Vox Charta » Solar and Stellar

3-D radiative transfer in clumped hot star winds I. Influence of clumping on the resonance line formation

astro-ph — Thu, 23 Feb 2012 01:58:00 +0000

The true mass-loss rates from massive stars are important for many branches of astrophysics. For the correct modeling of the resonance lines, which are among the key diagnostics of stellar mass-loss, the stellar wind clumping turned out to be very important. In order to incorporate clumping into radiative transfer calculation, 3-D models are required. Various properties of the clumps may have strong impact on the resonance line formation and, therefore, on the determination of empirical mass-loss rates. We incorporate the 3-D nature of the stellar wind clumping into radiative transfer calculations and investigate how different model parameters influence the resonance line formation. We develop a full 3-D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. The number density of clumps follows the mass conservation. For the first time, realistic 3-D models that describe the dense as well as the tenuous wind components are used to model the formation of resonance lines in a clumped stellar wind. At the same time, non-monotonic velocity fields are accounted for. The 3-D density and velocity wind inhomogeneities show very strong impact on the resonance line formation. The different parameters describing the clumping and the velocity field results in different line strengths and profiles. We present a set of representative models for various sets of model parameters and investigate how the resonance lines are affected. Our 3-D models show that the line opacity is reduced for larger clump separation and for more shallow velocity gradients within the clumps. Our new model demonstrates that to obtain empirically correct mass-loss rates from the UV resonance lines, the wind clumping and its 3-D nature must be taken into account.

Multiple outflows from the luminous YSO IRAS 07422-2001

astro-ph — Thu, 23 Feb 2012 01:55:39 +0000

The luminous Young Stellar Object (YSO) IRAS 07422-2001 is studied in the infrared. We discover star forming activity in embedded clusters located in a cloud detected at mid-IR wavelengths in emission. Multiple outflows are discovered from these clusters in the H_2 ro-vibrational line at 2.122 micron. We detect at least six outflows from the cluster associated with the IRAS source and another outflow from a source located in a cluster detected ~2.7 arcmin NE of the IRAS source. Additional star formation is taking place in two other cluster candidates within the cloud. Three of the YSOs in the cluster associated with the IRAS source are detected at 11.2 micron at an angular resolution of ~0.8 arcsec. We have a tentative detection of a circumstellar disk in this cluster, seen as an extinction lane in the J and H-band images. The spectral energy distributions (SEDs) of the dominant YSOs in the cluster associated with the IRAS source and in the NE cluster are studied using radiative transfer models and the properties of the YSOs are estimated. The YSO associated with the IRAS source is probably in a very early Class I stage of formation. The source identified as the dominant YSO in the NE cluster appears to be older than the dominant YSO in the cluster associated with the IRAS source, but its observed flux seems to be contaminated by extra emission, which suggests the presence of a young source contributing to the SED at far-IR wavelengths. The star formation observed in the field of IRAS 07422-2001 supports the idea of hierarchical formation of massive star clusters and the growth of massive young stellar objects near the centres of multiple sub-clusters in a star forming clump through competitive accretion.

First Science Observations with SOFIA/FORCAST: 6 TO 37 micron Imaging of Orion BN/KL

astro-ph — Thu, 23 Feb 2012 01:55:12 +0000

The BN/KL region of the Orion Nebula is the nearest region of high mass star formation in our galaxy. As such, it has been the subject of intense investigation at a variety of wavelengths, which have revealed it to be brightest in the infrared to sub-mm wavelength regime. Using the newly commissioned SOFIA airborne telescope and its 5-40 micron camera FORCAST, images of the entire BN/KL complex have been acquired. The 31.5 and 37.1 micron images represent the highest resolution observations (31.5 microns, and that this distinction goes instead to the source IRc4. It was determined from these images and derived dust color temperature maps that IRc4 is also likely to be self-luminous. A new source of emission has also been identified at wavelengths >31.5 microns that coincides with the northeastern outflow lobe from the protostellar disk associated with radio source I.

The Proposed Quadruple System SZ Herculis: Revised LITE Model and Orbital Stability Study

astro-ph — Thu, 23 Feb 2012 01:54:02 +0000

In a recent study, Lee et al. presented new photometric follow-up timing observations of the semi-detached binary system SZ Herculis and proposed the existence of two hierarchical cirumbinary companions. Based on the light-travel time effect, the two low-mass M-dwarf companions are found to orbit the binary pair on moderate to high eccentric orbits. The derived periods of these two companions are close to a 2:1 mean-motion orbital resonance. We have studied the stability of the system using the osculating orbital elements as presented by Lee et al. Results indicate an orbit-crossing architecture exhibiting short-term dynamical instabilities leading to the escape of one of the proposed companions. We have examined the system’s underlying model parameter-space by following a Monte Carlo approach and found an improved fit to the timing data. A study of the stability of our best-fitting orbits also indicates that the proposed system is generally unstable. If the observed anomalous timing variations of the binary period is due to additional circumbinary companions, then the resulting system should exhibit a long-term stable orbital configuration much different from the orbits suggested by Lee et al. We, therefore, suggest that based on Newtonian-dynamical considerations, the proposed quadruple system cannot exist. To uncover the true nature of the observed period variations of this system, we recommend future photometric follow-up observations that could further constrain eclipse-timing variations and/or refine light-travel time models.

The Origin of the EUV Late Phase: A Case Study of the C8.8 Flare on 2010 May 5

astro-ph — Thu, 23 Feb 2012 01:53:47 +0000

Since the launch of NASA’s Solar Dynamics Observatory on 2010 February 11, the Extreme ultraviolet Variability Experiment (EVE) has observed numerous flares. One interesting feature observed by EVE is that a subset of flares exhibit an additional enhancement of the 2-3 million K emission several hours after the flare’s soft X-ray emission. From the Atmospheric Imaging Assembly (AIA) images, we observe that this secondary emission, dubbed the EUV late phase, occurs in the same active region as the flare but not in the same coronal loops. Here, we examine the C8.8 flare that occurred on 2010 May 5 as a case study of EUV late phase flares. In addition to presenting detailed observations from both AIA and EVE, we develop a physical model of this flare and test it using the Enthalpy Based Thermal Evolution of Loops (EBTEL) model.

