 micron excesses. Among the (limited) set of configurations explored, the best candidates for hosting terrestrial planets at ~1 AU are stars older than 0.1-1 Gyr with bright debris disks at 70 micron but with no currently-known giant planets. These systems combine evidence for rocky building blocks, with giant planet properties least likely to undergo destructive dynamical evolution. We predict an anti-correlation between debris disks and eccentric giant planets, and a positive correlation between debris disks and terrestrial planets.

Orbital migration of interacting low-mass planets in evolutionary radiative turbulent models [Replacement]

astro-ph — Fri, 17 Feb 2012 01:32:41 +0000

The torques exerted by a locally isothermal disk on an embedded planet lead to rapid inward migration. Recent work has shown that modeling the thermodynamics without the assumption of local isothermality reveals regions where the net torque on an embedded planet is positive, leading to outward migration of the planet. When a region with negative torque lies directly exterior to this, planets in the inner region migrate outwards and planets in the outer region migrate inwards, converging where the torque is zero. We incorporate the torques from an evolving non-isothermal disk into an N-body simulation to examine the behavior of planets or planetary embryos interacting in the convergence zone. We find that mutual interactions do not eject objects from the convergence zone. Small numbers of objects in a laminar disk settle into near resonant orbits that remain stable over the 10 Myr periods that we examine. However, either or both increasing the number of planets or including a correlated, stochastic force to represent turbulence drives orbit crossings and mergers in the convergence zone. These processes can build gas giant cores with masses of order ten Earth masses from sub-Earth mass embryos in 2-3 Myr.

Indirect Dark Matter Detection in the Light of Sterile Neutrinos [Cross-Listing]

astro-ph — Fri, 17 Feb 2012 01:31:04 +0000

The recent global fit of short baseline neutrino oscillation data favors the presence of one (or more) sterile neutrino state which leads to new mass splitting \Delta m^2 ~1 eV^2. We consider the effect of this new states on the evolution of neutrinos from the dark matter annihilation inside the Sun. We show that neutrinos with energy E_\nu > 100 GeV undergo resonant active-sterile oscillation which depletes the flux of neutrinos arriving at the Earth. As an example of this effect, we present the oscillation probabilities for the case of monochromatic neutrinos from the direct annihilation of dark matter particles to neutrinos and the depletion due to the presence of sterile neutrinos.

The Great Escape II: Exoplanet Ejection from Dying Multiple Star Systems

astro-ph — Thu, 16 Feb 2012 02:02:34 +0000

Extrasolar planets and belts of debris orbiting post-main-sequence single stars may become unbound as the evolving star loses mass. In multiple star systems, the presence or co-evolution of the additional stars can significantly complicate the prospects for orbital excitation and escape. Here, we investigate the dynamical consequences of multi-phasic, nonlinear mass loss and establish a criterion for a system of any stellar multiplicity to retain a planet whose orbit surrounds all of the parent stars. For single stars which become white dwarfs, this criterion can be combined with the Chandrasekhar Limit to establish the maximum allowable mass loss rate for planet retention. We then apply the criterion to circumbinary planets in evolving binary systems over the entire stellar mass phase space. Through about 10^5 stellar evolutionary track realizations, we characterize planetary ejection prospects as a function of binary separation, stellar mass and metallicity. This investigation reveals that planets residing at just a few tens of AU from a central concentration of stars are susceptible to escape in a wide variety of multiple systems. Further, planets are significantly more susceptible to ejection from multiple star systems than from single star systems for a given system mass. For system masses greater than about 2 Solar masses, multiple star systems represent the greater source of free-floating planets.

The sources of HCN and CH3OH and the Rotational Temperature in Comet 103P/Hartley 2 from Time-Resolved Millimeter Spectroscopy

astro-ph — Thu, 16 Feb 2012 01:55:19 +0000

One of the least understood properties of comets is the compositional structure of their nuclei, which can either be homogeneous or heterogeneous. The nucleus structure can be conveniently studied at millimeter wavelengths, using velocity-resolved spectral time series of the emission lines, obtained simultaneously for multiple molecules as the body rotates. Using this technique, we investigated the sources of CH3OH and HCN in comet 103P/Hartley 2, the target of NASA’s EPOXI mission, which had an exceptionally favorable apparition in late 2010. Our monitoring at IRAM 30-m shows short-term variability of the spectral lines caused by nucleus rotation. The varying production rates generate changes in brightness by a factor of 5 for HCN and by a factor of 2 for CH3OH, and they are remarkably well correlated in time. With the addition of the velocity information from the line profiles, we identify the main sources of outgassing: two jets, oppositely directed in a radial sense, and icy grains, injected into the coma primarily through one of the jets. The mixing ratio of CH3OH and HCN is dramatically different in the two jets, which evidently shows large-scale chemical heterogeneity of the nucleus. We propose a network of identities linking the two jets with morphological features reported elsewhere, and postulate that the chemical heterogeneity may result from thermal evolution. The model-dependent average production rates are 2.10×10**26 molec/s for CH3OH and 1.25×10**25 molec/s for HCN and their ratio is typical of comets. The rotational temperature from CH3OH varied strongly, presumably due to nucleus rotation, with the average value 47 K.

The Change in Jupiter’s Moment of Inertia due to Core Erosion and Planetary Contraction

astro-ph — Thu, 16 Feb 2012 01:52:01 +0000

We explore the change in Jupiter’s normalized axial moment of inertia (NMOI) assuming that Jupiter undergoes core erosion. It is found that Jupiter’s contraction combined with an erosion of 20 M_Earth from a primordial core of 30 M_Earth can change Jupiter’s NMOI over time significantly. It is shown that Jupiter’s NMOI could have changed from ~0.235 to ~0.264 throughout its evolution. We find that a NMOI value of ~0.235 as suggested by dynamical models (Ward & Canup, 2006, ApJ, 640, L91) could, in principle, be consistent with Jupiter’s primordial internal structure. Low NMOI values, however, persist only for the first ~ 10^6 years of Jupiter’s evolution. Re-evaluation of dynamical stability models as well as more sophisticated evolution models of Jupiter with core erosion seem to be required in order to provide more robust estimates for Jupiter’s primordial NMOI.

2008 LC18: a potentially unstable Neptune Trojan

astro-ph — Thu, 16 Feb 2012 01:49:39 +0000

The recent discovery of the first Neptune Trojan at the planet’s trailing (L5) Lagrange point, 2008 LC18, offers an opportunity to confirm the formation mechanism of a member of this important tracer population for the Solar system’s dynamical history. We tested the stability of 2008 LC18’s orbit through a detailed dynamical study, using test particles spread across the \pm3{\sigma} range of orbital uncertainties in a, e, i and {\Omega}. This showed that the wide uncertainties of the published orbit span regions of both extreme dynamical instability, with lifetimes 1 Gyr lifetimes). The stability of 2008 LC18’s clones is greatly dependent on their semi-major axis and only weakly correlated with their eccentricity. Test particles on orbits with an initial semi-major axis less than 29.91 AU have dynamical half-lives shorter than 100 Myr; in contrast, particles with an initial semi-major axis greater than 29.91 AU exhibit such strong dynamical stability that almost all are retained over the 1 Gyr of our simulations. More observations of this object are necessary to improve the orbit. If 2008 LC18 is in the unstable region, then our simulations imply that it is either a temporary Trojan capture, or a representative of a slowly decaying Trojan population (like its sibling the L4 Neptunian Trojan 2001 QR322), and that it may not be primordial. Alternatively, if the orbit falls into the larger, stable region, then 2008 LC18 is a primordial member of the highly stable and highly inclined component of the Neptune Trojan population, joining 2005 TN53 and 2007 VL305. We attempted to recover 2008 LC18 using the 2.3 m telescope at Siding Spring Observatory to provide this astrometry, but were unsuccessful due to the high stellar density of its current sky location near the galactic centre. The recovery of this object will require a telescope in the 8m class.

The Influence of Atmospheric Scattering and Absorption on Ohmic Dissipation in Hot Jupiters

astro-ph — Thu, 16 Feb 2012 01:47:01 +0000

Using semi-analytical, one-dimensional models, we elucidate the influence of scattering and absorption on the degree of Ohmic dissipation in hot Jovian atmospheres. With the assumption of Saha equilibrium, the variation in temperature is the main driver of the variations in the electrical conductivity, induced current and Ohmic power dissipated. Atmospheres possessing temperature inversions tend to dissipate most of the Ohmic power superficially, at high altitudes, whereas those without temperature inversions are capable of greater dissipation deeper down. Scattering in the optical range of wavelengths tends to cool the lower atmosphere, thus reducing the degree of dissipation at depth. Purely absorbing cloud decks (in the infrared), of a finite extent in height, allow for localized reductions in dissipation and may reverse a temperature inversion if they are dense and thick enough, thus greatly enhancing the dissipation at depth. If Ohmic dissipation is the mechanism for inflating hot Jupiters, then variations in the atmospheric opacity (which may be interpreted as arising from variations in metallicity and cloud/haze properties) and magnetic field strength naturally produce a scatter in the measured radii at a given strength of irradiation. Future work will determine if these effects are dominant over evolutionary effects, which also contribute a scatter to the measured radii.

Observational and Dynamical Characterization of Main-Belt Comet P/2010 R2 (La Sagra) [Replacement]

astro-ph — Thu, 16 Feb 2012 01:38:28 +0000

We present observations of comet-like main-belt object P/2010 R2 (La Sagra) obtained by Pan-STARRS 1 and the Faulkes Telescope-North on Haleakala in Hawaii, the University of Hawaii 2.2 m, Gemini-North, and Keck I telescopes on Mauna Kea, the Danish 1.54 m telescope at La Silla, and the Isaac Newton Telescope on La Palma. An antisolar dust tail is observed from August 2010 through February 2011, while a dust trail aligned with the object’s orbit plane is also observed from December 2010 through August 2011. Assuming typical phase darkening behavior, P/La Sagra is seen to increase in brightness by >1 mag between August 2010 and December 2010, suggesting that dust production is ongoing over this period. These results strongly suggest that the observed activity is cometary in nature (i.e., driven by the sublimation of volatile material), and that P/La Sagra is therefore the most recent main-belt comet to be discovered. We find an approximate absolute magnitude for the nucleus of H_R=17.9+/-0.2 mag, corresponding to a nucleus radius of ~0.7 km, assuming an albedo of p=0.05. Using optical spectroscopy, we find no evidence of sublimation products (i.e., gas emission), finding an upper limit CN production rate of Q_CN<6×10^23 mol/s, from which we infer an H2O production rate of Q_H2O100 Myr, suggesting that it is likely native to its current location and that its composition is likely representative of other objects in the same region of the main belt, though the relatively close proximity of the 13:6 mean-motion resonance with Jupiter and the (3,-2,-1) three-body mean-motion resonance with Jupiter and Saturn mean that dynamical instability on larger timescales cannot be ruled out.

Habitability of Earth-type Planets and Moons in the Kepler-16 System [Replacement]

astro-ph — Thu, 16 Feb 2012 01:35:21 +0000

We demonstrate that habitable Earth-type planets and moons can exist in the Kepler-16 system by investigating their orbital stability in the standard and extended habitable zone (HZ). We find that Earth-type planets in S-type orbits are possible within the standard HZ in direct vicinity of Kepler-16b, thus constituting habitable exomoons. However, Earth-mass planets cannot exist in P-type orbits around the two stellar components within the standard HZ. Yet, P-type Earth-mass planets can exist superior to the giant planet in the extended HZ pertaining to considerably enhanced back-warming in the planetary atmosphere if facilitated. We briefly discuss the potential detectability of such habitable Earth-type moons and planets positioned in S-type and P-type orbits, respectively.

Asteroseismology of the solar analogs 16 Cyg A & B from Kepler observations [Replacement]

astro-ph — Thu, 16 Feb 2012 01:35:12 +0000

The evolved solar-type stars 16 Cyg A & B have long been studied as solar analogs, yielding a glimpse into the future of our own Sun. The orbital period of the binary system is too long to provide meaningful dynamical constraints on the stellar properties, but asteroseismology can help because the stars are among the brightest in the Kepler field. We present an analysis of three months of nearly uninterrupted photometry of 16 Cyg A & B from the Kepler space telescope. We extract a total of 46 and 41 oscillation frequencies for the two components respectively, including a clear detection of octupole (l=3) modes in both stars. We derive the properties of each star independently using the Asteroseismic Modeling Portal, fitting the individual oscillation frequencies and other observational constraints simultaneously. We evaluate the systematic uncertainties from an ensemble of results generated by a variety of stellar evolution codes and fitting methods. The optimal models derived by fitting each component individually yield a common age (t=6.8+/-0.4 Gyr) and initial composition (Z_i=0.024+/-0.002, Y_i=0.25+/-0.01) within the uncertainties, as expected for the components of a binary system, bolstering our confidence in the reliability of asteroseismic techniques. The longer data sets that will ultimately become available will allow future studies of differential rotation, convection zone depths, and long-term changes due to stellar activity cycles.

On The Effect of Giant Planets on the Scattering of Parent Bodies of Iron Meteorite from the Terrestrial Planet Region into the Asteroid Belt: A Concept Study

astro-ph — Wed, 15 Feb 2012 01:50:32 +0000

In their model for the origin of the parent bodies of iron meteorites, Bottke et al proposed differentiated planetesimals that were formed in the region of 1-2 AU during the first 1.5 Myr, as the parent bodies, and suggested that these objects and their fragments were scattered into the asteroid belt as a result of interactions with planetary embryos. Although viable, this model does not include the effect of a giant planet that might have existed or been growing in the outer regions. We present the results of a concept study where we have examined the effect of a planetary body in the orbit of Jupiter on the early scattering of planetesimals from terrestrial region into the asteroid belt. We integrated the orbits of a large battery of planetesimals in a disk of planetary embryos, and studied their evolutions for different values of the mass of the planet. Results indicate that when the mass of the planet is smaller than 10 Earth-masses, its effects on the interactions among planetesimals and planetary embryos is negligible. However, when the planet mass is between 10 and 50 Earth-masses, simulations point to a transitional regime with ~50 Earth-mass being the value for which the perturbing effect of the planet can no longer be ignored. Simulations also show that further increase of the mass of the planet strongly reduces the efficiency of the scattering of planetesimals from the terrestrial planet region into the asteroid belt. We present the results of our simulations and discuss their possible implications for the time of giant planet formation.

On the origin of planets at very wide orbits from re-capture of free floating planets

astro-ph — Tue, 14 Feb 2012 02:14:56 +0000

In recent years several planets have been discovered at wide orbits (>100 AU) around their host stars. Theoretical studies encounter difficulties in explaining their formation and origin. Here we propose a novel scenario for the production of planetary systems at such orbits, through the dynamical recapture of free floating planets (FFPs) in dispersing stellar clusters. This process is a natural extension of the recently suggested scenario for the formation of wide stellar binaries. We use N-body simulations of dispersing clusters with 10-1000 and f_FFP=0.5-2 to study this process. We find that planets are captured into wide orbits, ~100-10^6 AU, and a thermal eccentricity distribution. Typically, 3-6x(f_FFP/1) % of all stars capture a planetary companion (f_FFP is the number of FFP per star). The planetary capture efficiency is comparable to that of capture-formed stellar-binaries, and shows a similar dependence on the cluster size and structure. The capture efficiency is almost independent of the specific planetary mass; planets as well as sub-stellar companions of any mass can be captured, where the capture efficiency decreases with increasing cluster size. For a given cluster size the capture efficiency increases with the host/primary mass. More than one planet can be captured around the same host, and planets can be captured into binary systems. We also expect planets to be captured into pre-existing planetary systems as well as around compact objects, if these formed early enough before the cluster dispersal. In particular, stellar black holes have a high capture efficiency (>50 % and 5-10x(f_FFP/1) % for capture of stars and planetary companions, respectively) due to their large mass. Finally, although rare, two FFPs or brown dwarfs can become bound and form a FFP-binary system with no stellar host through this process.

The Habitable Zone Gallery

astro-ph — Tue, 14 Feb 2012 02:11:44 +0000

The Habitable Zone Gallery (www.hzgallery.org) is a new service to the exoplanet community which provides Habitable Zone (HZ) information for each of the exoplanetary systems with known planetary orbital parameters. The service includes a sortable table with information on the percentage of orbital phase spent within the HZ, planetary effective temperatures, and other basic planetary properties. In addition to the table, we also plot the period and eccentricity of the planets with respect to their time spent in the HZ. The service includes a gallery of known systems which plot the orbits and the location of the HZ with respect to those orbits. Also provided are animations which aid in orbit visualization and provide the changing effective temperature for those planets in eccentric orbits. Here we describe the science motivation, the under-lying calculations, and the structure of the web site.