The Unusual Temporal and Spectral Evolution of the Type IIn Supernova 2011ht

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

We present very early UV to optical photometric and spectroscopic observations of the peculiar Type IIn supernova (SN) 2011ht in UGC 5460. The UV observations of the rise to peak are only the second ever recorded for a Type IIn SN and are by far the most complete. The SN, first classified as a SN impostor, slowly rose to a peak of M_V \sim -17 in \sim55 days. In contrast to the \sim2 magnitude increase in the v-band light curve from the first observation until peak, the UV flux increased by >7 magnitudes. The optical spectra are dominated by strong, Balmer emission with narrow peaks (FWHM\sim600 km/s), very broad asymmetric wings (FWHM\sim4200 km/s), and blue shifted absorption (\sim300 km/s) superposed on a strong blue continuum. The UV spectra are dominated by FeII, MgII, SiII, and SiIII absorption lines broadened by \sim1500 km/s. Merged X-ray observations reveal a L_(0.2-10)=(1.0+/-0.2)x10^(39) erg/s. Some properties of SN 2011ht are similar to SN impostors, while others are comparable to Type IIn SNe. Early spectra showed features typical of luminous blue variables at maximum and during giant eruptions. However, the broad emission profiles coupled with the strong UV flux have not been observed in previous SN impostors. The absolute magnitude and energetics (~2.5×10^(49) ergs in the first 112 days) are reminiscent of normal Type IIn SN, but the spectra are of a dense wind. We suggest that the mechanism for creating this unusual profile could be a shock interacting with a shell of material that was ejected a year before the discovery of the SN.

Absolute dimensions of solar-type eclipsing binaries. EF Aquarii: a G0 test for stellar evolution models

astro-ph — Thu, 23 Feb 2012 01:52:41 +0000

Recent studies have shown that stellar chromospheric activity, and its effect on convective energy transport in the envelope, is most likely the cause of significant radius and temperature discrepancies between theoretical evolution models and observations. We aim to determine absolute dimensions and abundances for the solar-type detached eclipsing binary EF Aqr, and to perform a detailed comparison with results from recent stellar evolutionary models. uvby-beta standard photometry was obtained with the Stromgren Automatic Telescope. The broadening function formalism was applied on spectra observed with HERMES at the Mercator telescope in La Palma, to obtain radial velocity curves. Masses and radii with a precision of 0.6% and 1.0% respectively have been established for both components of EF Aqr. The active 0.956 M_sol secondary shows star spots and strong Ca II H and K emission lines. The 1.224 M_sol primary shows signs of activity as well, but at a lower level. An [Fe/H] abundance of 0.00+-0.10 is derived with similar abundances for Si, Ca, Sc, Ti, V, Cr, Co, and Ni. Solar calibrated evolutionary models such as Yonsei-Yale, Victoria-Regina and BaSTI isochrones and evolutionary tracks are unable to reproduce EF Aqr, especially for the secondary, which is 9% larger and 400 K cooler than predicted. Models adopting significantly lower mixing length parameters l/H_p remove these discrepancies, as seen in other solar type binaries. For the observed metallicity, Granada models with a mixing length of l/H_p=1.30 (primary) and 1.05 (secondary) reproduce both components at a common age of 1.5+-0.6 Gyr. Observations of EF Aqr suggests that magnetic activity, and its effect on envelope convection, is likely to be the cause of discrepancies in both radius and temperature, which can be removed by adjusting the mixing length parameter of the models downwards.

The Pisa Stellar Evolution Data Base for low-mass stars

astro-ph — Thu, 23 Feb 2012 01:52:10 +0000

The last decade showed an impressive observational effort from the photometric and spectroscopic point of view for ancient stellar clusters in our Galaxy and beyond. The theoretical interpretation of these new observational results requires updated evolutionary models and isochrones spanning a wide range of chemical composition. With this aim we built the new “Pisa Stellar Evolution Database” of stellar models and isochrones by adopting a well-tested evolutionary code (FRANEC) implemented with updated physical and chemical inputs. In particular, our code adopts realistic atmosphere models and an updated equation of state, nuclear reaction rates and opacities calculated with recent solar elements mixture. A total of 32646 models have been computed in the range of initial masses 0.30 – 1.10 Msun for a grid of 216 chemical compositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42. Models were computed for both solar-scaled and alpha-enhanced abundances with different external convection efficiencies. Correspondingly, 9720 isochrones were computed in the age range 8 – 15 Gyr, in time steps of 0.5 Gyr. The whole database is available to the scientific community on the web. Models and isochrones were compared with recent calculations available in the literature and with the color-magnitude diagram of selected Galactic globular clusters. The dependence of relevant evolutionary quantities on the chemical composition and convection efficiency were analyzed in a quantitative statistical way and analytical formulations were made available for reader’s convenience.

Sympathetic Partial and Full Filament Eruptions Observed in One Solar Breakout Event

astro-ph — Thu, 23 Feb 2012 01:51:54 +0000

We report two sympathetic solar eruptions, including a partial and a full flux rope eruption in a quadrupolar magnetic region, where a large and a small filament resided above the middle and the east neutral lines respectively. The large filament first rose slowly at a speed of 8 km/s for 23 minutes and then it was accelerated to 102 km/s. Finally, this filament erupted successfully and caused a coronal mass ejection. During the slow rising phase, various evidence for breakout-like external reconnection has been identified at high and low temperature lines. The eruption of the small filament started around the end of the large filament’s slow rising. This filament erupted partially and no associating coronal mass ejection could be detected. Based on a potential field extrapolation, we find that the topology of the three-dimensional coronal field above the source region is composed of three low-lying lobes and a large overlying flux system, and a null point located between the middle lobe and the overlying antiparallel flux system. We propose a possible mechanism within the framework of the magnetic breakout model to interpret the sympathetic filament eruptions, in which the magnetic implosion mechanism is thought to be a possible linkage between the sympathetic eruptions, and the external reconnection at the null point transfers field lines from the middle lobe to the lateral lobes and thereby leads to the full (partial) eruption of the observed large (small) filament. Other possible mechanisms are also discussed briefly. We conclude that the structural properties of coronal fields are important for producing sympathetic eruptions.

The fight for accretion: discovery of intermittent mass transfer in BB Doradus in the low state

astro-ph — Thu, 23 Feb 2012 01:47:24 +0000

Our long-term photometric monitoring of southern nova-like cataclysmic variables with the 1.3-m SMARTS telescope found BB Doradus fading from V ~ 14.3 towards a deep low state at V ~ 19.3 in April 2008. Here we present time-resolved optical spectroscopy of BB Dor in this faint state in 2009. The optical spectrum in quiescence is a composite of a hot white dwarf with Teff = 30000 +- 5000 K and a M3-4 secondary star with narrow emission lines (mainly of the Balmer series and HeI) superposed. We associate these narrow profiles with an origin on the donor star. Analysis of the radial velocity curve of the H-alpha emission from the donor star allowed the measurement of an orbital period of 0.154095 +- 0.000003 d (3.69828 +- 0.00007 h), different from all previous estimates. We detected episodic accretion events which veiled the spectra of both stars and radically changed the line profiles within a timescale of tens of minutes. This shows that accretion is not completely quenched in the low state. During these accretion episodes the line wings are stronger and their radial velocity curve is delayed by ~ 0.2 cycle, similar to that observed in SW Sex and AM Her stars in the high state, with respect to the motion of the white dwarf. Two scenarios are proposed to explain the extra emission: impact of the material on the outer edge of a cold, remnant accretion disc, or the combined action of a moderately magnetic white dwarf (B1 <~ 5 MG) and the magnetic activity of the donor star.

A Note on the Variability of V538 Cassiopeiae

astro-ph — Thu, 23 Feb 2012 01:47:00 +0000

CCD observations of V538 Cas have been made on nine nights during three weeks using the AAVSO Bright Star Monitor. No significant variations were found.

The Sun in Hidden Photons [Cross-Listing]

astro-ph — Thu, 23 Feb 2012 01:45:32 +0000

We present some aspects and first results of the emission of sub-eV mass hidden photons from the Sun. The contribution from a resonant region below the photosphere can be quite significant, raising previous estimates. This is relevant for the Telescope for Hidden Photon Search, TSHIPS I, currently targeting at meV-mass hidden photons with O(10^-6) kinetic mixing with the photon. These particles could account for the large effective number of neutrinos pointed at by the cosmic microwave background and other large-scale structure probes, and are motivated in some scenarios of string theory.

On the formation of glycolaldehyde in dense molecular cores

astro-ph — Thu, 23 Feb 2012 01:44:27 +0000

Glycolaldehyde is a simple monosaccharide sugar linked to prebiotic chemistry. Recently it was detected in a molecular core in the star-forming region G31.41+0.31 at a reasonably high abundance. We investigate the formation of glycolaldehyde at 10K to determine whether it can form efficiently under typical dense core conditions. Using an astrochemical model, we test five different reaction mechanisms that have been proposed in the astrophysical literature, finding that a gas-phase formation route is unlikely. Of the grain-surface formation routes, only two are efficient enough at very low temperatures to produce sufficient glycolaldehyde to match the observational estimates, with the mechanism culminating in CH3OH + HCO being favoured. However, when we consider the feasibility of these mechanisms from a reaction chemistry perspective, the second grain-surface route looks more promising, H3CO + HCO.

The rotation of field stars from CoRoT data

astro-ph — Thu, 23 Feb 2012 01:44:11 +0000

We present period measurements of a large sample of field stars in the solar neighbourhood, observed by CoRoT in two different directions of the Galaxy. The presence of a period was detected using the Scargle Lomb Normalized Periodogram technique and the autocorrelation analysis. The assessment of the results has been performed through a consistency verification supported by the folded light curve analysis. The data analysis procedure has discarded a non-negligible fraction of light curves due to instrumental artifacts, however it has allowed us to identify pulsators and binaries among a large number of field stars. We measure a wide range of periods, from 0.25 to 100 days, most of which are rotation periods. The final catalogue includes 1978 periods, with 1727 of them identified as rotational periods, 169 are classified as pulsations and 82 as orbital periods of binary systems. Our sample suffers from selection biases not easily corrected for, thus we do not use the distribution of rotation periods to derive the age distribution of the main-sequence population. Nevertheless, using rotation as a proxy for age, we can identify a sample of young stars (< 600 Myr), that will constitute a valuable sample, supported by further spectroscopic observations, to study the recent star formation history in the solar neighborhood.

Very Low-Mass Stellar and Substellar Companions to Solar-Like Stars from MARVELS I: A Low Mass Ratio Stellar Companion to TYC 4110-01037-1 in a 79-day Orbit

astro-ph — Thu, 23 Feb 2012 01:43:56 +0000

TYC 4110-01037-1 has a low-mass stellar companion, whose small mass ratio and short orbital period are atypical amongst solar-like (Teff ~< 6000 K) binary systems. Our analysis of TYC 4110-01037-1 reveals it to be a moderately aged (~0.087 +/- 0.003, places it at the lowest end of observed values for short period stellar companions to solar-like (Teff ~< 6000 K) stars. One possible way to create such a system would be if a triple-component stellar multiple broke up into a short period, low q binary during the cluster dispersal phase of its lifetime. A candidate tertiary body has been identified in the system via single-epoch, high contrast imagery. If this object is confirmed to be co-moving, we estimate it would be a dM4 star. We present these results in the context of our larger-scale effort to constrain the statistics of low mass stellar and brown dwarf companions to FGK-type stars via the MARVELS survey.

Break up of returning plasma after the 7 June 2011 filament eruption by Rayleigh-Taylor instabilities

astro-ph — Thu, 23 Feb 2012 01:41:34 +0000

A prominence eruption on 7 June 2011 produced spectacular curtains of plasma falling through the lower corona. At the solar surface they created an incredible display of extreme ultraviolet brightenings. The aim is to identify and analyze some of the local instabilities which produce structure in the falling plasma. The structures were investigated using SDO/AIA 171A and 193A images in which the falling plasma appeared dark against the bright coronal emission. Several instances of the Rayleigh-Taylor instability were investigated. In two cases the Alfven velocity associated with the dense plasma could be estimated from the separation of the Rayleigh-Taylor fingers. A second type of feature, which has the appearance of self-similar branching horns, is also discussed.