A revised asteroid polarization-albedo relationship using WISE/NEOWISE data

astro-ph — Tue, 14 Feb 2012 02:10:42 +0000

We present a reanalysis of the relationship between asteroid albedo and polarization properties using the albedos derived from the Wide-field Infrared Survey Explorer. We find that the function that best describes this relation is a three-dimensional linear fit in the space of log(albedo)-log(polarization slope)-log(minimum polarization). When projected to two dimensions the parameters of the fit are consistent with those found in previous work. We also define p* as the quantity of maximal polarization variation when compared with albedo and present the best fitting albedo-p* relation. Some asteroid taxonomic types stand out in this three-dimensional space, notably the E, B, and M Tholen types, while others cluster in clumps coincident with the S- and C-complex bodies. We note that both low albedo and small (D<30 km) asteroids are under-represented in the polarimetric sample, and we encourage future polarimetric surveys to focus on these bodies.

The HARPS search for southern extra-solar planets XXXV. Super-Earths around the M-dwarf neighbors Gl433 and Gl667C

astro-ph — Tue, 14 Feb 2012 02:07:26 +0000

M dwarfs have been found to often have super-Earth planets with short orbital periods. Such stars are thus preferential targets in searches for rocky or ocean planets in the solar neighbourhood. In a recent paper (Bonfils et al. 2011), we announced the discovery of respectively 1 and 2 low mass planets around the M1.5V stars Gl433 and Gl667C. We found those planets with the HARPS spectrograph on the ESO~3.6-m telescope at La Silla Observatory, from observations obtained during the Guaranteed Time Observing program of that instrument. We have obtained additional HARPS observations of those two stars, for a total of respectively 67 and 179 Radial Velocity measurements for Gl433 and Gl667C, and present here an orbital analysis of those extended data sets and our main conclusion about both planetary systems. One of the three planets, Gl667Cc, has a mass of only M2.sin(i)~4.25 M_earth and orbits in the central habitable zone of its host star. It receives just 10% less stellar energy from Gl667C than the Earth receives from the Sun. However planet evolution in habitable zone can be very different if the host star is a M dwarf or a solar-like star, without necessarily questioning the presence of water. The two other planets, Gl433b and Gl667Cb, both have M2.sin(i) of ~5.5 M_earth and periods of ~7 days. The Radial Velocity measurements of both stars contain longer time scale signals, which we fit as longer period Keplerians. For Gl433 that signal probably originates in a Magnetic Cycle, while a longer time span will be needed to conclude for Gl667C. The metallicity of Gl433 is close to solar, while Gl667C is metal poor with [Fe/H] ~ -0.6. This reinforces the recent conclusion that the occurence of Super-Earth planets does not strongly correlate with stellar metallicity.

The HARPS-TERRA project I. Description of the algorithms, performance and new measurements on a few remarkable stars observed by HARPS

astro-ph — Tue, 14 Feb 2012 02:02:55 +0000

Doppler spectroscopy has uncovered or confirmed all the known planets orbiting nearby stars. Two different approaches are used to obtain precision Doppler measurements at optical wavelengths. The first approach is the gas cell method, which is based on the least-squares matching of the absorption spectrum of Iodine over-imposed to the spectrum of the star. The second method relies on the construction of a stabilized spectrograph calibrated in wavelength with an externally fed calibration source. The most precise stabilized spectrometer in operation is HARPS, operated by ESO in La Silla/Chile. In the case of HARPS, the Doppler measurement is obtained using the so–called Cross-Correlation Function technique (CCF). It consists of multiplying the stellar spectrum with a binary mask and finding the minimum of such product as a function of the stellar Doppler shift. Such mask is weighted to account for the different depths of the stellar lines. It is known that CCF is suboptimal in exploiting the Doppler information in the stellar spectrum. Here, we describe an algorithm to obtain precision RV measurements based on least squares matching of each observation to a high signal-to-noise ratio template. Such algorithm is implemented in our software called HARPS-TERRA (Template Enhanced Radial velocity Re-analysis Application). We show that, compared to CCF, template matching provides a significant improvement in accuracy, specially when applied to M dwarfs. We conclude that other stabilized spectrographs should use a similar approach to achieve the sub \ms precision required to detect potentially habitable worlds around nearby stars.

Constrains on planets around beta Pic with Harps radial velocity data

astro-ph — Tue, 14 Feb 2012 01:59:37 +0000

Context. The {\beta} Pictoris system with its debris disk and a massive giant planet orbiting at \simeq 9 AU represents an ideal laboratory to study giant planet formation and evolution as well as planet-disk interactions. {\beta} Pic b can also help testing brightness-mass relations at young ages. Other planets, yet undetected, may of course be present in the system. Aims. We aim at putting direct constrains on the mass of {\beta} Pic b and at searching for additional jovian planets on orbits closer than typically 2 AU. Methods. We use high precision Harps data collected over 8 years since 2003 to measure and analyse {\beta} Pic radial velocities. Results. We show that the true mass of {\beta} Pic b is less than 10, 12, 15.5, 20 and 25 MJup if orbiting respectively at 8, 9, 10, 11 and 12 AU. This is the first direct constraint on the mass of an imaged planet. The upper mass found is well in the range predicted by brightness-mass relations provided by current “hot start” models. We also exclude the presence of giant planets more massive than 2.5 MJup with periods less than 100 days (hot Jupiters), more massive than 9 MJup for periods in the range 100-500 days. In the 500-1000 day range, the detection limit is in the brown dwarf domain. Beyond the intrinsic interest for {\beta} Pic, these results show the possibilities of precise RV measurements of early type, rapidly rotating stars.

The Maximal Runaway Temperature of Earth-like Planets

astro-ph — Tue, 14 Feb 2012 01:55:44 +0000

We generalize the problem of the semi-gray model to cases in which a non-negligible fraction of the stellar radiation falls on the long-wavelength range, and/or that the planetary long-wavelength emission penetrates into the transparent short wavelength domain of the absorption. Second, applying the most general assumptions and independently of any particular properties of an absorber, we show that the greenhouse effect saturates and any Earth-like planet has a maximal temperature which depends on the type of and distance to its main-sequence star, its albedo and the primary atmospheric components which determine the cutoff frequency below which the atmosphere is optically thick. For example, a hypothetical convection-less planet similar to Venus, that is optically thin in the visible, could have at most a surface temperature of 1200-1300K irrespective of the nature of the greenhouse gas. We show that two primary mechanisms are responsible for the saturation of the runaway greenhouse effect, depending on the value of the wavelength above which the atmosphere becomes optically thick. Unless this wavelength is small and resides in the optical region, saturation is achieved by radiating the thermal flux of the planet through the short wavelength tail of the thermal distribution. This has the observational implication, the radiation from such a planet should be skewed towards the NIR. Otherwise, saturation takes place by radiating through windows in the FIR.

Orbital characterization of the \beta Pictoris b giant planet

astro-ph — Tue, 14 Feb 2012 01:55:16 +0000

In June 2010, we confirmed the existence of a giant planet in the disk of the young star Beta Pictoris, located between 8 AU and 15 AU from the star. This young planet offers the rare opportunity to monitor a large fraction of the orbit using the imaging technique over a reasonably short timescale. Using the NAOS-CONICA adaptive-optics instrument (NACO) at the Very Large Telescope (VLT), we obtained repeated follow-up images of the Bpic system in the Ks and L’ filters at four new epochs in 2010 and 2011. Complementing these data with previous measurements, we conduct a homogeneous analysis, which covers more than eight yrs, to accurately monitor the Bpic b position relative to the star. On the basis of the evolution of the planet’s relative position with time, we derive the best-fit orbital solutions for our measurements. More reliable results are found with a Markov-chain Monte Carlo approach. The solutions favor a low-eccentricity orbit e < 0.17, with semi-major axis in the range 8–9 AU corresponding to orbital periods of 17–21 yrs. Our solutions favor a highly inclined solution with a peak around i=88.5+-1.7 deg, and a longitude of ascending node tightly constrained at Omega = -147.5+-1.5 deg. These results indicate that the orbital plane of the planet is likely to be above the midplane of the main disk, and compatible with the warp component of the disk being tilted between 3.5 deg and 4.0 deg. This suggests that the planet plays a key role in the origin of the inner warped-disk morphology of the Bpic disk. Finally, these orbital parameters are consistent with the hypothesis that the planet is responsible for the transit-like event observed in November 1981, and also linked to the cometary activity observed in the Bpic system.

Extrasolar Planet Transits Observed at Kitt Peak National Observatory

astro-ph — Tue, 14 Feb 2012 01:47:16 +0000

We obtained J-, H- and JH-band photometry of known extrasolar planet transiting systems at the 2.1-m Kitt Peak National Observatory Telescope using the FLAMINGOS infrared camera between October 2008 and October 2011. From the derived lightcurves we have extracted the mid-transit times, transit depths and transit durations for these events. The precise mid-transit times obtained help improve the orbital periods and also constrain transit-time variations of the systems. For most cases the published system parameters successfully accounted for our observed lightcurves, but in some instances we derive improved planetary radii and orbital periods. We complemented our 2.1-m infrared observations using CCD z’-band and B-band photometry (plus two Hydrogen Alpha filter observations) obtained with the Kitt Peak Visitor’s Center telescope, and with four H-band transits observed in October 2007 with the NSO’s 1.6-m McMath-Pierce Solar Telescope. The principal highlights of our results are: 1) our ensemble of J-band planetary radii agree with optical radii, with the best-fit relation being: (Rp/R*)J = 0.0017 + 0.979 (Rp/R*)optical, 2) We observe star spot crossings during the transit of WASP-11/HAT-P-10, 3) we detect star spot crossings by HAT-P-11b (Kepler-3b), thus confirming that the magnetic evolution of the stellar active regions can be monitored even after the Kepler mission has ended, and 4) we confirm a grazing transit for HAT-P-27/WASP-40. In total we present 57 individual transits of 32 known exoplanet systems.

Comparing HARPS and Kepler surveys: On the alignment of multiple-planet systems

astro-ph — Tue, 14 Feb 2012 01:47:02 +0000

Aims. We study a subset of the planetary population characterized both by HARPS and Kepler surveys. We compare the statistical properties of planets in systems with m.sin i >5-10 M_Earth and R>2 R_Earth. If we assume that the underlying population has the same characteristics, the different detection sensitivity to the orbital inclination relative to the line of sight allows us to probe the planets’ mutual inclination. Methods. We considered the frequency of systems with one, two and three planets as dictated by HARPS data. We used Kepler’s planetary period and host mass and radii distributions (corrected from detection bias) to model planetary systems in a simple yet physically plausible way. We then varied the mutual inclination between planets in a system according to different prescriptions (completely aligned, Rayleigh distributions and isotropic) and compared the transit frequencies with one, two or three planets with those measured by Kepler. Results. The results show that the two datasets are compatible, a remarkable result especially because there are no tunable knobs other than the assumed inclination distribution. For m.sin i cutoffs of 7-10 M_Earth, which are those expected to correspond to the radius cutoff of 2 R_Earth, we conclude that the results are better described by a Rayleigh distribution with mode of 1 deg or smaller. We show that the best-fit scenario only becomes a Rayleigh distribution with mode of 5 deg if we assume a rather extreme mass-radius relationship for the planetary population. Conclusions. These results have important consequences for our understanding of the role of several proposed formation and evolution mechanisms. They confirm that planets are likely to have been formed in a disk and show that most planetary systems evolve quietly without strong angular momentum exchanges (abridged).

Solar System Signatures of Impacts by Compact Ultra Dense Objects [Replacement]

astro-ph — Tue, 14 Feb 2012 01:43:05 +0000

We investigate the hypothesis that dark matter is found in compact ultra dense objects with nuclear density or greater (CUDOs). We discuss signatures of CUDOs’ collision with the Earth and other rocky bodies in the solar system. Some of these signatures can persist over geologic time, meaning that solar system rocky bodies are integrating detectors of dark matter in CUDO form. We discuss CUDO impacts which have the potential to resolve some geological riddles and help explain catastrophic events which marked the evolution of life on Earth.

Influence of the coorbital resonance on the rotation of the Trojan satellites of Saturn [Replacement]

astro-ph — Tue, 14 Feb 2012 01:39:14 +0000

The Cassini spacecraft collects high resolution images of the saturnian satellites and reveals the surface of these new worlds. The shape and rotation of the satellites can be determined from the Cassini Imaging Science Subsystem data, employing limb coordinates and stereogrammetric control points. This is the case for Epimetheus (Tiscareno et al. 2009) that opens elaboration of new rotational models (Tiscareno et al. 2009; Noyelles 2010; Robutel et al. 2011). Especially, Epimetheus is characterized by its horseshoe shape orbit and the presence of the swap is essential to introduce explicitly into rotational models. During its journey in the saturnian system, Cassini spacecraft accumulates the observational data of the other satellites and it will be possible to determine the rotational parameters of several of them. To prepare these future observations, we built rotational models of the coorbital (also called Trojan) satellites Telesto, Calypso, Helene, and Polydeuces, in addition to Janus and Epimetheus. Indeed, Telesto and Calypso orbit around the L_4 and L_5 Lagrange points of Saturn-Tethys while Helene and Polydeuces are coorbital of Dione. The goal of this study is to understand how the departure from the Keplerian motion induced by the perturbations of the coorbital body, influences the rotation of these satellites. To this aim, we introduce explicitly the perturbation in the rotational equations by using the formalism developed by Erdi (1977) to represent the coorbital motions, and so we describe the rotational motion of the coorbitals, Janus and Epimetheus included, in compact form.

Discovery of Main-Belt Comet P/2006 VW139 by Pan-STARRS1

astro-ph — Mon, 13 Feb 2012 01:58:15 +0000

Main belt asteroid (300163) 2006 VW139 (later designated P/2006 VW139) was discovered to exhibit comet-like activity by the Pan-STARRS1 survey telescope using automated point-spread-function analyses performed by PS1’s Moving Object Processing System. Deep follow-up observations show both a short (\sim 10″) antisolar dust tail and a longer (\sim 60″) dust trail aligned with the object’s orbit plane, similar to the morphology observed for another main-belt comet, P/2010 R2 (La Sagra), and other well-established comets, implying the action of a long-lived, sublimation-driven emission event. Photometry showing the brightness of the near-nucleus coma remaining constant over \sim 30 days provides further evidence for this object’s cometary nature, suggesting it is in fact a main-belt comet, and not a disrupted asteroid. A spectroscopic search for CN emission was unsuccessful, though we find an upper limit CN production rate of Q_CN < 1.3×10^24 mol/s, from which we infer a water production rate of Q_H2O 100 Myr, while a search for a potential asteroid family around the object reveals a cluster of 24 asteroids within a cutoff distance of 68 m/s. At 70 m/s, this cluster merges with the Themis family, suggesting that it could be similar to the Beagle family to which another main-belt comet, 133P/Elst-Pizarro, belongs.

Heating in collisions of solids:possible application to impact craters

astro-ph — Mon, 13 Feb 2012 01:56:55 +0000

Due to the importance of collisions and impacts in early phases of the evolution of the planetary system, it is interesting to estimate the heating of a solid target due to an impact in it . A physically simple calculation of the temperature to which a solid target heats up after the impact of a projectile with mass $m$ and speed $v$ is performed,and possibilities for the application of this result in planetology are pointed out.

Current state of the modelling of photospheric activity

astro-ph — Mon, 13 Feb 2012 01:50:48 +0000

I briefly review the current state of the modelling of photospheric activity based on the high-precision optical light curves obtained with MOST, CoRoT, and Kepler. These models can be used to search for active longitudes where activity is preferentially concentrated, estimate the amplitude of stellar differential rotation, and look for short-term activity cycles as, e.g., in the case of CoRoT-2. In the case of a late-type star accompanied by a transiting hot Jupiter, the small light modulations observed during transits when a dark spot is occulted by the disc of the planet are also briefly considered. They can be used to derive information on individual active regions as well as on stellar rotation and the spin-orbit alignment of the system.

Spectral properties of oscillatory and non-oscillatory {\alpha}^2-dynamos

astro-ph — Mon, 13 Feb 2012 01:49:37 +0000

Depths of the Relief Compensation and the Anomalous Structure of Crust and Mantle of Mars

astro-ph — Mon, 13 Feb 2012 01:48:13 +0000

This study determined the contribution of Martian topography and the density jump at the Mohorovicic discontinuity (M) to the gravity in a quadratic approximation. It also resolved the problem of determining the possible depths of compensation for topography harmonics of various degrees and orders. It shows that all the topography compensation is within the depth range from 0 to 1400 km. Different topographic irregularities are most likely to be compensated at depths that correspond to the upper crust (d =(4.5 +- 3.7)km), crust-mantle transition layer (d = (78 +- 24)km), lithospheric boundary (d = (200 +- 34)km), upper-middle mantle transition layer (d = (400 +- 70)km), or middle-lower mantle transition layer (d =(1120 +- 180)km). This paper presents the lateral distributions of compensation masses for these depths and the respective maps. According to calculations, stresses in the Martian crust and mantle may be as high as 10^8 Pa. This paper shows that the topographic anomalies of the Tharsis volcanic plateau and the symmetric formation in the eastern hemisphere could have originated and be dynamically maintained by two plumes of melted mantle substance enriched with fluids; these plumes may have their origin at the boundary of the lower mantle.