Negative effective magnetic pressure in turbulent convection [Replacement]

astro-ph — Thu, 23 Feb 2012 01:39:07 +0000

We investigate the effects of weakly and strongly stratified turbulent convection on the mean effective Lorentz force, and especially on the mean effective magnetic pressure. Earlier studies with isotropically forced non-stratified and stratified turbulence have shown that the contribution of the turbulence to the mean magnetic pressure is negative for mean horizontal magnetic fields that are smaller than the equipartition strength, so that the effective mean magnetic pressure that takes into account the turbulence effects, can be negative. Compared with earlier cases of forced turbulence with an isothermal equation of state, we find that the turbulence effect is similar to or even stronger in the present case of turbulent convection. This is argued to be due to the anisotropy of turbulence in the vertical direction. Another important difference compared with earlier studies is the presence of an evolution equation for the specific entropy. Mean-field modelling with entropy evolution indicates that the negative effective magnetic pressure can still lead to a large-scale instability which forms local flux concentrations, even though the specific entropy evolution tends to have a stabilizing effect when applied to a stably stratified (e.g., isothermal) layer. It is argued that this large-scale instability could be important for the formation of solar large-scale magnetic structures such as active regions.

Spindown of Isolated Neutron Stars: Gravitational Waves or Magnetic Braking? [Replacement]

astro-ph — Thu, 23 Feb 2012 01:38:53 +0000

We study the spindown of isolated neutron stars from initially rapid rotation rates, driven by two factors: (i) gravitational wave emission due to r-modes and (ii) magnetic braking. In the context of isolated neutron stars, we present the first study including self-consistently the magnetic damping of r-modes in the spin evolution. We track the spin evolution employing the RNS code, which accounts for the rotating structure of neutron stars for various equations of state. We find that, despite the strong damping due to the magnetic field, r-modes alter the braking rate from pure magnetic braking for B<10^{13}G. For realistic values of the saturation amplitude, the r-mode can also decrease the time to reach the threshold central density for quark deconfinement. Within a phenomenological model, we assess the gravitational waveform that would result from r-mode driven spindown of a magnetized neutron star. To contrast with the persistent signal during the spindown phase, we also present a preliminary estimate of the transient gravitational wave signal from an explosive quark-hadron phase transition, which can be a signal for the deconfinement of quarks inside neutron stars.

Mean-field transport in stratified and/or rotating turbulence [Replacement]

astro-ph — Thu, 23 Feb 2012 01:38:24 +0000

We investigate the mean electromotive force in the kinematic framework, that is, ignoring the back-reaction of the magnetic field on the fluid velocity, under the assumption of axisymmetric turbulence determined by the presence of either rotation, density stratification, or both. We use an analogous approach for the mean passive scalar flux. As an alternative to convection, we consider forced turbulence in an isothermal layer. When using standard ansatzes, the mean magnetic transport is then determined by nine, and the mean passive scalar transport by four coefficients. We give results for all these transport coefficients. We use the test-field method and the test-scalar method, where transport coefficients are determined by solving sets of equations with properly chosen mean magnetic fields or mean scalars. These methods are adapted to mean fields which may depend on all three space coordinates. We find the anisotropy of turbulent diffusion to be moderate in spite of rapid rotation or strong density stratification. Contributions to the mean electromotive force determined by the symmetric part of the gradient tensor of the mean magnetic field, which were ignored in several earlier investigations, turn out to be important. In stratified rotating turbulence, the $\alpha$ effect is strongly anisotropic, suppressed along the rotation axis on large length scales, but strongly enhanced at intermediate length scales. Also the $\OO\times\meanJJ$ effect is enhanced at intermediate length scales. The turbulent passive scalar diffusivity is typically almost twice as large as the turbulent magnetic diffusivity. Both magnetic and passive scalar diffusion are slightly enhanced along the rotation axis, but decreased if there is gravity.

Possible optical counterpart of PSR J1357-6429 [Replacement]

astro-ph — Thu, 23 Feb 2012 01:34:15 +0000

(Abridged) PSR J1357-6429 is a Vela-like radio pulsar that has been recently detected in X-rays and gamma-rays. It powers a compact tail-like X-ray pulsar wind nebula and X-ray-radio plerion associated with an extended TeV source HESS J1356-645. We present our deep optical observations with the Very Large Telescope to search for an optical counterpart of the pulsar and its nebula. We detect a point-like source in V, R, and I bands whose position is within the 1-sigma error circle of the X-ray position of the pulsar, and whose colours are distinct from those of ordinary stars. We consider it as a candidate optical counterpart of the pulsar. If it is indeed the counterpart, its 5-sigma offset from the radio pulsar position, measured about 9 yr earlier, implies that the transverse velocity of the pulsar is in the range of 1600–2000 km s^{-1} at the distance of 2–2.5 kpc, making it the fastest moving pulsar known. The direction of the estimated proper motion coincides with the extension of the pulsar’s X-ray tail, suggesting that this is a jet. The tentative optical luminosity and efficiency of the pulsar are similar to those of the Vela pulsar, which also supports the optical identification. However, the candidate undergoes an unusually steep dereddened flux increase towards the infrared with a spectral index of about 5, that is not typical of optical pulsars. It implies a strong double-knee spectral break in the pulsar emission between the optical and X-rays. The reasons for the spectral steepness are unclear. It may be caused by a nebula knot projected onto the jet and strongly overlapping with the pulsar, as observed for the Crab, where the knot has a significantly steeper spectrum than the pulsar. We find no other signs of the pulsar nebula in the optical. Alternatively, the detected source may be a faint AGN, that has not yet been seen at other wavelengths.

Critical Conditions for Core-Collapse Supernovae [Replacement]

astro-ph — Thu, 23 Feb 2012 01:33:41 +0000

The explosion of a core-collapse supernova is approximated by the breakdown of the steady-state solution for accretion onto a proto-neutron-star (PNS). We analytically show that as the neutrino luminosity exceeds a critical value L_c, the neutrinosphere pressure exceeds the hydrostatic limit even for an optimal shock radius R. This yields L_c \propto M^2 T^2 (with logarithmic corrections) and R \propto M/T, in agreement with numerical results, where M, T are the PNS mass, neutrino temperature. The near-critical flow can be approximated as a ballistic shell on top of an isothermal layer.

Mergers and ejections of black holes in globular clusters [Replacement]

astro-ph — Thu, 23 Feb 2012 01:31:55 +0000

We report on results of fully consistent N-body simulations of globular cluster models with N = 100 000 members containing neutron stars and black holes. Using the improved `algorithmic regularization’ method of Hellstrom and Mikkola for compact subsystems, the new code NBODY7 enables for the first time general relativistic coalescence to be achieved for post-Newtonian terms and realistic parameters. Following an early stage of mass segregation, a few black holes form a small dense core which usually leads to the formation of one dominant binary. The subsequent evolution by dynamical shrinkage involves the competing processes of ejection and mergers by radiation energy loss. Unless the binary is ejected, long-lived triple systems often exhibit Kozai cycles with extremely high inner eccentricity (e > 0.999) which may terminate in coalescence at a few Schwarzschild radii. A characteristic feature is that ordinary stars as well as black holes and even BH binaries are ejected with high velocities. On the basis of the models studied so far, the results suggest a limited growth of a few remaining stellar mass black holes in globular clusters.

Nuclear symmetry energy and the r-mode instability of neutron stars [Cross-Listing]

astro-ph — Thu, 23 Feb 2012 01:31:05 +0000

We analyze the role of the symmetry energy slope parameter $L$ on the {\it r}-mode instability of neutron stars. Our study is performed using the microscopic Brueckner–Hartree–Fock approach of the nuclear equation of state, and several phenomenological Skyrme forces and relativistic mean field models. Our results show that the {\it r}-mode instability region is smaller for those models which give larger values of $L$. The reason is that both bulk ($\xi$) and shear ($\eta$) viscosities increase with $L$ and, therefore, the damping of the mode is more efficient for the models with larger $L$. We show also that the dependence of both viscosities on $L$ can be described at each density by simple power-laws of the type $\xi=A_{\xi}L^{B_\xi}$ and $\eta=A_{\eta}L^{B_\eta}$. Using the measured spin frequency and the estimated core temperature of the pulsar in the low-mass X-ray binary 4U 1608-52, we conclude that observational data seems to favor larger values of $L$ if the radius of this object is in the range $11.5-12$(10-12) km and its mass $1.4M_\odot$($2M_\odot$). Outside this range it is not possible to draw any conclusion on $L$ from this pulsar.

Stellar lifetime and ultraviolet properties of the old metal-rich Galactic open cluster NGC6791: a pathway to understand the UV upturn of elliptical galaxies

astro-ph — Wed, 22 Feb 2012 02:05:55 +0000

The evolutionary properties of the old metal-rich Galactic open cluster NGC6791 are assessed, based on deep UB photometry and 2Mass JK data. For 4739 stars in the cluster, bolometric luminosity and effective temperature have been derived from theoretical (U-B) and (J-K) color fitting. The derived H-R diagram has been matched with the UVBLUE grid of synthetic stellar spectra to obtain the integrated SED of the system, together with a full set UV (Fanelli) and optical (Lick) narrow-band indices. The cluster appears to be a fairly good proxy of standard elliptical galaxies, although with significantly bluer infrared colors, a shallower 4000A Balmer break, and a lower Mg2 index. The confirmed presence of a dozen hot stars, along their EHB evolution, leads the cluster SED to consistently match the properties of the most active UV-upturn galaxies, with 1.7+/-0.4% of the total bolometric luminosity emitted shortward of 2500A. The cluster Helium abundance results Y=0.30 +/-0.04, while the Post-MS implied stellar lifetime from star number counts fairly agrees with the theoretical expectations from both the Padova and BASTI stellar tracks. A Post-MS fuel consumption of 0.43 +/- 0.01 M_sun is found for NGC6791 stars, in close agreement with the estimated mass of cluster He-rich white dwarfs. Such a tight figure may lead to suspect that a fraction of the cluster stellar population does actually not reach the minimum mass required to effectively ignite He in the stellar core.

Rebirth of X-ray Emission from the Born-Again Planetary Nebula A 30

astro-ph — Wed, 22 Feb 2012 02:05:41 +0000

The planetary nebula (PN) A 30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch HST images we have detected the angular expansion of these knots and derived an age of 850+280-150 yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A 30. The X-ray emission from A 30 can be separated into two components: a point-source at the central star and diffuse X-ray emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A 30 using a non-LTE model fit to its optical and UV spectrum. The spatial distribution and spectral properties of the diffuse X-ray emission is highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. Charge-exchange reactions between the ions of the stellar winds and neutral material of the born-again ejecta seem the most likely mechanism for the production of diffuse X-ray emission. Shock-heated plasma may also contribute to this emission, as the hydrogen-poor knots are ablated by the stellar winds, in which case efficient mass-loading of the stellar winds is needed to raise the density and damp the velocity of the stellar winds to reproduce the spectral properties of the diffuse emission. The origin of the X-ray emission from the central star of A 30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible.

Modelling fully convective stars in eclipsing binaries: KOI-126 and CM Draconis

astro-ph — Wed, 22 Feb 2012 02:04:45 +0000

We present models of the components of the systems KOI-126 and CM Draconis, the two eclipsing binary systems known to date to contain stars with masses low enough to have fully convective interiors. We are able to model satisfactorily the system KOI-126, finding consistent solutions for the radii and surface temperatures of all three components, using a solar-like value of the mixing-length parameter \alpha in the convection zone, and PHOENIX NextGen 1D model atmospheres for the surface boundary conditions. Depending on the chemical composition, we estimate the age of the system to be in the range 3-5 Gyr. For CM Draconis, on the other hand, we cannot reconcile our models with the observed radii and T_eff using the current metal-poor composition estimate based on kinematics. Higher metallicities lessen but do not remove the discrepancy. We then explore the effect of varying the mixing length parameter \alpha. As previously noted in the literature, a reduced \alpha can be used as a simple measure of the lower convective efficiency due to rotation and induced magnetic fields. Our models show a sensitivity to \alpha (for \alpha < 1.0) sufficient to partially account for the radius discrepancies. It is, however, impossible to reconcile the models with the observations on the basis of the effect of the reduced \alpha alone. We therefore suggest that the combined effects of high metallicity and \alpha reduction could explain the observations of CM Draconis. For example, increasing the metallicity of the system towards super-solar values (i.e. Z = 2 Z_sun) yields an agreement within 2 \sigma with \alpha = 1.0.