An improved model of the Edgeworth-Kuiper debris disk

astro-ph — Mon, 13 Feb 2012 01:46:57 +0000

(Abridged) We access the expected EKB dust disk properties by modeling. We treat the debiased population of the known transneptunian objects (TNOs) as parent bodies and generate the dust with our collisional code. The resulting dust distributions are modified to take into account the influence of gravitational scattering and resonance trapping by planets on migrating dust grains as well as the effect of sublimation. A difficulty is that the amount and distribution of dust are largely determined by sub-kilometer-sized bodies. These are directly unobservable, and their properties cannot be accessed by collisional modeling, because objects larger than 10…60m in the present-day EKB are not in a collisional equilibrium. To place additional constraints, we use in-situ measurements of the New Horizons spacecraft within 20AU. We show that the TNO population has to have a break in the size distribution at s10AU)~10^-6 and their fractional luminosity is f_d~10^-7. Planets and sublimation are found to have little effect on dust impact fluxes and dust thermal emission. The spectral energy distribution of an EKB analog, as would be seen from 10pc distance, peaks at wavelengths of 40…50\mum at F~0.5mJy, which is less than 1% of the photospheric flux at those wavelengths. Therefore, exact EKB analogs cannot be detected with present-day instruments such as Herschel/PACS.

Asteroid taxonomic signatures from photometric phase curves

astro-ph — Mon, 13 Feb 2012 01:46:20 +0000

We explore the correlation between an asteroid’s taxonomy and photometric phase curve using the H, G12 photometric phase function, with the shape of the phase function described by the single parameter G12. We explore the usability of G12 in taxonomic classification for individual objects, asteroid families, and dynamical groups. We conclude that the mean values of G12 for the considered taxonomic complexes are statistically different, and also discuss the overall shape of the G12 distribution for each taxonomic complex. Based on the values of G12 for about half a million asteroids, we compute the probabilities of C, S, and X complex membership for each asteroid. For an individual asteroid, these probabilities are rather evenly distributed over all of the complexes, thus preventing meaningful classification. We then present and discuss the G12 distributions for asteroid families, and predict the taxonomic complex preponderance for asteroid families given the distribution of G12 in each family. For certain asteroid families, the probabilistic prediction of taxonomic complex preponderance can clearly be made. The Nysa-Polana family shows two distinct regions in the proper element space with different G12 values dominating in each region. We conclude that the G12-based probabilistic distribution of taxonomic complexes through the main belt agrees with the general view of C complex asteroid proportion increasing towards the outer belt. We conclude that the G12 photometric parameter cannot be used in determining taxonomic complex for individual asteroids, but it can be utilized in the statistical treatment of asteroid families and different regions of the main asteroid belt.

Production of N2 Vegard-Kaplan and other triplet band emissions in the dayglow of Titan

astro-ph — Mon, 13 Feb 2012 01:45:48 +0000

Recently the Cassini Ultraviolet Imaging Spectrograph has revealed the presence of N2 Vegard-Kaplan band emissions in Titan’s dayglow limb observation. We present model calculations for the production of various N2 triplet states in the upper atmosphere of Titan. The Analytical Yield Spectra technique is used to calculate steady state photoelectron fluxes in Titan’s atmosphere, which are in agreement with those observed by the Cassini’s CAPS instrument. Considering direct electron impact excitation, inter-state cascading, and quenching effects, the population of different levels of N2 triplet states are calculated under statistical equilibrium. Densities of all vibrational levels of each triplet state and volume production rates for various triplet states are calculated in the model. Vertically integrated overhead intensities for the same date and lighting conditions as the reported by UVIS observations for N2 VK, 1P, 2P, Wu-Benesch, and Reverse First Positive bands of N2 are found to be 132, 114, 19, 22, and 22 R, respectively. Overhead intensities are calculated for each vibrational transition of all the triplet band emissions of N2, which span a wider spectrum of wavelengths from ultraviolet to infrared. The calculated limb intensities of total and prominent transitions of VK band are presented. The model limb intensity of VK emission within the 150-190 nm wavelength region is in good agreement with the Cassini UVIS observed limb profile. An assessment of the impact of solar EUV flux on the N2 triplet band emission intensity has been made by using three different solar flux models, viz., Solar EUV Experiment, SOLAR2000 model of Tobiska (2004), and HEUVAC model of Richards et al, (2006). The calculated N2 VK band intensity at the peak of limb intensity due to S2K and HEUVAC solar flux models is a factor of 1.2 and 0.9, respectively, of that obtained using SEE solar EUV flux.

Considerations on the accretion of Uranus and Neptune by mutual collisions of planetary embryos in the vicinity of Jupiter and Saturn [Replacement]

astro-ph — Mon, 13 Feb 2012 01:40:16 +0000

Modeling the formation of the ice giants Uranus and Neptune is a long-lasting problem in planetary science. Due to gas-drag, collisional damping, and resonant shepherding, the planetary embryos repel the planetesimals away from their reach and thus they stop growing (Levison et al. 2010). This problem persists independently of whether the accretion took place at the current locations of the ice giants or closer to the Sun. Instead of trying to push the runaway/oligarchic growth of planetary embryos up to 10-15 Earth masses, we envision the possibility that the planetesimal disk could generate a system of planetary embryos of only 1-3 Earth masses. Then we investigate whether these embryos could have collided with each other and grown enough to reach the masses of current Uranus and Neptune. Our results point to two major problems. First, there is typically a large difference in mass between the first and the second most massive core formed and retained beyond Saturn. Second, in many simulations the final planetary system has more than two objects beyond Saturn. The growth of a major planet from a system of embryos requires strong damping of eccentricities and inclinations from the disk of gas. But strong damping also favors embryos and cores to find a stable resonant configuration, so that systems with more than two surviving objects are found. In addition to these problems, in order to have substantial mutual accretion among embryos, it is necessary to assume that the surface density of the gas was several times higher than that of the minimum-mass solar nebula. However this contrasts with the common idea that Uranus and Neptune formed in a gas-starving disk, which is suggested by the relatively small amount of hydrogen and helium contained in the atmospheres of these planets. Only one of our simulations “by chance” successfully reproduced the structure of the outer Solar System.

Scaling and intermittency in incoherent \alpha-shear dynamo [Replacement]

astro-ph — Mon, 13 Feb 2012 01:40:03 +0000

We consider mean-field dynamo models with fluctuating \alpha effect, both with and without shear. The \alpha effect is chosen to be Gaussian white noise with zero mean and given covariance. We show analytically that the mean magnetic field does not grow, but, in an infinitely large domain, the mean-squared magnetic field shows exponential growth of the fastest growing mode at a rate proportional to the shear rate, which agrees with earlier numerical results of Yousef et al (2008) and recent analytical treatment by Heinemann et al (2011) who use a method different from ours. In the absence of shear, an incoherent \alpha^2 dynamo may also be possible. We further show by explicit calculation of the growth rate of third and fourth order moments of the magnetic field that the probability density function of the mean magnetic field generated by this dynamo is non-Gaussian.

The KELT-South Telescope

astro-ph — Fri, 10 Feb 2012 01:56:11 +0000

The Kilodegree Extremely Little Telescope (KELT) project is a survey for new transiting planets around bright stars. KELT-South is a small-aperture, wide-field automated telescope located at Sutherland, South Africa. The telescope surveys a set of 26 degree by 26 degree fields around the southern sky, and targets stars in the range of 8 < V < 10 mag, searching for transits by Hot Jupiters. This paper describes the KELT-South system hardware and software and discusses the quality of the observations. We show that KELT-South is able to achieve the necessary photometric precision to detect transits of Hot Jupiters around solar-type main-sequence stars.

Orbital migration of interacting low-mass planets in evolutionary radiative turbulent models

astro-ph — Fri, 10 Feb 2012 01:53:11 +0000

A Two-Dimensional Infrared Map of the Extrasolar Planet HD 189733b

astro-ph — Fri, 10 Feb 2012 01:51:49 +0000

We derive the first secondary eclipse map of an exoplanet, HD 189733b, based on Spitzer IRAC 8 micron data. We develop two complementary techniques for deriving the two dimensional planet intensity: regularized slice mapping and spherical harmonic mapping. Both techniques give similar derived intensity maps for the infrared day-side flux of the planet, while the spherical harmonic method can be extended to include phase variation data which better constrain the map. The longitudinal offset of the day-side hot spot is consistent with that found in prior studies, strengthening the claim of super-rotating winds, and eliminating the possibility of phase variations being caused by stellar variability. The latitude of the hot-spot is within 12.5 deg (68% confidence) of the planet’s equator, confirming the predictions of general circulation models for hot Jupiters and indicative of a small planet obliquity.

The long sunspot cycle 23 predicts a significant temperature decrease in cycle 24

astro-ph — Fri, 10 Feb 2012 01:47:19 +0000

Relations between the length of a sunspot cycle and the average temperature in the same and the next cycle are calculated for a number of meteorological stations in Norway and in the North Atlantic region. No significant trend is found between the length of a cycle and the average temperature in the same cycle, but a significant negative trend is found between the length of a cycle and the temperature in the next cycle. This provides a tool to predict an average temperature decrease of at least 1.0 “C from solar cycle 23 to 24 for the stations and areas analyzed. We find for the Norwegian local stations investigated that 25-56% of the temperature increase the last 150 years may be attributed to the Sun. For 3 North Atlantic stations we get 63-72% solar contribution. This points to the Atlantic currents as reinforcing a solar signal.

A quantitative comparison of lunar orbital neutron data [Replacement]

astro-ph — Fri, 10 Feb 2012 01:38:55 +0000

Data from the Lunar Exploration Neutron Detector (LEND) Collimated Sensors for Epithermal Neutrons (CSETN) are used in conjunction with a model based on results from the Lunar Prospector mission to quantify the extent of the background in the LEND CSETN. A simple likelihood analysis implies that at least 90% of the lunar component of the LEND CSETN flux results from high energy epithermal neutrons passing through the walls of the collimator. Thus, the effective full-width at half-maximum of the LEND CSETN is comparable with that of the omni-directional Lunar Prospector Neutron Spectrometer. The resulting map of high energy epithermal neutrons offers the opportunity to probe the hydrogen abundance at low latitudes, and provide constraints on the distribution of lunar water.

The formation heritage of Jupiter Family Comet 10P/Tempel 2 as revealed by infrared spectroscopy [Replacement]

astro-ph — Fri, 10 Feb 2012 01:36:02 +0000

We present spectral and spatial information for major volatile species in Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT 2010 July 26 (heliocentric distance Rh = 1.44 AU) and September 18 (Rh = 1.62 AU), following the comet’s perihelion passage on UT 2010 July 04. The total production rate for water on July 26 was (1.90 +/- 0.12) x 10^28 molecules s-1, and abundances of six trace gases (relative to water) were: CH3OH (1.58% +/- 0.23), C2H6 (0.39% +/- 0.04), NH3 (0.83% +/- 0.20), and HCN (0.13% +/- 0.02). A detailed analysis of intensities for water emission lines provided a rotational temperature of 35 +/- 3 K. The mean OPR is consistent with nuclear spin populations in statistical equilibrium (OPR = 3.01 +/- 0.18), and the (1-sigma) lower bound corresponds to a spin temperature > 38 K. Our measurements were contemporaneous with a jet-like feature observed at optical wavelengths. The spatial profiles of four primary volatiles display strong enhancements in the jet direction, which favors release from a localized vent on the nucleus. The measured IR continuum is much more sharply peaked and is consistent with a dominant contribution from the nucleus itself. The peak intensities for H2O, CH3OH, and C2H6 are offset by ~200 km in the jet direction, suggesting the possible existence of a distributed source, such as the release of icy grains that subsequently sublimed in the coma. On UT September 18, no obvious emission lines were present in our spectra, nevertheless we obtained a 3-sigma upper limit Q(H2O) < 2.86 x 10^27 molecules s-1.

SDSS Observations of Kuiper Belt Objects: Colors and Variability

astro-ph — Thu, 09 Feb 2012 01:55:49 +0000

Colors of Trans Neptunian Objects (TNOs) are used to study the evolutionary processes of bodies in the outskirts of the Solar System, and to test theories regarding their origin. Here I describe a search for serendipitous Sloan Digital Sky Survey (SDSS) observations of known TNOs and Centaurs. I present a catalog of SDSS photometry, colors and astrometry of 388 measurements of 42 outer Solar-System objects. I find a weak evidence, at the ~2-sigma level (per trial), for a correlation between the g-r color and inclination of scattered disk objects and hot classical KBOs. I find a correlation between the g-r color and the angular momentum in the z direction of all the objects in this sample. Light curves as a function of phase angle are constructed for 13 objects. The steepness of the slopes of these light curves suggests that the coherent backscatter mechanism plays a major role in the reflectivity of outer Solar-System small objects at small phase angles. I find a weak evidence for an anti-correlation, significant at the 2-sigma confidence level (per trial), between the g-band phase angle slope parameter and the semi-major axis, as well as the aphelion distance, of these objects. I discuss the origin of this possible correlation and argue that if this correlation is real it probably indicates that “Sedna”-like objects have a different origin than other classes of TNOs. Finally, I identify several objects with large variability amplitudes (abridged).

Astrophysical Reconnection and Particle Acceleration

astro-ph — Thu, 09 Feb 2012 01:54:24 +0000

Astrophysical reconnection takes place in a turbulent medium. The turbulence in most cases is pre-existing, not caused by the reconnection itself. The model of magnetic reconnection in Lazarian & Vishniac (1999) predicts that in the presence of turbulence the reconnection becomes fast, i.e. it is independent of resistivity, but dependent on the level of turbulence. Magnetic reconnection injects energy into plasmas through a turbulent outflow from the reconnection region and this outflow can enhance the level of turbulence creating bursts of reconnection. Magnetic reconnection in the presence of turbulence can accelerate energetic particles through the first order Fermi mechanism, as was discussed in Gouveia dal Pino & Lazarian (2005). We discuss new numerical results on particle acceleration in turbulent reconnection, compare the acceleration arising from turbulent reconnection to the acceleration of energetic particles in turbulent medium.

Spitzer/MIPS 24 micron Observations of HD 209458b: 2.5 transits, 3 eclipses, and a Phase Curve Corrupted by Instrumental Sensitivity Variations

astro-ph — Thu, 09 Feb 2012 01:54:06 +0000

We report the results of an analysis of all Spitzer/MIPS 24 micron observations of HD 209458b, one of the touchstone objects in the study of irradiated giant planet atmospheres. Altogether we analyze 2.5 transits, 3 eclipses, and a 58-hour near-continuous observation designed to detect the planet’s thermal phase curve. The results of our analysis are: (1) A mean transit depth of 1.484% +/- 0.035%, consistent with previous measurements and showing no evidence of variability in transit depth at the 3% level. (2) A mean eclipse depth of 0.332% +/- 0.026%, somewhat higher than that previously reported for this system; this new value brings observations into better agreement with models. The dayside flux shows no evidence of variability at the 12% level. (3) Eclipses in the system occur 32 s +/- 129 s earlier than would be expected from a circular orbit, which constrains the orbital quantity (e cos omega) to be 0.00004 +/- 0.00033. This result is fully consistent with a circular orbit and sets an upper limit of 140 m/s (3 sigma) on any eccentricity-induced velocity offset during transit. The phase curve observations (including one of the transits) exhibit an anomalous trend similar to the detector ramp seen in previous Spitzer/IRAC observations; by modeling this ramp we recover the system parameters. The photometry which follows the ramp and transit exhibits a gradual, ~0.2% decrease in flux, similar to that seen in pre-launch calibration data. The large uncertainties associated with this poorly-understood, likely instrumental effect prevent us from usefully constraining the planet’s thermal phase curve. Our observations highlight the need for a thorough understanding of detector-related instrumental effects on long time scales when making the high-precision mid-infrared measurements planned for future missions such as EChO, SPICA, and JWST. [abridged]

Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program II. Activity and radial velocity

astro-ph — Thu, 09 Feb 2012 01:53:23 +0000

Due to their low mass and luminosity, M dwarfs are ideal targets if one hopes to find low-mass planets similar to Earth by using the radial velocity (RV) method. However, stellar magnetic cycles could add noise or even mimic the RV signal of a long-period companion. Following our previous work that studied the correlation between activity cycles and long-term RV variations for K dwarfs we now expand that research to the lower-end of the main sequence. Our objective is to detect any correlations between long-term activity variations and the observed RV of a sample of M dwarfs. We used a sample of 27 M-dwarfs with a median observational timespan of 5.9 years. The cross-correlation function (CCF) with its parameters RV, bisector inverse slope (BIS), full-width-at-half- maximum (FWHM) and contrast have been computed from the HARPS spectrum. The activity index have been derived using the Na I D doublet. These parameters were compared with the activity level of the stars to search for correlations. We detected RV variations up to ~5 m/s that we can attribute to activity cycle effects. However, only 36% of the stars with long-term activity variability appear to have their RV affected by magnetic cycles, on the typical timescale of ~6 years. Therefore, we suggest a careful analysis of activity data when searching for extrasolar planets using long-timespan RV data.

A massive exoplanet candidate around KOI-13: Independent confirmation by ellipsoidal variations

astro-ph — Thu, 09 Feb 2012 01:41:50 +0000

We present an analysis of the KOI-13.01 candidate exoplanet system included in the September 2011 Kepler data release. The host star is a known and relatively bright $(m_{\rm KP} = 9.95)$ visual binary with a separation significantly smaller (0.8 arcsec) than the size of a Kepler pixel (4 arcsec per pixel). The Kepler light curve shows both primary and secondary eclipses, as well as significant out-of-eclipse light curve variations. We confirm that the transit occurs round the brighter of the two stars. We model the relative contributions from (i) thermal emission from the companion, (ii) planetary reflected light, (iii) Doppler beaming, and (iv) ellipsoidal variations in the host-star arising from the tidal distortion of the host star by its companion. Our analysis, based on the light curve alone, enables us to constrain the mass of the KOI-13.01 companion to be $M_{\rm C} = 8.3 \pm 1.25M_{\rm J}$ and thus demonstrates that the transiting companion is a planet (rather than a brown dwarf which was recently proposed by \cite{b7}). The high temperature of the host star (Spectral Type A5-7V, $T_{\rm eff} = 8511-8020$ K), combined with the proximity of its companion KOI-13.01, may make it one of the hottest exoplanets known, with a detectable thermal contribution to the light curve even in the Kepler optical passband. However, the single passband of the Kepler light curve does not enable us to unambiguously distinguish between the thermal and reflected components of the planetary emission. Infrared observations may help to break the degeneracy, while radial velocity follow-up with $\sigma \sim$ 100 m s$^{-1}$ precision should confirm the mass of the planet.

The effect of local optically thick regions in the long-wave emission of young circumstellar disks

astro-ph — Thu, 09 Feb 2012 01:39:20 +0000

Multi-wavelength observations of protoplanetary disks in the sub-millimeter continuum have measured spectral indices values which are significantly lower than what is found in the diffuse interstellar medium. Under the assumption that mm-wave emission of disks is mostly optically thin, these data have been generally interpreted as evidence for the presence of mm/cm-sized pebbles in the disk outer regions. In this work we investigate the effect of possible local optically thick regions on the mm-wave emission of protoplanetary disks without mm/cm-sized grains. A significant local increase of the optical depth in the disk can be caused by the concentration of solid particles, as predicted to result from a variety of proposed physical mechanisms. We calculate the filling factors and implied overdensities these optically thick regions would need to significantly affect the millimeter fluxes of disks, and we discuss their plausibility. We find that optically thick regions characterized by relatively small filling factors can reproduce the mm-data of young disks without requesting emission from mm/cm-sized pebbles. However, these optically thick regions require dust overdensities much larger than what predicted by any of the physical processes proposed in the literature to drive the concentration of solids. We find that only for the most massive disks it is possible and plausible to imagine that the presence of optically thick regions in the disk is responsible for the low measured values of the mm spectral index. For the majority of the disk population, optically thin emission from a population of large mm-sized grains remains the most plausible explanation. The results of this analysis further strengthen the scenario for which the measured low spectral indices of protoplanetary disks at mm wavelengths are due to the presence of large mm/cm-sized pebbles in the disk outer regions.

Kinematics of Ionized Gas at 0.01 AU of TW Hya [Replacement]

astro-ph — Thu, 09 Feb 2012 01:32:33 +0000

We report two-dimensional spectroastrometry of Br gamma emission of TW Hya to study the kinematics of the ionized gas in the star-disk interface region. The spectroastrometry with the integral field spectrograph SINFONI at the Very Large Telescope is sensitive to the positional offset of the line emission down to the physical scale of the stellar diameter (~0.01 AU). The centroid of Br gamma emission is displaced to the north with respect to the central star at the blue side of the emission line, and to the south at the red side. The major axis of the centroid motion is P.A.= -20 degrees, which is nearly equal to the major axis of the protoplanetary disk projected on the sky, previously reported by CO sub millimeter spectroscopy (P.A.= -27 degrees) The line-of-sight motion of the Br gamma emission, in which the northern side of the disk is approaching toward us, is also consistent with the direction of the disk rotation known from the CO observation. The agreement implies that the kinematics of Br gamma emission is accounted for by the ionized gas in the inner edge of the disk. A simple modeling of the astrometry, however, indicates that the accretion inflow similarly well reproduces the centroid displacements of Br gamma, but only if the position angles of the centroid motion and the projected disk ellipse is a chance coincidence. No clear evidence of disk wind is found.

The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids [Replacement]

astro-ph — Thu, 09 Feb 2012 01:32:18 +0000

Recently the first Earth Trojan has been observed (Mainzer et al., ApJ 731) and found to be on an interesting orbit close to the Lagrange point L4 (Connors et al., Nature 475). In the present study we therefore perform a detailed investigation on the stability of its orbit and moreover extend the study to give an idea of the probability to find additional Earth-Trojans. Our results are derived using different approaches: a) we derive an analytical mapping in the spatial elliptic restricted three-body problem to find the phase space structure of the dynamical problem. We explore the stability of the asteroid in the context of the phase space geometry, including the indirect influence of the additional planets of our Solar system. b) We use precise numerical methods to integrate the orbit forward and backward in time in different dynamical models. Based on a set of 400 clone orbits we derive the probability of capture and escape of the Earth Trojan asteroids 2010 TK7. c) To this end we perform an extensive numerical investigation of the stability region of the Earth’s Lagrangian points. We present a detailed parameter study in the regime of possible stable tadpole and horseshoe orbits of additional Earth-Trojans, i.e. with respect to the semi-major axes and inclinations of thousands of fictitious Trojans. All three approaches underline that the Earth Trojan asteroid 2010 TK7 finds himself in an unstable region on the edge of a stable zone; additional Earth-Trojan asteroids may be found in this regime of stability.

The Arecibo Detection of the Coolest Radio-flaring Brown Dwarf

astro-ph — Wed, 08 Feb 2012 01:59:25 +0000

Quantifying Jupiter’s influence on the Earth’s impact flux: Implications for planetary habitability

astro-ph — Wed, 08 Feb 2012 01:56:14 +0000

It has long been thought that the presence of a giant planet is a pre-requisite for the development of life on potentially habitable planets. Without Jupiter, it was argued, the Earth would have been subject to a punishing impact regime, which would have significantly retarded or outright prevented the development of life on our planet. Although this idea is widely embraced, little research has previously been carried out to support it. Here, we present the results of several suites of dynamical integrations used to model the influence of Jupiter’s mass and orbit on the impact rate that would be experienced by the Earth. We find that, far from being a simple shield, Jupiter’s role in determining the terrestrial impact flux is significantly more complicated than previously thought. Far from being a simple friend, such giant planets are perhaps more likely to imperil the development of life on otherwise habitable planets.

"TNOs are Cool": A Survey of the Transneptunian Region IV. Size/albedo characterization of 15 scattered disk and detached objects observed with Herschel Space Observatory-PACS

astro-ph — Wed, 08 Feb 2012 01:42:29 +0000

Physical characterization of Trans-Neptunian objects, a primitive population of the outer solar system, may provide constraints on their formation and evolution. The goal of this work is to characterize a set of 15 scattered disk (SDOs) and detached objects, in terms of their size, albedo, and thermal properties. Thermal flux measurements obtained with the Herschel-PACS instrument at 70, 100 and 160 \mu m, and whenever applicable, with Spitzer-MIPS at 24 and 70 \mu m, are modeled with radiometric techniques, in order to derive the objects’ individual size, albedo and when possible beaming factor. Error bars are obtained from a Monte-Carlo approach. We look for correlations between these and other physical and orbital parameters. Diameters obtained for our sample range from 100 to 2400 km, and the geometric albedos (in V band) vary from 3.8 % to 84.5 %. The unweighted mean V geometric albedo for the whole sample is 11.2 % (excluding Eris); 6.9 % for the SDOs, and 17.0 % for the detached objects (excluding Eris). We obtain new bulk densities for three binary systems: Ceto/Phorcys, Typhon/Echidna and Eris/Dysnomia. Apart from correlations clearly due to observational bias, we find significant correlations between albedo and diameter (more reflective objects being bigger), and between albedo, diameter and perihelion distance (brighter and bigger objects having larger perihelia). We discuss possible explanations for these correlations.

Mercury and frame-dragging in light of the MESSENGER flybys: conflict with general relativity, poor knowledge of the physical properties of the Sun, data reduction artifact, or still insufficient observations? [Replacement]

astro-ph — Wed, 08 Feb 2012 01:37:20 +0000

The Lense-Thirring precession of the longitude of perihelion of Mercury, as predicted by general relativity by using the value of the Sun’s angular momentum S = 190 x 10^39 kg m^2 s^-1 from helioseismology, is -2.0 milliarcseconds per century, computed in a celestial equatorial reference frame. It disagrees at 4-{\sigma} level with the correction 0.4 +/- 0.6 milliarcseconds per century to the standard Newtonian/Einsteinian precession. The supplementary precession was recently determined in a global fit with the INPOP10a ephemerides to a long planetary data record (1914-2010) including also 3 data points collected in 2008-2009 from the MESSENGER spacecraft. The INPOP10a models did not include the solar gravitomagnetic field at all, so that its signature might have partly been removed in the data reduction process. On the other hand, the Lense-Thirring precession may have been canceled to a certain extent by the competing precession caused by a small mismodeling in the quadrupole mass moment of the Sun, actually modeled in INPOP10a, of the order of (0.1-0.2) x 10^-7. On the contrary, the oblateness of Mercury itself has a negligible impact on its perihelion. The same holds for the mismodelled actions of both the largest individual asteroids and the ring of the minor asteroids. Future analysis of more observations from the currently ongoing MESSENGER mission will shed further light on such an issue which, if confirmed, might potentially challenge our present-day picture of the currently accepted laws of gravitation and/or of the physical properties of the Sun.

Laser frequency comb techniques for precise astronomical spectroscopy

astro-ph — Tue, 07 Feb 2012 02:11:30 +0000

Precise astronomical spectroscopic analyses routinely assume that individual pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons. Intra-pixel sensitivity (IPS) variations may already cause small systematic errors in, for example, studies of extra-solar planets via stellar radial velocities and cosmological variability in fundamental constants via quasar spectroscopy, but future experiments requiring velocity precisions approaching ~1 cm/s will be more strongly affected. Laser frequency combs have been shown to provide highly precise wavelength calibration for astronomical spectrographs, but here we show that they can also be used to measure IPS variations in astronomical CCDs in situ. We successfully tested a laser frequency comb system on the Ultra-High Resolution Facility spectrograph at the Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded in a single CCD exposure, we find that the average IPS deviates by <8 per cent if it is assumed to vary symmetrically about the pixel centre. We also demonstrate that series of comb exposures with absolutely known offsets between them can yield tighter constraints on symmetric IPS variations from ~100 pixels. We discuss measurement of asymmetric IPS variations and absolute wavelength calibration of astronomical spectrographs and CCDs using frequency combs.

The Hubble Wide Field Camera 3 Test of Surfaces in the Outer Solar System: The Compositional Classes of the Kuiper Belt

astro-ph — Tue, 07 Feb 2012 02:10:55 +0000

We present the first results of the Hubble Wide Field Camera 3 Test of Surfaces in the Outer Solar System (H/WTSOSS). The purpose of this survey was to measure the surface properties of a large number of Kuiper belt objects and attempt to infer compositional and dynamical correlations. We find that the Centaurs and the low-perihelion scattered disk and resonant objects exhibit virtually identical bifurcated optical colour distributions and make up two well defined groups of object. Both groups have highly correlated optical and NIR colours which are well described by a pair of two component mixture models that have different red components, but share a common neutral component. The small, $H_{606}\gtrsim5.6$ high-perihelion excited objects are entirely consistent with being drawn from the two branches of the mixing model suggesting that the colour bifurcation of the Centaurs is apparent in all small excited objects. On the other hand, objects larger than $H_{606}\sim5.6$ are not consistent with the mixing model, suggesting some evolutionary process avoided by the smaller objects. The existence of a bifurcation amongst all excited populations argues that the two separate classes of object existed in the primordial disk before the excited Kuiper belt was populated. The cold classical objects exhibit a different type of surface which has colours that are consistent with being drawn from the red branch of the mixing model, but with much higher albedos.

One or more bound planets per Milky Way star from microlensing observations

astro-ph — Tue, 07 Feb 2012 02:07:50 +0000

Most known extrasolar planets (exoplanets) have been discovered using the radial velocity$^{\bf 1,2}$ or transit$^{\bf 3}$ methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17–30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing$^{\bf 6\rm{\bf -}\bf 9}$, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing$^{\bf 10}$. These planets are at least as numerous as the stars in the Milky Way$^{\bf 10}$. Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10 AU (Sun–Earth distance) from their stars. We find that 17$_{\bf -9}^{\bf +6}$% of stars host Jupiter-mass planets (0.3–10 $\MJ$, where $\MJ {\bf = 318}$ $\Mearth$ and $\Mearth$ is Earth’s mass). Cool Neptunes (10–30 $\Mearth$) and super-Earths (5–10 $\Mearth$) are even more common: their respective abundances per star are 52$_{\bf -29}^{\bf +22}$% and 62$_{\bf -37}^{\bf +35}$%. We conclude that stars are orbited by planets as a rule, rather than the exception.

Kinetic simulation of the electron-cyclotron maser instability: effect of a finite source size

astro-ph — Tue, 07 Feb 2012 02:07:24 +0000

The electron-cyclotron maser instability is widespread in the Universe, producing, e.g., radio emission of the magnetized planets and cool substellar objects. Diagnosing the parameters of astrophysical radio sources requires comprehensive nonlinear simulations of the radiation process. We simulate the electron-cyclotron maser instability in a very low-beta plasma. The model used takes into account the radiation escape from the source region and the particle flow through this region. We developed a kinetic code to simulate the time evolution of an electron distribution in a radio emission source. The model includes the terms describing the particle injection to and escape from the emission source region. The spatial escape of the emission from the source is taken into account by using a finite amplification time. The unstable electron distribution of the horseshoe type is considered. A number of simulations were performed for different parameter sets typical of the magnetospheres of planets and ultracool dwarfs. The generated emission (corresponding to the fundamental extraordinary mode) has a frequency close to the electron cyclotron frequency and propagates across the magnetic field. Shortly after the onset of a simulation, the electron distribution reaches a quasi-stationary state. If the emission source region is relatively small, the resulting electron distribution is similar to that of the injected electrons; the emission intensity is low. In larger sources, the electron distribution may become nearly flat due to the wave-particle interaction, while the conversion efficiency of the particle energy flux into waves reaches 10-20%. We found good agreement of our model with the in situ observations in the source regions of auroral radio emissions of the Earth and Saturn. The expected characteristics of the electron distributions in the magnetospheres of ultracool dwarfs were obtained.

The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids

astro-ph — Tue, 07 Feb 2012 01:59:04 +0000

Recently the first Earth Trojan has been observed (Connors et al., Nature 475) and found to be on an interesting orbit close to the Lagrange point L4. In the present study we therefore perform a detailed investigation on the stability of its orbit and moreover extend the study to give an idea of the probability to find additional Earth-Trojans. Our results are derived using different approaches: a) we derive an analytical mapping in the spatial elliptic restricted three-body problem to find the phase space structure of the dynamical problem. We explore the stability of the asteroid in the context of the phase space geometry, including the indirect influence of the additional planets of our Solar system. b) We use precise numerical methods to integrate the orbit forward and backward in time in different dynamical models. Based on a set of 400 clone orbits we derive the probability of capture and escape of the Earth Trojan asteroids 2010 TK7. c) To this end we perform an extensive numerical investigation of the stability region of the Earth’s Lagrangian points. We present a detailed parameter study in the regime of possible stable tadpole and horseshoe orbits of additional Earth-Trojans, i.e. with respect to the semi-major axes and inclinations of thousands of fictitious Trojans. All three approaches underline that the Earth Trojan asteroid 2010 TK7 finds himself in an unstable region on the edge of a stable zone; additional Earth-Trojan asteroids may be found in this regime of stability.