Vsini-s for late-type stars from spectral synthesis in K-band region

astro-ph — Wed, 22 Feb 2012 02:04:25 +0000

We analyse medium-resolution spectra (R\sim 18000) of 19 late type dwarfs in order to determine vsini-s using synthetic rather than observational template spectra. For this purpose observational data around 2.2 $\mu$m of stars with spectral classes from G8V to M9.5V were modelled. We find that the Na I (2.2062 and 2.2090 $\mu$m) and $^{12}$CO 2-0 band features are modelled well enough to use for vsini determination without the need for a suitable observational template spectra. Within the limit of the resolution of our spectra, we use synthetic spectra templates to derive vsini values consistent with those derived in the optical regime using observed templates. We quantify the errors in our vsini determination due to incorrect choice of model parameters \Teff, log $g$, $v_{\rm micro}$, [Fe/H] or FWHM and show that they are typically less than 10 per cent. We note that the spectral resolution of our data(\sim 16 km/s) limited this study to relatively fast rotators and that resolutions of 60000 will required to access most late-type dwarfs.

Magnetic field, differential rotation and activity of the hot-Jupiter hosting star HD 179949

astro-ph — Wed, 22 Feb 2012 02:03:33 +0000

HD 179949 is an F8V star, orbited by a giant planet at ~8 R* every 3.092514 days. The system was reported to undergo episodes of stellar activity enhancement modulated by the orbital period, interpreted as caused by Star-Planet Interactions (SPIs). One possible cause of SPIs is the large-scale magnetic field of the host star in which the close-in giant planet orbits. In this paper we present spectropolarimetric observations of HD 179949 during two observing campaigns (2009 September and 2007 June). We detect a weak large-scale magnetic field of a few Gauss at the surface of the star. The field configuration is mainly poloidal at both observing epochs. The star is found to rotate differentially, with a surface rotation shear of dOmega=0.216\pm0.061 rad/d, corresponding to equatorial and polar rotation periods of 7.62\pm0.07 and 10.3\pm0.8 d respectively. The coronal field estimated by extrapolating the surface maps resembles a dipole tilted at ~70 degrees. We also find that the chromospheric activity of HD 179949 is mainly modulated by the rotation of the star, with two clear maxima per rotation period as expected from a highly tilted magnetosphere. In September 2009, we find that the activity of HD 179949 shows hints of low amplitude fluctuations with a period close to the beat period of the system.

Constraints on the Formation of the Galactic Bulge from Na, Al, and Heavy Element Abundances in Plaut’s Field

astro-ph — Wed, 22 Feb 2012 02:02:49 +0000

We report chemical abundances of Na, Al, Zr, La, Nd, and Eu for 39 red giant branch (RGB) stars and 23 potential inner disk red clump stars located in Plaut-s low extinction window. We also measure lithium for a super Li-rich RGB star. The abundances were determined by spectrum synthesis of high resolution (R~25,000), high signal-to-noise (S/N~50-100 pixel-1) spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. For the bulge RGB stars, we find a general increase in the [Na/Fe] and [Na/Al] ratios with increasing metallicity, and a similar decrease in [La/Fe] and [Nd/Fe]. Additionally, the [Al/Fe] and [Eu/Fe] abundance trends almost identically follow those of the {\alpha}-elements, and the [Zr/Fe] ratios exhibit relatively little change with [Fe/H]. The consistently low [La/Eu] ratios of the RGB stars indicate that at least a majority of bulge stars formed rapidly (<1 Gyr) and before the main s-process could become a significant pollution source. In contrast, we find that the potential inner disk clump stars exhibit abundance patterns more similar to those of the thin and thick disks. Comparisons between the abundance trends at different bulge locations suggest that the inner and outer bulge formed on similar timescales. However, we find evidence of some abundance differences between the most metal-poor and metal-rich stars in various bulge fields. The data also indicate that the halo may have had a more significant impact on the outer bulge initial composition than the inner bulge composition. The [Na/Fe] and to a lesser extent [La/Fe] abundances further indicate that the metal-poor bulge, at least at ~1 kpc from the Galactic center, and thick disk may not share an identical chemistry.

3-D Monte Carlo radiative transfer calculation of resonance line formation in the inhomogeneous expanding stellar wind

astro-ph — Wed, 22 Feb 2012 01:59:38 +0000

We study the effects of optically thick clumps, non-void inter-clump medium, variation of the onset of clumping, and velocity dispersion inside clumps on the formation of resonance lines. For this purpose we developed a full 3-D Monte Carlo Radiative Transfer (MCRT) code that is able to handle 3-D shapes of clumps and arbitrary 3-D velocity fields. The method we developed allows us to take into account contributions from density and velocity wind inhomogeneities to the total opacity very precisely. The first comparison with observation shows that 3-D density and velocity wind inhomogeneities have a very strong influence on the resonance line formation, and that they have to be accounted for in order to obtain reliable mass-loss rate determinations.