Oscillations in the Habitable Zone around Alpha Centauri B

astro-ph — Tue, 07 Feb 2012 01:49:02 +0000

The Alpha Centauri AB system is an attractive one for radial velocity observations to detect potential exoplanets. The high metallicity of both Alpha Centauri A and B suggest that they could have possessed circumstellar discs capable of forming planets. As the closest star system to the Sun, with well over a century of accurate astrometric measurements (and Alpha Centauri B exhibiting low chromospheric activity) high precision surveys of Alpha Centauri B’s potential exoplanetary system are possible with relatively cheap instrumentation. Authors studying habitability in this system typically adopt habitable zones (HZs) based on global radiative balance models that neglect the radiative perturbations of Alpha Centauri A. We investigate the habitability of planets around Alpha Centauri B using 1D latitudinal energy balance models (LEBMs), which fully incorporate the presence of Alpha Centauri A as a means of astronomically forcing terrestrial planet climates. We find that the extent of the HZ is relatively unchanged by the presence of Alpha Centauri A, but there are variations in fractional habitability for planets orbiting at the boundaries of the zone due to Alpha Centauri A, even in the case of zero eccentricity. Temperature oscillations of a few K can be observed at all planetary orbits, the strength of which varies with the planet’s ocean fraction and obliquity.

Teorie lunari, effemeridi e sistemi di coordinate (Lunar theories, ephemerides and coordinates systems)

astro-ph — Tue, 07 Feb 2012 01:48:38 +0000

This work deals with the comparison of different parameters (French/IMCCE and US/JPL ephemerides) used to calculate the extension of the umbral shadow and the location of the centre line in the total solar eclipse that took place on March, 29th 2006. We needed to know the exact points where the predicted duration of the solar eclipse was 0 seconds. This phenomenon allows, observed on this particular position, the measurement of the solar diameter. The work is composed by three parts. In the first one is described the evolution of the lunar theory from Hipparchus to the current theory (Bureau de Longitudes/IMCCE, Chapront). The second part is dedicated to geodesy (WGS84 datum) and there are examples of longitude’s calculations. In the third one there is the analysis of the parameters to understand the reasons about the different extensions of the umbral shadow and the different positions of the centre line in the solar eclipse calculated respectively with French and American ephemerides. The use of the center of the figure of the Moon with respect to the center of mass produces the differences in the ephemerides. This work prepared the observational mission in Egypt to observe the total solar eclipse from the shadow’s edges in Zawayet al Mahtallah, near Sidi Barrani, whose results were after published in Solar Physics vol. 259 p. 189 (2009).

LETSGO: a spacecraft-based mission to accurately measure the solar angular momentum with frame-dragging [Replacement]

astro-ph — Tue, 07 Feb 2012 01:48:01 +0000

LETSGO (LEnse-Thirring Sun-Geo Orbiter) is a proposed space-based mission involving the use of a spacecraft moving along a highly eccentric heliocentric orbit perpendicular to the ecliptic. It aims to accurately measure some important physical properties of the Sun and to test some post-Newtonian features of its gravitational field by continuously monitoring the Earth-probe range. Preliminary sensitivity analyses show that, by assuming a cm-level accuracy in ranging to the spacecraft, it would be possible to detect, in principle, the Lense-Thirring effect on it at a 10^-3-10^-4 level over a timescale of 2 yr, while the larger Schwarzschild component of the solar gravitational field may be sensed with a relative accuracy of about 10^-8-10^-9 during the same temporal interval. The competing range perturbation due to the non-sphericity of the Sun would be a source of systematic error, but it turns out that all the three dynamical features of motion examined affect the Earth-probe range in different ways, allowing for a separation in data analyses. The high eccentricity would help in reducing the impact of the non-gravitational perturbations whose impact would certainly be severe when LETSGO would approach the Sun at just a few solar radii. Further studies should be devoted to investigate both the consequences of the non-conservative forces and the actual measurability of the effects of interest by means of extensive numerical data simulations, parameter estimations and covariance analyses. Also an alternative, fly-by configuration is worth of consideration.

Global Models of Runaway Accretion in White Dwarf Debris Disks

astro-ph — Mon, 06 Feb 2012 01:46:30 +0000

A growing sample of white dwarfs (WDs) with metal-enriched atmospheres are accompanied by excess infrared emission, indicating that they are encircled by a compact dusty disk of solid debris. Such `WD debris disks’ are thought to originate from the tidal disruption of asteroids or other minor bodies, but the precise mechanism(s) responsible for transporting matter to the WD surface remains unclear, especially in those systems with the highest inferred metal accretion rates dM_Z/dt ~ 1e8-1e10 g/s. Here we present global time-dependent calculations of the coupled evolution of the gaseous and solid components of WD debris disks. Solids transported inwards (initially due to PR drag) sublimate at tens of WD radii, producing a source of gas that accretes onto the WD surface and viscously spreads outwards in radius, where it overlaps with the solid disk. If the aerodynamic coupling between the solids and gaseous disks is sufficiently strong (and/or the gas viscosity sufficiently weak), then gas builds up near the sublimation radius faster than it can viscously spread away. Since the rate of drag-induced solid accretion increases with gas density, this results in a runaway accretion process, during which the WD accretion rate reaches values orders of magnitude higher than can be achieved by PR drag alone. We explore the evolution of WD debris disks across a wide range of physical conditions and calculate the predicted distribution of observed accretion rates dM_Z/dt, finding reasonable agreement with the current sample. Although the conditions necessary for runaway accretion are at best marginally satisfied given the minimal level of aerodynamic drag between circular gaseous and solid disks, the presence of other stronger forms of solid-gas coupling—such as would result if the gaseous disk is only mildly eccentric—substantially increase the likelihood of runaway accretion.

Dead zones as safe-havens for planetesimals: influence of disc mass and external magnetic field

astro-ph — Mon, 06 Feb 2012 01:39:05 +0000

(Abridged) Planetesimals embedded in a protoplanetary disc are stirred by gravitational torques exerted by density fluctuations in the surrounding turbulence. In particular, planetesimals in a disc supporting fully developed magneto-rotational turbulence are readily excited to velocity dispersions above the threshold for catastrophic disruption, halting planet formation. We aim to examine the stirring of planetesimals lying instead in a magnetically-decoupled midplane dead zone, stirred only by spiral density waves propagating out of the disc’s magnetically-coupled turbulent surface layers. We extend previous studies to include a wider range of disc models, and explore the effects of varying the disc column density and external magnetic field strength. [...] The strength of the stirring is found to be independent of the gas surface density, which is contrary to the increase with disc mass expected from a simple linear wave picture. The discrepancy arises from the shearing out of density waves as they propagate into the dead zone, resulting in density structures near the midplane that exert weaker stochastic torques on average. We provide a simple analytic fit to our numerically obtained torque amplitudes that accounts for this effect. The stirring on the other hand depends sensitively on the net vertical magnetic flux, up to a saturation level above which magnetic forces dominate in the turbulent layers. For the majority of our models, the equilibrium planetesimal velocity dispersions lie between the thresholds for disrupting strong and weak aggregates, suggesting that collision outcomes will depend on material properties. However, discs with relatively weak magnetic fields yield reduced stirring, and their dead zones provide safe-havens even for the weakest planetesimals against collisional destruction.

A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone

astro-ph — Fri, 03 Feb 2012 01:46:45 +0000

We re-analyze 4 years of HARPS spectra of the nearby M1.5 dwarf GJ 667C available through the ESO public archive. The new radial velocity (RV) measurements were obtained using a new data analysis technique that derives the Doppler measurement and other instrumental effects using a least-squares approach. Combining these new 143 measurements with 41 additional RVs from the Magellan/PFS and Keck/HIRES spectrometers, reveals 3 additional signals beyond the previously reported 7.2-day candidate, with periods of 28 days, 75 days, and a secular trend consistent with the presence of a gas giant (Period sim 10 years). The 28-day signal implies a planet candidate with a minimum mass of 4.5 Mearth orbiting well within the canonical definition of the star’s liquid water habitable zone, this is, the region around the star at which an Earth-like planet could sustain liquid water on its surface. Still, the ultimate water supporting capability of this candidate depends on properties that are unknown such as its albedo, atmospheric composition and interior dynamics. The 75-day signal is less certain, being significantly affected by aliasing interactions among a potential 91-day signal, and the likely rotation period of the star at 105 days detected in two activity indices. GJ 667C is the common proper motion companion to the GJ 667AB binary, which is metal poor compared to the Sun. The presence of a super-Earth in the habitable zone of a metal poor M dwarf in a triple star system, supports the evidence that such worlds should be ubiquitous in the Galaxy.

Limb darkening laws for two exoplanet host stars derived from 3D stellar model atmospheres

astro-ph — Fri, 03 Feb 2012 01:37:29 +0000

We compare limb darkening laws derived from 3D hydrodynamical model atmospheres and 1D hydrostatic MARCS models for the host stars of the two transiting exoplanet systems HD 209458 and HD 189733. The surface brightness distribution of the stellar disks is calculated using 3D LTE spectrum formation and opacity sampling. We test our predictions using least-squares fits of model light curves to primary eclipses that were observed with the Hubble Space Telescope (HST). The limb darkening law derived from the 3D model of HD 209458 between 2900 A and 5700 A produces significantly better fits to the HST data, removing systematic residuals that were previously observed for model light curves based on 1D predictions. This difference arises mainly from the shallower mean temperature structure of the 3D model, which is a consequence of the explicit simulation of surface granulation. In the case of HD 189733, the model atmospheres produce practically equivalent limb darkening curves between 2900 A and 5700 A, partly due to obstruction by spectral lines, and the data are not sufficient to distinguish between the light curves. We also analyze HST observations between 5350 A and 10500 A for this star; the 3D model leads to a better fit compared to 1D predictions. The significant improvement of fit quality for the HD 209458 system demonstrates the higher degree of realism of 3D models and the importance of surface granulation for the formation of the atmospheric radiation field of late-type stars. This result agrees well with recent investigations of limb darkening in the solar continuum and other observational tests. The case of HD 189733 is no contradiction as the model light curves are less sensitive to the temperature stratification of the atmosphere and the observed data in the 2900 A – 5700 A region are not sufficient to distinguish more clearly between 3D and 1D limb darkening.

The Curious Case of HU Aquarii – Dynamically Testing Proposed Planetary Systems [Replacement]

astro-ph — Fri, 03 Feb 2012 01:31:45 +0000

In early 2011, the discovery of two planets moving on surprisingly extreme orbits around the eclipsing polar cataclysmic variable system HU Aquraii was announced based on variations in the timing of mutual eclipses between the two central stars. We perform a detailed dynamical analysis of the stability of the exoplanet system as proposed in that work, revealing that it is simply dynamically unfeasible. We then apply the latest rigorous methods used by the Anglo-Australian Planet Search to analyse radial velocity data to re-examine the data used to make the initial claim. Using that data, we arrive at a significantly different orbital solution for the proposed planets, which we then show through dynamical analysis to be equally unfeasible. Finally, we discuss the need for caution in linking eclipse-timing data for cataclysmic variables to the presence of planets, and suggest a more likely explanation for the observed signal.

Ohmic Heating Suspends, not Reverses, the Cooling Contraction of Hot Jupiters

astro-ph — Thu, 02 Feb 2012 01:49:57 +0000

We study the radius evolution of close-in extra-solar jupiters under Ohmic heating, a mechanism that was recently proposed to explain the large observed sizes of many of these planets. Planets are born with high entropy and they subsequently cool and contract. We focus on two cases: first, that ohmic heating commences when the planet is hot (high entropy); and second, that it commences after the planet has cooled. In the former case, we use analytical scalings and numerical experiments to confirm that Ohmic heating is capable of suspending the cooling as long as a few percent of the stellar irradiation is converted into Ohmic heating, and the planet has a surface wind that extends to pressures of ~10 bar or deeper. For these parameters, the radii at which cooling is stalled are consistent with (or larger than) the observed radii of most planets. The only two exceptions are WASP-17b and HAT-P-32b. In contrast to the high entropy case, we show that Ohmic heating cannot significantly re-inflate planets after they have already cooled. This leads us to suggest that the diversity of radii observed in hot jupiters may be partially explained by the different epochs at which they are migrated to their current locations.

The GJ 436 System: Directly Determined Astrophysical Parameters of an M-Dwarf and Implications for the Transiting Hot Neptune

astro-ph — Thu, 02 Feb 2012 01:46:58 +0000

The late-type dwarf GJ 436 is known to host a transiting Neptune-mass planet in a 2.6-day orbit. We present results of our interferometric measurements to directly determine the stellar diameter ($R_{\star} = 0.455 \pm 0.018 R_{\odot}$) and effective temperature ($T_{\rm EFF} = 3416 \pm 54$ K). We combine our stellar parameters with literature time-series data, which allows us to calculate physical and orbital system parameters, including GJ 436’s stellar mass ($M_{\star} = 0.472^{+ 0.0636}_{- 0.0566} M_{\odot}$), planetary radius ($R_{p} = 0.370^{+ 0.0149}_{- 0.0145} R_{Jupiter}$), planetary mass ($M_{p} = 0.075^{+ 0.0076}_{- 0.0072} M_{Jupiter}$), implying a mean planetary density of $\rho_{p} = 1.48^{+ 0.116}_{- 0.103} \rho_{Jupiter}$. These values are generally in good agreement with previous literature estimates based on assumed stellar mass and photometric light curve fitting. Finally, we examine the expected phase curves of the hot Neptune GJ 436b, based on various assumptions concerning the efficiency of energy redistribution in the planetary atmosphere, and find that it could be constrained with {\it Spitzer} monitoring observations.

The chemical composition of CO-rich comet C/2009 P1 (Garradd) at Rh = 2.4 and 2.0 AU before perihelion

astro-ph — Thu, 02 Feb 2012 01:39:40 +0000

We quantified ten parent volatiles in comet C/2009 P1 (Garradd) before perihelion, through high-dispersion infrared spectra acquired with CRIRES at ESO’s VLT on UT 2011 August 07 (Rh = 2.4 AU) and September 17-21 (Rh = 2.0 AU). On August 07, water was searched but not detected at an upper limit (3{\sigma}) of 2.1 \times 10^28 s-1, while ethane was detected with a production rate of 6.1 \times 10^26 s-1 (apparent mixing ratio > 2.90%). On September 17-21, the mean production rate for water was 8.4 \times 10^28 s-1, and abundance ratios (relative to water) of detected trace species were: CO (12.51%), CH3OH (3.90%), CH4 (1.24%), C2H6 (1.01%) and HCN (0.36%). Upper limits (3{\sigma}) to abundances for four minor species were: NH3 (1.55%), C2H2 (0.13%), HDO (0.89%) and OCS (0.20%). Given the relatively large heliocentric distance, we explored the effect of water not being fully sublimated within our FOV and identified the ‘missing’ water fraction needed to reconcile the retrieved abundance ratios with the mean values found for “organics-normal”. The individual spatial profiles of parent volatiles and the continuum displayed rather asymmetric outgassing. Indications of H2O and CO gas being released in different directions suggest different active vents and/or the possible existence of polar and apolar ice aggregates in the nucleus. The high fractional abundance of CO identifies comet C/2009 P1 as a CO-rich comet.

The McDonald Observatory Planet Search: New Long-Period Giant Planets, and Two Interacting Jupiters in the HD 155358 System

astro-ph — Thu, 02 Feb 2012 01:37:08 +0000

We present high-precision radial velocity (RV) observations of four solar-type (F7-G5) stars – HD 79498, HD 155358, HD 197037, and HD 220773 – taken as part of the McDonald Observatory Planet Search Program. For each of these stars, we see evidence of Keplerian motion caused by the presence of one or more gas giant planets in long-period orbits. We derive orbital parameters for each system, and note the properties (composition, activity, etc.) of the host stars. While we have previously announced the two-gas-giant HD 155358 system, we now report a shorter period for planet c. This new period is consistent with the planets being trapped in mutual 2:1 mean-motion resonance. We therefore perform an in-depth stability analysis, placing additional constraints on the orbital parameters of the planets. These results demonstrate the excellent long-term RV stability of the spectrometers on both the Harlan J. Smith 2.7 m telescope and the Hobby-Eberly telescope.

Kepler-20: A Sun-like Star with Three Sub-Neptune Exoplanets and Two Earth-size Candidates [Replacement]

astro-ph — Thu, 02 Feb 2012 01:32:15 +0000

We present the discovery of the Kepler-20 planetary system, which we initially identified through the detection of five distinct periodic transit signals in the Kepler light curve of the host star 2MASSJ19104752+4220194. We find a stellar effective temperature Teff=5455+-100K, a metallicity of [Fe/H]=0.01+-0.04, and a surface gravity of log(g)=4.4+-0.1. Combined with an estimate of the stellar density from the transit light curves we deduce a stellar mass of Mstar=0.912+-0.034 Msun and a stellar radius of Rstar=0.944^{+0.060}_{-0.095} Rsun. For three of the transit signals, our results strongly disfavor the possibility that these result from astrophysical false positives. We conclude that the planetary scenario is more likely than that of an astrophysical false positive by a factor of 2e5 (Kepler-20b), 1e5 (Kepler-20c), and 1.1e3 (Kepler-20d), sufficient to validate these objects as planetary companions. For Kepler-20c and Kepler-20d, the blend scenario is independently disfavored by the achromaticity of the transit: From Spitzer data gathered at 4.5um, we infer a ratio of the planetary to stellar radii of 0.075+-0.015 (Kepler-20c) and 0.065+-0.011 (Kepler-20d), consistent with each of the depths measured in the Kepler optical bandpass. We determine the orbital periods and physical radii of the three confirmed planets to be 3.70d and 1.91^{+0.12}_{-0.21} Rearth for Kepler-20b, 10.85 d and 3.07^{+0.20}_{-0.31} Rearth for Kepelr-20c, and 77.61 d and 2.75^{+0.17}_{-0.30} Rearth for Kepler-20d. From multi-epoch radial velocities, we determine the masses of Kepler-20b and Kepler-20c to be 8.7\+-2.2 Mearth and 16.1+-3.5 Mearth, respectively, and we place an upper limit on the mass of Kepler-20d of 20.1 Mearth (2 sigma).

The sub-Saturn Mass Transiting Planet HAT-P-12b

astro-ph — Wed, 01 Feb 2012 01:49:40 +0000

We present new photometric data of the transiting planet HAT-P-12b observed in 2011. Our three transit curves are modelled using the JKTEBOP code and adopting the quadratic limb-darkening law. Including our measurements, 18 transit times spanning about 4.2 yr were used to determine the improved ephemeris with a transit epoch of 2,454,187.85560$\pm$0.00011 BJD and an orbital period of 3.21305961$\pm$0.00000035 d. The physical properties of the star-planet system are computed using empirical calibrations from eclipsing binary stars and stellar evolutionary models, combined with both our transit parameters and previously-known spectroscopic results. We found that the absolute dimensions of the host star are $M_{\rm A}$=0.73$\pm$0.02 M$_\odot$, $R_{\rm A}$=0.70$\pm$0.01 R$_\odot$, log $g_{\rm A}$=4.61$\pm$0.02, $\rho_{\rm A}$=2.10$\pm$0.09 $\rho_{\odot}$, and $L_{\rm A}$=0.21$\pm$0.01 L$_\odot$. The planetary companion has $M_{\rm b}$=0.21$\pm$0.01 M$_{\rm Jup}$, $R_{\rm b}$=0.94$\pm$0.01 R$_{\rm Jup}$, log $g_{\rm b}$=2.77$\pm$0.02, $\rho_{\rm b}$=0.24$\pm$0.01 $\rho_{\rm Jup}$, and $T_{\rm eq}$=960$\pm$14 K. Our results agree well with standard models of irradiated gas giants with a core mass of 11.3 M$_{\oplus}$.

Probing the haze in the atmosphere of HD 189733b with HST/WFC3 transmission spectroscopy

astro-ph — Wed, 01 Feb 2012 01:45:36 +0000

We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting exoplanet HD 189733b, using Wide Field Camera 3. This consists of time-series spectra of two transits, used to measure the wavelength dependence of the planetary radius. These observations aim to test whether the Rayleigh scattering haze detected at optical wavelengths extends into the near-infrared, or if it becomes transparent leaving molecular features to dominate the transmission spectrum. Due to saturation and non-linearity affecting the brightest (central) pixels of the spectrum, light curves were extracted from the blue and red ends of the spectra only, corresponding to wavelength ranges of 1.099-1.168 um and 1.521-1.693 um, respectively, for the first visit, and 1.082-1.128 um and 1.514-1.671 um for the second. The light curves were fitted using a Gaussian process model to account for instrumental systematics whilst simultaneously fitting for the transit parameters. This gives values of the planet-to-star radius ratio for the blue and red light curves of 0.15650\pm0.00048 and 0.15634\pm0.00032, respectively, for visit one and 0.15716\pm0.00078 and 0.15630\pm0.00037 for visit 2 (using a quadratic limb darkening law). The planet-to-star radius ratios measured in both visits are consistent, and we see no evidence for the drop in absorption expected if the haze that is observed in the optical becomes transparent in the infrared. This tentatively suggests that the haze dominates the transmission spectrum of HD 189733b into near-infrared wavelengths, although more robust observations are required to provide conclusive evidence.

Exoplanets Bouncing Between Binary Stars

astro-ph — Wed, 01 Feb 2012 01:45:20 +0000

Exoplanetary systems are found not only among single stars, but also binaries of widely varying parameters. Binaries with separations of 100–1000 au are prevalent in the Solar neighborhood; at these separations planet formation around a binary member may largely proceed as if around a single star. During the early dynamical evolution of a planetary system, planet–planet scattering can eject planets from a star’s grasp. In a binary, the motion of a planet ejected from one star has effectively entered a restricted three-body system consisting of itself and the two stars, and the equations of motion of the three body problem will apply as long as the ejected planet remains far from the remaining planets. Depending on its energy, escape from the binary as a whole may be impossible or delayed until the three-body approximation breaks down, and further close interactions with its planetary siblings boost its energy when it passes close to its parent star. Until then this planet may be able to transition from the space around one star to the other, and chaotically `bounce’ back and forth. In this paper we directly simulate scattering planetary systems that are around one member of a circular binary, and quantify the frequency of bouncing in scattered planets. We find that a great majority (70 to 85 per cent) of ejected planets will pass at least once through the space of it’s host’s binary companion, and depending on the binary parameters about 45 to 75 per cent will begin bouncing. The time spent bouncing is roughly log-normally distributed with a peak at about $10^4$ years, with only a small percentage bouncing for more than a Myr. This process may perturb and possibly incite instability among existing planets around the companion star. In rare cases, the presence of multiple planets orbiting both stars may cause post-bouncing capture or planetary swapping.

A natural formation scenario for misaligned and short-period eccentric extrasolar planets [Replacement]

astro-ph — Wed, 01 Feb 2012 01:39:49 +0000

Recent discoveries of strongly misaligned transiting exoplanets pose a challenge to the established planet formation theory which assumes planetary systems to form and evolve in isolation. However, the fact that the majority of stars actually do form in star clusters raises the question how isolated forming planetary systems really are. Besides radiative and tidal forces the presence of dense gas aggregates in star-forming regions are potential sources for perturbations to protoplanetary discs or systems. Here we show that subsequent capture of gas from large extended accretion envelopes onto a passing star with a typical circumstellar disc can tilt the disc plane to retrograde orientation, naturally explaining the formation of strongly inclined planetary systems. Furthermore, the inner disc regions may become denser, and thus more prone to speedy coagulation and planet formation. Pre-existing planetary systems are compressed by gas inflows leading to a natural occurrence of close-in misaligned hot Jupiters and short-period eccentric planets. The likelihood of such events mainly depends on the gas content of the cluster and is thus expected to be highest in the youngest star clusters.

Asteroseismology of the solar analogs 16 Cyg A & B from Kepler observations

astro-ph — Tue, 31 Jan 2012 01:55:21 +0000

Detailed compositional analysis of the heavily polluted DBZ white dwarf SDSS J073842.56+183509.06: A window on planet formation?

astro-ph — Tue, 31 Jan 2012 01:44:22 +0000

We present a new model atmosphere analysis of the most metal contaminated white dwarf known, the DBZ SDSS J073842.56+183509.06. Using new high resolution spectroscopic observations taken with Keck and Magellan, we determine precise atmospheric parameters and measure abundances of 14 elements heavier than helium. We also report new Spitzer mid-infrared photometric data that are used to better constrain the properties of the debris disk orbiting this star. Our detailed analysis, which combines data taken from 7 different observational facilities (GALEX, Gemini, Keck, Magellan, MMT, SDSS and Spitzer) clearly demonstrate that J0738+1835 is accreting large amounts of rocky terrestrial-like material that has been tidally disrupted into a debris disk. We estimate that the body responsible for the photospheric metal contamination was at least as large Ceres, but was much drier, with less than 1% of the mass contained in the form of water ice, indicating that it formed interior to the snow line around its parent star. We also find a correlation between the abundances (relative to Mg and bulk Earth) and the condensation temperature; refractory species are clearly depleted while the more volatile elements are possibly enhanced. This could be the signature of a body that formed in a lower temperature environment than where Earth formed. Alternatively, we could be witnessing the remains of a differentiated body that lost a large part of its outer layers.

Model for the Production of CO Cameron band emission in Comet 1P/Halley

astro-ph — Tue, 31 Jan 2012 01:42:57 +0000

The abundance of CO2 in comets has been derived using CO Cameron band (a3pi –> X1Sigma+) emission assuming that photodissociative excitation of CO2 is the main production process of CO(a3pi). On comet 1P/Halley the Cameron (1-0) band has been observed by International Ultraviolet Explorer (IUE) on several days in March 1986. A coupled chemistry-emission model is developed for comet 1P/Halley to assess the importance of various production and loss mechanisms of CO(a3pi) and to calculate the intensity of Cameron band emission on different days of IUE observation. Two different solar EUV flux models, EUVAC of Richards et al. (1994) and SOLAR2000 of Tobiska (2004), and different relative abundances of CO and CO2, are used to evaluate the role of photon and photoelectron in producing CO molecule in a3pi state in the cometary coma. It is found that in comet 1P/Halley 60–70% of the total intensity of the Cameron band emission is contributed by electron impact excitation of CO and CO2, while the contribution from photodissociative excitation of CO2 is small (20–30%). Thus, in the comets where CO and CO2 relative abundances are comparable, the Cameron band emission is largely governed by electron impact excitation of CO, and not by the photodissociative excitation of CO2 as assumed earlier. Model calculated Cameron band 1-0 emission intensity (40 R) is consistent with the observed IUE slit-averaged brightness (37 +/- 6 R) using EUVAC model solar flux on 13 March 1986, and also on other days of observations. Since electron impact excitation is the major production mechanism, the Cameron emission can be used to derive photoelectron density in the inner coma rather than the CO2 abundance.

Orbital perturbations due to massive rings [Replacement]

astro-ph — Tue, 31 Jan 2012 01:31:06 +0000

We analytically work out the long-term orbital perturbations induced by a homogeneous circular ring of radius Rr and mass mr on the motion of a test particle in the cases (I): r > R_r and (II): r < R_r. In order to extend the validity of our analysis to the orbital con?gurations of, e.g., some proposed spacecraftbased mission for fundamental physics like LISA and ASTROD, of possible annuli around the supermassive black hole in Sgr A* coming from tidal disruptions of incoming gas clouds, and to the e?ect of arti?cial space debris belts around the Earth, we do not restrict ourselves to the case in which the ring and the orbit of the perturbed particle lie just in the same plane. From the corrections to the standard secular perihelion precessions, recently determined by a team of astronomers for some planets of the Solar System, we infer upper bounds on mr for various putative and known annular matter distributions of natural origin (close circumsolar ring with R_r = 0.02-0.13 au, dust ring with R_r = 1 au, minor asteroids, Trans-Neptunian Objects). We find m_r <= 8.4 10^-4 m_E (circumsolar ring with R_r = 0.02 au), m_r <= 1.6 10^-5 m_E (circumsolar ring with R_r = 0.13 au), m_r <= 8.8 10^-7 m_E (ring with R_r = 1 au), m_r <= 7.3 10^-12 M_S (asteroidal ring with R_r = 2.80 au), m_r <= 1.1 <= 10^-11 M_S (asteroidal ring with R_r = 3.14 au), m_r <= 2.0 10^-8 M_S (TNOs ring with R_r = 43 au). In principle, our analysis is valid both for baryonic and non-baryonic Dark Matter distributions.

The Curious Case of HU Aquarii – Dynamically Testing Proposed Planetary Systems

astro-ph — Mon, 30 Jan 2012 01:43:17 +0000

A catalogue of rotation and activity in early-M stars

astro-ph — Mon, 30 Jan 2012 01:41:35 +0000

We present a catalogue of rotation and chromospheric activity in a sample of 334 M dwarfs of spectral types M0–M4.5 populating the parameter space around the boundary to full convection. We obtained high-resolution optical spectra for 206 targets and determined projected rotational velocity, vsini, and Halpha emission. The data are combined with measurements of vsini in field stars of the same spectral type from the literature. Our sample adds 157 new rotation measurements to the existing literature and almost doubles the sample of available vsini. The final sample provides a statistically meaningful picture of rotation and activity at the transition to full convection in the solar neighborhood. We confirm the steep rise in the fraction of active stars at the transition to full convection known from earlier work. In addition, we see a clear rise in rotational velocity in the same stars. In very few stars, no chromospheric activity but a detection of rotational broadening was reported. We argue that all of them are probably spurious detections; we conclude that in our sample all significantly rotating stars are active, and all active stars are significantly rotating. The rotation-activity relation is valid in partially and in fully convective stars. Thus, we do not observe any evidence for a transition from a rotationally dominated dynamo in partially convective stars to a rotation-independent turbulent dynamo in fully convective stars; turbulent dynamos in fully convective stars of spectral types around M4 are still driven by rotation. Finally, we compare projected rotational velocities of 33 stars to rotational periods derived from photometry in the literature and determine inclinations for a few of them.

A simple model for the evolution of the dust population in protoplanetary disks

astro-ph — Mon, 30 Jan 2012 01:39:52 +0000

Context: The global size and spatial distribution of dust is an important ingredient in the structure and evolution of protoplanetary disks and in the formation of larger bodies, such as planetesimals. Aims: We aim to derive simple equations that explain the global evolution of the dust surface density profile and the upper limit of the grain size distribution and which can readily be used for further modeling or for interpreting of observational data. Methods: We have developed a simple model that follows the upper end of the dust size distribution and the evolution of the dust surface density profile. This model is calibrated with state-of-the-art simulations of dust evolution, which treat dust growth, fragmentation, and transport in viscously evolving gas disks. Results: We find very good agreement between the full dust-evolution code and the toy model presented in this paper. We derive analytical profiles that describe the dust-to-gas ratios and the dust surface density profiles well in protoplanetary disks, as well as the radial flux by solid material “rain out”, which is crucial for triggering any gravity assisted formation of planetesimals. We show that fragmentation is the dominating effect in the inner regions of the disk leading to a dust surface density exponent of -1.5, while the outer regions at later times can become drift-dominated, yielding a dust surface density exponent of -0.75. Our results show that radial drift is not efficient in fragmenting dust grains. This supports the theory that small dust grains are resupplied by fragmentation due to the turbulent state of the disk.

Thermal Phase Variations of WASP-12b: Defying Predictions [Replacement]

astro-ph — Mon, 30 Jan 2012 01:33:15 +0000

[Abridged] We report Warm Spitzer full-orbit phase observations of WASP-12b at 3.6 and 4.5 micron. We are able to measure the transit depths, eclipse depths, thermal and ellipsoidal phase variations at both wavelengths. The large amplitude phase variations, combined with the planet’s previously-measured day-side spectral energy distribution, is indicative of non-zero Bond albedo and very poor day-night heat redistribution. The transit depths in the mid-infrared indicate that the atmospheric opacity is greater at 3.6 than at 4.5 micron, in disagreement with model predictions, irrespective of C/O ratio. The secondary eclipse depths are consistent with previous studies. We do not detect ellipsoidal variations at 3.6 micron, but our parameter uncertainties -estimated via prayer-bead Monte Carlo- keep this non-detection consistent with model predictions. At 4.5 micron, on the other hand, we detect ellipsoidal variations that are much stronger than predicted. If interpreted as a geometric effect due to the planet’s elongated shape, these variations imply a 3:2 ratio for the planet’s longest:shortest axes and a relatively bright day-night terminator. If we instead presume that the 4.5 micron ellipsoidal variations are due to uncorrected systematic noise and we fix the amplitude of the variations to zero, the best fit 4.5 micron transit depth becomes commensurate with the 3.6 micron depth, within the uncertainties. The relative transit depths are then consistent with a Solar composition and short scale height at the terminator. Assuming zero ellipsoidal variations also yields a much deeper 4.5 micron eclipse depth, consistent with a Solar composition and modest temperature inversion. We suggest future observations that could distinguish between these two scenarios.