Physical Processes of Interstellar Turbulence

astro-ph — Wed, 22 Feb 2012 01:58:54 +0000

I discuss the role of self-gravity and radiative heating and cooling in shaping the nature of the turbulence in the interstellar medium (ISM) of our galaxy. The heating and cooling cause it to be highly compressible, and, in some regimes of density and temperature, to become thermally unstable, tending to spontaneously segregate into warm/diffuse and cold/dense phases. On the other hand, turbulence is an inherently mixing process, tending to replenish the density and temperature ranges that would be forbidden under thermal processes alone. The turbulence in the ionized ISM appears to be transonic (i.e, with Mach numbers $\Ms \sim 1$), and thus to behave essentially incompressibly. However, in the neutral medium, thermal instability causes the sound speed of the gas to fluctuate by up to factors of $\sim 30$, and thus the flow can be highly supersonic with respect to the dense/cold gas, although numerical simulations suggest that this behavior corresponds more to the ensemble of cold clumps than to the clumps’ internal velocity dispersion. Finally, coherent large-scale compressions in the warm neutral medium (induced by, say, the passage of spiral arms or by supernova shock waves) can produce large, dense molecular clouds that are subject to their own self-gravity, and begin to contract gravitationally. Because they are populated by nonlinear density fluctuations, whose local free-fall times are significantly smaller than that of the whole cloud, the fluctuations terminate their collapse earlier, giving rise to a regime of hierarchical gravitational fragmentation, with small-scale collapses occurring within larger-scale ones. Thus, the “turbulence” in molecular clouds may be dominated by a gravitationally contracting component at all scales.

Revision of Solar Spicule Classification

astro-ph — Wed, 22 Feb 2012 01:56:49 +0000

Solar spicules are the fundamental magnetic structures in the chromosphere and considered to play a key role in channelling the chromosphere and corona. Recently, it was suggested by De Pontieu et al. that there were two types of spicules with very different dynamic properties, which were detected by space- time plot technique in the Ca ii H line (3968 A) wavelength from Hinode/SOT observations. ‘Type I’ spicule, with a 3-7 minute lifetime, undergoes a cycle of upward and downward motion; in contrast, ‘Type II’ spicule fades away within dozens of seconds, without descending phase. We are motivated by the fact that for a spicule with complicated 3D motion, the space-time plot, which is made through a slit on a fixed position, could not match the spicule behavior all the time and might lose its real life story. By revisiting the same data sets, we identify and trace 105 and 102 spicules in quiet sun (QS) and coronal hole (CH), respectively, and obtain their statistical dynamic properties. First, we have not found a single convincing example of ‘Type II’ spicules. Secondly, more than 60% of the identified spicules in each region show a complete cycle, i.e., majority spicules are ‘Type I’. Thirdly, the lifetime of spicules in QS and CH are 148 s and 112 s, respectively, but there is no fundamental lifetime difference between the spicules in QS and CH reported earlier. Therefore, the suggestion of coronal heating by ‘Type II’ spicules should be taken with cautions. Subject headings: Sun: chromosphere Sun:transition region Sun:corona

New activity in the large circumstellar disk of the Be-shell star 48 Lib

astro-ph — Wed, 22 Feb 2012 01:56:16 +0000

Spectroscopic, polarimetric, and high spectral resolution interferometric data covering the period 1995-2011 are analyzed to document the transition into a new phase of circumstellar disk activity in the classical Be-shell star 48 Lib. The long-term disk evolution is described using the V/R ratio of the violet and red emission components of H alpha and Br gamma, radial velocities and profiles of He I and optical metal shell lines, multi-band BVRI polarimetry, broad-band and high-resolution interferometric visibilities and phases. Spectroscopic signatures of disk asymmetries in 48 Lib vanished in the late nineties but recovered some time between 2004 and 2007. Variations in the radial velocity and line profile of conventional shell lines correlate with the V/R behavior. They are shared by narrow absorption cores superimposed on otherwise seemingly photospheric He I lines, which may form in high-density gas at the inner disk close to the photosphere. Large radial velocity variations continued also during the V/R-quiet years. The broad H-band interferometry gives a disk diameter of (1.72+-0.2) mas (equivalent to 15 stellar radii), position angle of the disk (50+-9) deg and a relatively low disk flattening of 1.66+-0.3. Within the errors the same disk position angle is derived from polarimetric observations and from photocenter shifts across Br gamma. The high-resolution interferometric visibility and phase profiles show a double or even multiple-component structure. A preliminary estimate based on the size of the Br gamma emitting region indicates a large diameter for the disk (tens of stellar radii).

Generation of strong magnetic fields via the small-scale dynamo during the formation of the first stars

astro-ph — Wed, 22 Feb 2012 01:55:56 +0000

Here we summarize our recent results of high-resolution computer simulations on the turbulent amplification of weak magnetic seed fields showing that such fields will be exponentially amplified also during the gravitational collapse reminiscent to the situation during primordial star formation. The exponential magnetic field amplification is driven by the turbulent small-scale dynamo that can be only observed in computer simulations if the turbulent motions in the central core are sufficiently resolved. We find that the Jeans length, which determines the central core region, has to be resolved by at least 30 grid cells to capture the dynamo activity. We conclude from our studies that strong magnetic fields will be unavoidably created already during the formation of the first stars in the Universe, potentially influencing their evolution and mass distribution.

Spectroscopic detection of Carbon Monoxide in the Young Supernova Remnant Cassiopeia A

astro-ph — Wed, 22 Feb 2012 01:55:18 +0000

We report the detection of carbon monoxide (CO) emission from the young supernova remnant Cassiopeia A (Cas A) at wavelengths corresponding to the fundamental vibrational mode at 4.65 micron. We obtained AKARI Infrared Camera spectra towards 4 positions which unambiguously reveal the broad characteristic CO ro-vibrational band profile. The observed positions include unshocked ejecta at the center, indicating that CO molecules form in the ejecta at an early phase. We extracted a dozen spectra across Cas A along the long 1 arcmin slits, and compared these to simple CO emission models in Local Thermodynamic Equilibrium to obtain first-order estimates of the excitation temperatures and CO masses involved. Our observations suggest that significant amounts of carbon may have been locked up in CO since the explosion 330 years ago. Surprisingly, CO has not been efficiently destroyed by reactions with ionized He or the energetic electrons created by the decay of the radiative nuclei. Our CO detection thus implies that less carbon is available to form carbonaceous dust in supernovae than is currently thought and that molecular gas could lock up a significant amount of heavy elements in supernova ejecta.