The Effects of Irradiation on Hot Jovian Atmospheres: Heat Redistribution and Energy Dissipation

astro-ph — Fri, 27 Jan 2012 01:56:20 +0000

Hot Jupiters, due to the proximity to their parent stars, are subjected to a strong irradiating flux which governs their radiative and dynamical properties. We compute a suite of 3D circulation models with dual-band radiative transfer, exploring a relevant range of irradiation temperatures (770K <~ Tirr <~ 3000K), both with and without temperature inversions. We find that, for irradiation temperatures Tirr <~ 2000K, heat redistribution is very efficient, producing comparable day- and night-side fluxes. For Tirr ~ 2200-2400K, redistribution starts to break down, resulting in a high day-night flux contrast. Our simulations support the physical intuition that the efficiency of heat transfer is primarily governed by the ratio of advective to radiative timescales. For the same Tirr, models with temperature inversions display a higher day-night contrast, but we find this opacity-driven effect to be secondary to irradiation. The hotspot offset from the substellar point is large when insolation is weak and redistribution is efficient, and decreases as redistribution breaks down. We further explore the importance of various dissipation mechanisms with the strength of the irradiating flux. The atmospheric flow can be potentially subjected to the Kelvin-Helmholtz instability only in the uppermost layers, with a depth that penetrates to pressures of a few millibars at most. Shocks penetrate deeper, down to several bars in the hottest model. For a B ~ a few Gauss, Ohmic dissipation generally occurs down to deeper levels than shock dissipation (to tens of bars), but the penetration depth varies with the atmospheric opacity. The total dissipated Ohmic power increases steeply with the strength of the irradiating flux and the dissipation depth recedes into the atmosphere, favoring radius inflation in the most irradiated objects. (Abridged)

Transit Timing Observations from Kepler: II. Confirmation of Two Multiplanet Systems via a Non-parametric Correlation Analysis

astro-ph — Fri, 27 Jan 2012 01:53:47 +0000

We present a new method for confirming transiting planets based on the combination of transit timingn variations (TTVs) and dynamical stability. Correlated TTVs provide evidence that the pair of bodies are in the same physical system. Orbital stability provides upper limits for the masses of the transiting companions that are in the planetary regime. This paper describes a non-parametric technique for quantifying the statistical significance of TTVs based on the correlation of two TTV data sets. We apply this method to an analysis of the transit timing variations of two stars with multiple transiting planet candidates identified by Kepler. We confirm four transiting planets in two multiple planet systems based on their TTVs and the constraints imposed by dynamical stability. An additional three candidates in these same systems are not confirmed as planets, but are likely to be validated as real planets once further observations and analyses are possible. If all were confirmed, these systems would be near 4:6:9 and 2:4:6:9 period commensurabilities. Our results demonstrate that TTVs provide a powerful tool for confirming transiting planets, including low-mass planets and planets around faint stars for which Doppler follow-up is not practical with existing facilities. Continued Kepler observations will dramatically improve the constraints on the planet masses and orbits and provide sensitivity for detecting additional non-transiting planets. If Kepler observations were extended to eight years, then a similar analysis could likely confirm systems with multiple closely spaced, small transiting planets in or near the habitable zone of solar-type stars.

Transit Timing Observations from Kepler: III. Confirmation of 4 Multiple Planet Systems by a Fourier-Domain Study of Anti-correlated Transit Timing Variations

astro-ph — Fri, 27 Jan 2012 01:53:28 +0000

We present a method to confirm the planetary nature of objects in systems with multiple transiting exoplanet candidates. This method involves a Fourier-Domain analysis of the deviations in the transit times from a constant period that result from dynamical interactions within the system. The combination of observed anti-correlations in the transit times and mass constraints from dynamical stability allow us to claim the discovery of four planetary systems Kepler-25, Kepler-26, Kepler-27, and Kepler-28, containing eight planets and one additional planet candidate.

Transit Timing Observations from Kepler: IV. Confirmation of 4 Multiple Planet Systems by Simple Physical Models

astro-ph — Fri, 27 Jan 2012 01:52:34 +0000

Eighty planetary systems of two or more planets are known to orbit stars other than the Sun. For most, the data can be sufficiently explained by non-interacting Keplerian orbits, so the dynamical interactions of these systems have not been observed. Here we present 4 sets of lightcurves from the Kepler spacecraft, which each show multiple planets transiting the same star. Departure of the timing of these transits from strict periodicity indicates the planets are perturbing each other: the observed timing variations match the forcing frequency of the other planet. This confirms that these objects are in the same system. Next we limit their masses to the planetary regime by requiring the system remain stable for astronomical timescales. Finally, we report dynamical fits to the transit times, yielding possible values for the planets’ masses and eccentricities. As the timespan of timing data increases, dynamical fits may allow detailed constraints on the systems’ architectures, even in cases for which high-precision Doppler follow-up is impractical.

Almost All of Kepler’s Multiple Planet Candidates are Planets

astro-ph — Fri, 27 Jan 2012 01:52:16 +0000

We present a statistical analysis that demonstrates that the overwhelming majority of Kepler candidate multiple transiting systems (multis) indeed represent true, physically-associated transiting planets. Binary stars provide the primary source of false positives among Kepler planet candidates, implying that false positives should be nearly randomly-distributed among Kepler targets. In contrast, true transiting planets would appear clustered around a smaller number of Kepler targets if detectable planets tend to come in systems and/or if the orbital planes of planets encircling the same star are correlated. There are more than one hundred times as many Kepler planet candidates in multi-candidate systems as would be predicted from a random distribution of candidates, implying that the vast majority are true planets. Most of these multis are multiple planet systems orbiting the Kepler target star, but there are likely cases where (a) the planetary system orbits a fainter star, and the planets are thus significantly larger than has been estimated, or (b) the planets orbit different stars within a binary/multiple star system. We use the low overall false positive rate among Kepler multis, together with analysis of Kepler spacecraft and ground-based data, to validate the closely-packed Kepler-33 planetary system, which orbits a star that has evolved somewhat off of the main sequence. Kepler-33 hosts five transiting planets with periods ranging from 5.67 to 41 days.

WASP-36b: A new transiting planet around a metal-poor G-dwarf, and an investigation into analyses based on a single transit light curve [Replacement]

astro-ph — Fri, 27 Jan 2012 01:36:34 +0000

We report the discovery, from WASP and CORALIE, of a transiting exoplanet in a 1.54-d orbit. The host star, WASP-36, is a magnitude V = 12.7, metal-poor G2 dwarf (Teff = 5959 \pm 134 K), with [Fe/H] = -0.26 \pm 0.10. We determine the planet to have mass and radius respectively 2.30 \pm 0.07 and 1.28 \pm 0.03 times that of Jupiter. We have eight partial or complete transit light curves, from four different observatories, which allows us to investigate the potential effects on the fitted system parameters of using only a single light curve. We find that the solutions obtained by analysing each of these light curves independently are consistent with our global fit to all the data, despite the apparent presence of correlated noise in at least two of the light curves.

Benchmark cool companions: Ages and abundances for the PZ Tel system [Replacement]

astro-ph — Fri, 27 Jan 2012 01:35:53 +0000

We present new ages and abundance measurements for the pre-main sequence star PZ Tel. PZ Tel was recently found to host a young and low-mass companion. Using FEROS spectra we have measured atomic abundances (e.g. Fe and Li) and chromospheric activity for PZ Tel and used these to obtain metallicity and age estimates for the companion. We find PZ Tel to be a rapidly rotating (vsini=73\pm5km/s), ~solar metallicity star (logN(Fe)=-4.37 dex or [Fe/H]=0.05 dex) with a measured mean logR’HK of -4.12. We measure a NLTE lithium abundance of logN(Li)=3.1\pm0.1dex, which from depletion models gives rise to an age of 7+4-2 Myrs for the system. The measured chromospheric activity returns an age of 26\pm2Myrs, as does fitting pre-main sequence evolutionary tracks (Tau_evol=22\pm3Myrs), both of which are in disagreement with the lithium age. We speculate on reasons for this difference and introduce new models for lithium depletion that incorporates both rotation and magnetic field affects. We also synthesize solar, metal-poor and metal-rich substellar evolutionary models to better determine the bulk properties of PZ Tel B, showing that PZ Tel B is probably more massive than previous estimates, meaning the companion is not a giant exoplanet. We show how PZ Tel B compares to other currently known age and metallicity benchmark systems and try to empirically test the effects of dust opacity as a function of metallicity on the near infrared colours of brown dwarfs. Current models suggest that in the near infrared observations are more sensitive to low-mass companions orbiting more metal-rich stars. We also look for trends between infrared photometry and metallicity amongst a growing population of substellar benchmark objects, and identify the need for more data in mass-age-metallicity parameter space. [Abridged]

Demonstration of On-Sky Calibration of Astronomical Spectra using a 25 GHz near-IR Laser Frequency Comb [Cross-Listing]

astro-ph — Thu, 26 Jan 2012 01:48:00 +0000

We describe and characterize a 25 GHz laser frequency comb based on a cavity-filtered erbium fiber mode-locked laser. The comb provides a uniform array of optical frequencies spanning 1450 nm to 1700 nm, and is stabilized by use of a global positioning system referenced atomic clock. This comb was deployed at the 9.2 m Hobby-Eberly telescope at the McDonald Observatory where it was used as a radial velocity calibration source for the fiber-fed Pathfinder near-infrared spectrograph. Stellar targets were observed in three echelle orders over four nights, and radial velocity precision of ~10 m/s (~6 MHz) was achieved from the comb-calibrated spectra.

The formation heritage of Jupiter Family Comet 10P/Tempel 2 as revealed by infrared spectroscopy

astro-ph — Thu, 26 Jan 2012 01:45:09 +0000

We present spectral and spatial information for major volatile species in Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT 2010 July 26 (heliocentric distance Rh = 1.44 AU) and September 18 (Rh = 1.62 AU), following the comet’s perihelion passage on UT 2010 July 04. The total production rate for water on July 26 was (1.90 \pm 0.12) \times 10^28 molecules s-1, and abundances of six trace gases (relative to water) were: CH3OH (1.58% \pm 0.23), C2H6 (0.39% \pm 0.04), NH3 (0.83% \pm 0.20), and HCN (0.13% \pm 0.02). A detailed analysis of intensities for water emission lines provided a rotational temperature of 35 \pm 3 K. The mean OPR is consistent with nuclear spin populations in statistical equilibrium (OPR = 3.01 \pm 0.18), and the (1{\sigma}) lower bound corresponds to a spin temperature > 38 K. Our measurements were contemporaneous with a jet-like feature observed at optical wavelengths. The spatial profiles of four primary volatiles display strong enhancements in the jet direction, which favors release from a localized vent on the nucleus. The measured IR continuum is much more sharply peaked and is consistent with a dominant contribution from the nucleus itself. The peak intensities for H2O, CH3OH, and C2H6 are offset by ~200 km in the jet direction, suggesting the possible existence of a distributed source, such as the release of icy grains that subsequently sublimed in the coma. On UT September 18, no obvious emission lines were present in our spectra, nevertheless we obtained a 3{\sigma} upper limit Q(H2O) < 2.86 \times 10^27 molecules s-1.

A low mass for Mars from Jupiter’s early gas-driven migration

astro-ph — Thu, 26 Jan 2012 01:44:50 +0000

Jupiter and Saturn formed in a few million years (Haisch et al. 2001) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ~100,000 years (Armitage 2007). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration (Masset & Snellgrove 2001, Morbidelli & Crida 2007, Pierens & Nelson 2008). The terrestrial planets finished accreting much later (Klein et al. 2009), and their characteristics, including Mars’ small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (Wetherill 1978, Hansen 2009) (1 AU is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 AU, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 AU; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 AU and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.

Symplectic integration of space debris motion considering several Earth’s shadowing models

astro-ph — Thu, 26 Jan 2012 01:41:39 +0000

In this work, we present a symplectic integration scheme to numerically compute space debris motion. Such an integrator is particularly suitable to obtain reliable trajectories of objects lying on high orbits, especially geostationary ones. Indeed, it has already been demonstrated that such objects could stay there for hundreds of years. Our model takes into account the Earth’s gravitational potential, luni-solar and planetary gravitational perturbations and direct solar radiation pressure. Based on the analysis of the energy conservation and on a comparison with a high order non-symplectic integrator, we show that our algorithm allows us to use large time steps and keep accurate results. We also propose an innovative method to model Earth’s shadow crossings by means of a smooth shadow function. In the particular framework of symplectic integration, such a function needs to be included analytically in the equations of motion in order to prevent numerical drifts of the energy. For the sake of completeness, both cylindrical shadows and penumbra transitions models are considered. We show that both models are not equivalent and that big discrepancies actually appear between associated orbits, especially for high area-to-mass ratios.

Orbital perturbations due to massive rings [Cross-Listing]

astro-ph — Thu, 26 Jan 2012 01:39:43 +0000

We analytically work out the long-term orbital perturbations induced by a homogeneous circular ring of radius Rr and mass mr on the motion of a test particle in the cases (I): r > R_r and (II): r < R_r. In order to extend the validity of our analysis to the orbital con?gurations of, e.g., some proposed spacecraftbased mission for fundamental physics like LISA and ASTROD, of possible annuli around the supermassive black hole in Sgr A* coming from tidal disruptions of incoming gas clouds, and to the e?ect of arti?cial space debris belts around the Earth, we do not restrict ourselves to the case in which the ring and the orbit of the perturbed particle lie just in the same plane. From the corrections to the standard secular perihelion precessions, recently determined by a team of astronomers for some planets of the Solar System, we infer upper bounds on mr for various putative and known annular matter distributions of natural origin (close circumsolar ring with R_r = 0.02-0.13 au, dust ring with R_r = 1 au, minor asteroids, Trans-Neptunian Objects). We find m_r <= 8.4 10^-4 m_E (circumsolar ring with R_r = 0.02 au), m_r <= 1.6 10^-5 m_E (circumsolar ring with R_r = 0.13 au), m_r <= 8.8 10^-7 m_E (ring with R_r = 1 au), m_r <= 7.3 10^-12 M_S (asteroidal ring with R_r = 2.80 au), m_r <= 1.1 <= 10^-11 M_S (asteroidal ring with R_r = 3.14 au), m_r <= 2.0 10^-8 M_S (TNOs ring with R_r = 43 au). In principle, our analysis is valid both for baryonic and non-baryonic Dark Matter distributions.

On the interaction between tides and convection

astro-ph — Wed, 25 Jan 2012 01:45:41 +0000

We study the interaction between tides and convection in astrophysical bodies by analysing the effect of a homogeneous oscillatory shear on a fluid flow. This model can be taken to represent the interaction between a large-scale periodic tidal deformation and a smaller-scale convective motion. We first consider analytically the limit in which the shear is of low amplitude and the oscillation period is short compared to the timescales of the unperturbed flow. In this limit there is a viscoelastic response and we obtain expressions for the effective elastic modulus and viscosity coefficient. The effective viscosity is inversely proportional to the square of the oscillation frequency, with a coefficient that can be positive, negative or zero depending on the properties of the unperturbed flow. We also carry out direct numerical simulations of Boussinesq convection in an oscillatory shearing box and measure the time-dependent Reynolds stress. The results indicate that the effective viscosity of turbulent convection falls rapidly as the oscillation frequency is increased, attaining small negative values in the cases we have examined, although significant uncertainties remain because of the turbulent noise. We discuss the implications of this analysis for astrophysical tides.

HAT-P-38b: A Saturn-Mass Planet Transiting a Late G Star

astro-ph — Wed, 25 Jan 2012 01:41:32 +0000

We report the discovery of HAT-P-38b, a Saturn-mass exoplanet transiting the V=12.56 dwarf star GSC 2314-00559 on a P = 4.6404 d circular orbit. The host star is a 0.89Msun late G-dwarf, with solar metallicity, and a radius of 0.92Rsun. The planetary companion has a mass of 0.27MJ, and radius of 0.82RJ. HAT-P-38b is one of the closest planets in mass and radius to Saturn ever discovered.

High-resolution Fourier-transform XUV photoabsorption spectroscopy of 14N15N [Cross-Listing]

astro-ph — Wed, 25 Jan 2012 01:39:55 +0000

The first comprehensive high-resolution photoabsorption spectrum of 14N15N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet (XUV) and span 100,000-109,000 cm-1 (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N2 that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schroedinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.

Infrared Non-detection of Fomalhaut b — Implications for the Planet Interpretation

astro-ph — Tue, 24 Jan 2012 02:06:28 +0000

The nearby A4-type star Fomalhaut hosts a debris belt in the form of an eccentric ring, which is thought to be caused by dynamical influence from a giant planet companion. In 2008, a detection of a point-source inside the inner edge of the ring was reported and was interpreted as a direct image of the planet, named Fomalhaut b. The detection was made at ~600–800 nm, but no corresponding signatures were found in the near-infrared range, where the bulk emission of such a planet should be expected. Here we present deep observations of Fomalhaut with Spitzer/IRAC at 4.5 um, using a novel PSF subtraction technique based on ADI and LOCI, in order to substantially improve the Spitzer contrast at small separations. The results provide more than an order of magnitude improvement in the upper flux limit of Fomalhaut b and exclude the possibility that any flux from a giant planet surface contributes to the observed flux at visible wavelengths. This renders any direct connection between the observed light source and the dynamically inferred giant planet highly unlikely. We discuss several possible interpretations of the total body of observations of the Fomalhaut system, and find that the interpretation that best matches the available data for the observed source is scattered light from transient or semi-transient dust cloud.

Coagulation Calculations of Icy Planet Formation at 15–150 AU: A Correlation Between the Maximum Radius and the Slope of the Size Distribution for Transneptunian Objects

astro-ph — Tue, 24 Jan 2012 02:05:17 +0000

We investigate whether coagulation models of planet formation can explain the observed size distributions of transneptunian objects (TNOs). Analyzing published and new calculations, we demonstrate robust relations between the size of the largest object and the slope of the size distribution for sizes 0.1 km and larger. These relations yield clear, testable predictions for TNOs and other icy objects throughout the solar system. Applying our results to existing observations, we show that a broad range of initial disk masses, planetesimal sizes, and fragmentation parameters can explain the data. Adding dynamical constraints on the initial semimajor axis of `hot’ KBOs along with probable TNO formation times of 10-700 Myr restricts the viable models to those with a massive disk composed of relatively small (1-10 km) planetesimals.

A new vision on giant planet interiors: the impact of double diffusive convection

astro-ph — Tue, 24 Jan 2012 01:58:02 +0000

While conventional interior models for Jupiter and Saturn are based on the simplistic assumption of a solid core surrounded by a homogeneous gaseous envelope, we derive new models with an inhomogeneous distribution of heavy elements, i.e. a gradient of composition, within these planets. Such a compositional stratification hampers large scale convection which turns into double-diffusive convection, yielding an inner thermal profile which departs from the traditionally assumed adiabatic interior, affecting these planet heat content and cooling history. To address this problem, we develop an analytical approach of layered double-diffusive convection and apply this formalism to Solar System gaseous giant planet interiors. These models satisfy all observational constraints and yield a metal enrichment for our gaseous giants up to 30 to 60% larger than previously thought. The models also constrain the size of the convective layers within the planets. As the heavy elements tend to be redistributed within the gaseous envelope, the models predict smaller than usual central cores inside Saturn and Jupiter, with possibly no core for this latter. These models open a new window and raise new challenges on our understanding of the internal structure of giant (solar and extrasolar) planets, in particular on the determination of their heavy material content, a key diagnostic for planet formation theories.

Evaporation of extrasolar planets

astro-ph — Tue, 24 Jan 2012 01:47:34 +0000

This article presents a review on the observations and theoretical modeling of the evaporation of extrasolar planets. The observations and the resulting constraints on the upper atmosphere (thermosphere and exosphere) of the “hot-Jupiters”. are described. The early observations of the first discovered transiting extrasolar planet, HD209458b, allowed the discovery that this planet has an extended atmosphere of escaping hydrogen. Subsequent observations showed the presence of oxygen and carbon at very high altitude. These observations give unique constraints on the escape rate and mechanism in the atmosphere of hot-Jupiters. The most recent Lyman-alpha HST observations of HD189733b and MgII observations of Wasp-12b allow for the first time comparison of the evaporation from different planets in different environments. Models to quantify the escape rate from the measured occultation depths, and an energy diagram to describe the evaporation state of hot-Jupiters are presented. Using this diagram, it is shown that few already known planets like GJ876d or CoRot-7b could be remnants of formerly giant planets.

Ejection and Capture Dynamics in Restricted Three-Body Encounters

astro-ph — Tue, 24 Jan 2012 01:40:36 +0000

We study the tidal disruption of binaries by a massive point mass (e.g. the black hole at the Galactic center), and we discuss how the ejection and capture preference between unequal-mass binary members depends on which orbit they approach the massive object. We show that the restricted three-body approximation provides a simple and clear description of the dynamics. The orbit of a binary with mass m around a massive object M should be almost parabolic with an eccentricity |1-e| < (m/M)^{1/3} << 1 for a member to be captured, while the other is ejected. Indeed, the energy change of the members obtained for a parabolic orbit can be used to describe non-parabolic cases. If a binary has an encounter velocity much larger than (M/m)^{1/3} times the binary rotation velocity, it would be abruptly disrupted, and the energy change at the encounter can be evaluated in a simple disruption model. We evaluate the probability distributions for the ejection and capture of circular binary members and for the final energies. In principle, for any hyperbolic (elliptic) orbit, the heavier member has more chance to be ejected (captured), because it carries a larger fraction of the orbital energy. However, if the orbital energy is close to zero, the difference between the two members becomes small, and there is practically no ejection and capture preference. The preference becomes significant when the orbital energy is comparable to the typical energy change at the encounter. We discuss its implications to hypervelocity stars and irregular satellites around giant planets.

First sub-millimeter detection of the TWA brown dwarf disc 2MASSW J1207334-393254

astro-ph — Mon, 23 Jan 2012 01:54:38 +0000

We present Herschel/SPIRE observations for the 2MASS1207334-393254 (2M1207) system. Based on radiative transfer modeling of near-infrared to sub-millimeter data, we estimate a disc mass of 3+/-2 M_Jup and an outer disc radius of 50–100 AU for the 2M1207A disc. The relative disc mass for 2M1207A is similar to the T Tauri star TW Hya, which indicates that massive discs are not underabundant around sub-stellar objects. In probing the various formation mechanisms for this system, we find that core accretion is highly uncertain mainly due to the large separation between the primary and the companion. Disk fragmentation could be a likely scenario based on analytical models, and if the disc initially was more massive than its current estimate. Considering that the TWA is sparsely populated, this system could have formed via one of the known binary formation mechanisms (e.g. turbulent fragmentation of a core) and survived disruption at an early stage.

The Relation between Ion Temperature Anisotropy and Formation of Slow Shocks in Collisionless Magnetic Reconnection

astro-ph — Mon, 23 Jan 2012 01:50:55 +0000

We perform a two-dimensional simulation by using an electromagnetic hybrid code to study the formation of slow-mode shocks in collisionless magnetic reconnection in low beta plasmas, and we focus on the relation between the formation of slow shocks and the ion temperature anisotropy enhanced at the shock downstream region. It is known that as magnetic reconnection develops, the parallel temperature along the magnetic field becomes large in association with the anisotropic PSBL (plasma sheet boundary layer) ion beams, and this temperature anisotropy has a tendency to suppress the formation of slow shocks. Based on our simulation result, we found that the slow shock formation is suppressed due to the large temperature anisotropy near the X-type region, but the ion temperature anisotropy relaxes with increasing the distance from the magnetic neutral point. As a result, two pairs of current structures, which are the strong evidence of dissipation of magnetic field in slow shocks, are formed at the distance x > 115 ion inertial lengths from the neutral point.

Planetesimal formation by sweep-up: How the bouncing barrier can be beneficial to growth

astro-ph — Mon, 23 Jan 2012 01:48:48 +0000

The formation of planetesimals is often accredited to collisional sticking of dust grains. The exact process is unknown, as collisions between larger aggregates tend to lead to fragmentation or bouncing rather than sticking. Recent laboratory experiments have however made great progress in the understanding and mapping of the complex physics involved in dust collisions. We want to study the possibility of planetesimal formation using the results from the latest laboratory experiments, particularly by including the fragmentation with mass transfer effect, which might lead to growth even at high impact velocities. We present a new experimentally and physically motivated dust collision model capable of predicting the outcome of a collision between two particles of arbitrary masses and velocities. It is used together with a continuum dust-size evolution code that is both fast in terms of execution time and able to resolve the dust well at all sizes, allowing for all types of interactions to be studied without biases. We find that for the general dust population, bouncing collisions prevent the growth above millimeter-sizes. However, if a small number of cm-sized particles are introduced, for example due to vertical mixing or radial drift, they can act as a catalyst and start to sweep up the smaller particles. At a distance of 3 AU, 100-meter-sized bodies are formed on a timescale of 1 Myr. We conclude that direct growth of planetesimals might be a possibility thanks to a combination of the existence of a bouncing barrier and the fragmentation with mass transfer effect. The bouncing barrier is here even beneficial, as it prevents the growth of too many large particles that would otherwise only fragment among each other, and creates a reservoir of small particles that can be swept up by larger bodies. However, for this process to work, a few seeds of cm in size or larger have to be introduced.

Ammonia and other parent molecules in comet 10P/Tempel 2 from Herschel/HIFI and ground-based radio observations

astro-ph — Mon, 23 Jan 2012 01:47:51 +0000

The Jupiter-family comet 10P/Tempel 2 was observed during its 2010 return with the Herschel Space Observatory. We present here the observation of the (J, K) = (1, 0)-(0, 0) transition of ammonia at 572 GHz in this comet with the Heterodyne Instrument for the Far Infrared (HIFI) of Herschel. We also report on radio observations of other molecules (HCN, CH3OH, H2S and CS) obtained during the 1999 return of the comet with the CSO telescope and the JCMT, and during its 2010 return with the IRAM 30-m telescope. Molecular abundances relative to water are 0.09%, 1.8%, 0.4%, and 0.08% for HCN, CH3OH, H2S, and CS, respectively. An abundance of 0.5% for NH3 is obtained, which is similar to the values measured in other comets. The hyperfine structure of the ammonia line is resolved for the first time in an astronomical source. Strong anisotropy in the outgassing is present in all observations from 1999 to 2010 and is modelled to derive the production rates.

An Aladin-based search for proper-motion companions to young stars in the Local Association, Tucana-Horologium and beta Pictoris

astro-ph — Mon, 23 Jan 2012 01:46:32 +0000

We have used the Aladin sky atlas of the Virtual Observatory to look for new common proper-motion pairs in three young stellar kinematic groups: Local Association (~10-120 Ma), Tucana-Horologium (~30 Ma) and ? Pictoris (~12 Ma). We have found 9 new and 14 known common proper-motion companions to the 210 investigated stars. With the CAFOS instrument at the 2.2m Calar Alto telescope, we have investigated in detail one of the new pairs, the HD 143809 AB system, which is formed by a bright G0V primary star and a previously unknown young M1.0-1.5Ve star.

The physics of wind-blown sand and dust

astro-ph — Mon, 23 Jan 2012 01:45:57 +0000

The transport of dust and sand by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This article presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.

Spectroscopy across the brown dwarf/planetary mass boundary – I. Near-infrared JHK spectra

astro-ph — Fri, 20 Jan 2012 02:02:43 +0000

With a uniform VLT SINFONI data set of nine targets, we have developed an empirical grid of J,H,K spectra of the atmospheres of objects estimated to have very low substellar masses of \sim5-20 MJup and young ages of \sim1-50 Myr. Most of the targets are companions, objects which are especially valuable for comparison with atmosphere and evolutionary models, as they present rare cases in which the age is accurately known from the primary. Based on the sample youth, all objects are expected to have low surface gravity, and this study investigates the critical early phases of the evolution of substellar objects. The spectra are compared with grids of five different theoretical atmosphere models. This analysis represents the first systematic model comparison with infrared spectra of young brown dwarfs. The fits to the full JHK spectra of each object result in a range of best fit effective temperatures of +/-150-300K whether or not the full model grid or a subset restricted to lower log(g) values is used. This effective temperature range is significantly larger than the uncertainty typically assigned when using a single model grid. Fits to a single wavelength band can vary by up to 1000K using the different models. Since the overall shape of these spectra is governed more by the temperature than surface gravity, unconstrained model fits did not find matches with low surface gravity or a trend in log(g) with age. This suggests that empirical comparison with spectra of unambiguously young objects targets (such as these SINFONI data) may be the most reliable method to search for indications of low surface gravity and youth. For two targets, the SINFONI data are a second epoch and the data show no variations in morphology over time. The analysis of two other targets, AB Pic B and CT Cha B, suggests that these objects may have lower temperatures, and consequently lower masses, than previously estimated.

Planetesimal and Protoplanet Dynamics in a Turbulent Protoplanetary Disk: Ideal Stratified Disks [Replacement]

astro-ph — Fri, 20 Jan 2012 01:41:35 +0000

Due to the gravitational influence of density fluctuations driven by magneto-rotational instability in the gas disk, planetesimals and protoplanets undergo diffusive radial migration as well as changes in other orbital properties. The magnitude of the effect on particle orbits can have important consequences for planet formation scenarios. We use the local-shearing-box approximation to simulate an ideal, isothermal, magnetized gas disk with vertical density stratification and simultaneously evolve numerous massless particles moving under the gravitational field of the gas and the host star. We measure the evolution of the particle orbital properties, including mean radius, eccentricity, inclination, and velocity dispersion, and its dependence on the disk properties and the particle initial conditions. Although the results converge with resolution for fixed box dimensions, we find the response of the particles to the gravity of the turbulent gas correlates with the horizontal box size, up to 16 disk scale heights. This correlation indicates that caution should be exercised when interpreting local-shearing-box models involving gravitational physics of magneto-rotational turbulence. Based on heuristic arguments, nevertheless, the criterion L_h / R ~ O(1), where L_h is the horizontal box size and R is the distance to the host star, is proposed to possibly circumvent this conundrum. If this criterion holds, we can still conclude that magneto-rotational turbulence seems likely to be ineffective at driving either diffusive migration or collisional erosion under most circumstances.

Day and night side core cooling of a strongly irradiated giant planet [Replacement]

astro-ph — Fri, 20 Jan 2012 01:36:37 +0000

The internal heat loss, or cooling, of a planet determines its structure and evolution. We study the effects of irradiation, metallicity of the atmosphere, heat redistribution, stratospheres, and the depth where the heat redistribution takes place on the atmospheric structure, the core entropy, and subsequently on the cooling of the interior of the planet. We address in a consistent fashion the coupling between the day and the night sides of a planet by means of model atmosphere calculations with heat redistribution. We assume that strong convection leads to the same entropy on the day and night sides and that gravity is the same on both hemispheres. We argue that the core cooling rates from the two hemispheres of a strongly irradiated planet may not be the same and that the difference depends on several important parameters. If the day-night heat redistribution is very efficient or if it takes place at the large optical depth, then the day-side and the night-side cooling may be comparable. However, if the day-night heat transport is not efficient or if it takes place at a shallow optical depth then there can be a large difference between the day- and the night-side cooling and the night side will cool more efficiently. If stellar irradiation becomes stronger, e.g. owing to stellar evolution or migration, cooling from both the day and the night sides is reduced. Enhanced metallicity of the atmosphere would act as an added “blanket” and reduces both the day- and the night-side cooling. However, a stratosphere on the planetary day side can enhance day-side cooling since its opacity acts as a “sunshade” that screens the stellar irradiation. These effects may also influence the well-known gravity darkening and bolometric albedo effects in interacting binaries, especially for strongly irradiated cold components.

Detection of Potential Transit Signals in the First Three Quarters of Kepler Mission Data [Replacement]

astro-ph — Fri, 20 Jan 2012 01:33:24 +0000

We present the results of a search for potential transit signals in the first three quarters of photometry data acquired by the Kepler Mission. The targets of the search include 151,722 stars which were observed over the full interval and an additional 19,132 stars which were observed for only 1 or 2 quarters. From this set of targets we find a total of 5,392 detections which meet the Kepler detection criteria: those criteria are periodicity of the signal, an acceptable signal-to-noise ratio, and a composition test which rejects spurious detections which contain non-physical combinations of events. The detected signals are dominated by events with relatively low signal-to-noise ratio and by events with relatively short periods. The distribution of estimated transit depths appears to peak in the range between 40 and 100 parts per million, with a few detections down to fewer than 10 parts per million. The detected signals are compared to a set of known transit events in the Kepler field of view which were derived by a different method using a longer data interval; the comparison shows that the current search correctly identified 88.1% of the known events. A tabulation of the detected transit signals, examples which illustrate the analysis and detection process, a discussion of future plans and open, potentially fruitful, areas of further research are included.