Kinematics and helicity evolution of a loop-like eruptive prominence

astro-ph — Wed, 22 Feb 2012 01:54:22 +0000

We aim at investigating the morphology, kinematic and helicity evolution of a loop-like prominence during its eruption. We use multi-instrument observations from AIA/SDO, EUVI/STEREO and LASCO/SoHO. The kinematic, morphological, geometrical, and helicity evolution of a loop-like eruptive prominence are studied in the context of the magnetic flux rope model of solar prominences. The prominence eruption evolved as a height expanding twisted loop with both legs anchored in the chromosphere of a plage area. The eruption process consists of a prominence activation, acceleration, and a phase of constant velocity. The prominence body was composed of left-hand (counter-clockwise) twisted threads around the main prominence axis. The twist during the eruption was estimated at 6pi (3 turns). The prominence reached a maximum height of 526 Mm before contracting to its primary location and partially reformed in the same place two days after the eruption. This ejection, however, triggered a CME seen in LASCO C2. The prominence was located in the northern periphery of the CME magnetic field configuration and, therefore, the background magnetic field was asymmetric with respect to the filament position. The physical conditions of the falling plasma blobs were analysed with respect to the prominence kinematics. The same sign of the prominence body twist and writhe, as well as the amount of twisting above the critical value of 2pi after the activation phase indicate that possibly conditions for kink instability were present. No signature of magnetic reconnection was observed anywhere in the prominence body and its surroundings. The filament/prominence descent following the eruption and its partial reformation at the same place two days later suggest a confined type of eruption. The asymmetric background magnetic field possibly played an important role in the failed eruption.

Validity of clumping approximations for mass-loss rates determination

astro-ph — Wed, 22 Feb 2012 01:53:49 +0000

Clumping in stellar winds of hot stars is a possible consequence of radiative-acoustic instability appearing in solutions of radiative-hydrodynamical equations. However, clumping is usually included to stellar atmosphere modeling and radiative transfer calculations in a highly approximate way via a global free parameter called the clumping factor. Using different values of clumping factors many researchers succeeded to fit the observed spectra better and to correct empirical mass loss rates. This usually leads to a conclusion that the stellar wind is clumped. To understand how clumping may influence theoretical predictions of mass-loss rates, different clumping properties have to be taken into account. If clumping appears already below the critical point, the mass-loss rates is changed.

Probing the properties of Be star discs with spectroastrometry and NLTE radiative transfer modelling: beta CMi

astro-ph — Wed, 22 Feb 2012 01:52:45 +0000

While the presence of discs around classical Be stars is well established, their origin is still uncertain. To understand what processes result in the creation of these discs and how angular momentum is transported within them, their physical properties must be constrained. This requires comparing high spatial and spectral resolution data with detailed radiative transfer modelling. We present a high spectral resolution, R~80,000, sub milli-arcsecond precision, spectroastrometric study of the circumstellar disc around the Be star beta CMi. The data are confronted with three-dimensional, NLTE radiative transfer calculations to directly constrain the properties of the disc. Furthermore, we compare the data to disc models featuring two velocity laws; Keperian, the prediction of the viscous disc model, and angular momentum conserving rotation. It is shown that the observations of beta CMi can only be reproduced using Keplerian rotation. The agreement between the model and the observed SED, polarisation and spectroastrometric signature of beta CMi confirms that the discs around Be stars are well modelled as viscous decretion discs.

Magnetic fields of an active region filament from full Stokes analysis of Si I 1082.7 nm and He I 1083.0 nm

astro-ph — Wed, 22 Feb 2012 01:52:24 +0000

Vector magnetic fields of an active region filament in the photosphere and upper chromosphere are obtained from spectro-polarimetric observations recorded with the Tenerife Infrared Polarimeter (TIP II) at the German Vacuum Tower Telescope (VTT). We apply Milne-Eddington inversions on full Stokes vectors of the photospheric Si I 1082.7 nm and the upper chromospheric He I triplet at 1083.0 nm to obtain magnetic field vector and velocity maps in two atmosphere layers. We find that: (1)A complete filament was already present in H$\alpha$ at the beginning of the TIP II data acquisition. Only a partially formed one, composed of multiple small threads, was present in He I. (2) The AR filament comprises two sections. One shows strong magnetic field intensities, about 600 – 800 G in the upper chromosphere and 800 – 1000 G in the photosphere. The other exhibits only comparatively weak magnetic field strengths in both layers. (3) The Stokes V signal is indicative of a dip in the magnetic field strength close to the chromospheric PIL. (3) In the chromosphere consistent upflows are found along the PIL flanked by downflows. (4) The transversal magnetic field is nearly parallel to the PIL in the photosphere and inclined by 20 – 30 degree in the chromosphere. (5) The chromospheric magnetic field around the filament is found to be in normal configuration, while the photospheric field presents a concave magnetic topology. The observations are consistent with the emergence of a flux rope with a subsequent formation of a filament.

Presupernova evolution and explosive nucleosynthesis of zero metal massive stars

astro-ph — Wed, 22 Feb 2012 01:50:28 +0000

We present a new set of zero metallicity models in the range 13-80 $\rm M_\odot$ together to the associated explosive nucleosynthesis. These models are fully homogeneous with the solar metallicity set we published in Limongi & Chieffi (2006) and will be freely available at the web site http://www.iasf-roma.inaf.it./orfeo/public{\_}html. A comparison between these yields and an average star that represents the average behavior of most of the very metal poor stars in the range $\rm -5.0<[Fe/H]<-2.5$ confirms previous findings that only a fraction of the elemental [X/Fe] may be fitted by the ejecta of $\it standard$ core collapse supernovae.

Surface Brightness Variation of the Contact Binary SW Lac: Clues From Doppler Imaging

astro-ph — Wed, 22 Feb 2012 01:49:15 +0000

In this study, we present the preliminary light curve analysis of the contact binary SW Lac, using B, V light curves of the system spanning 2 years (2009 – 2010). During the spot modeling process, we used the information coming from the Doppler maps of the system, which was performed using the high resolution and phase dependent spectra obtained at the 2.1 m Otto Struve Telescope of the McDonald Observatory, in 2009. The results showed that the spot modeling from the light curve analysis are in accordance with the Doppler maps, while the non-circular spot modeling technique is needed in order to obtain much better and reliable spot models.

Symmetry energy effects on location of the inner edge of neutron star crusts [Cross-Listing]

astro-ph — Wed, 22 Feb 2012 01:47:37 +0000

The symmetry energy effects on the location of the inner edge of neutron star crusts are studied. Three phenomenological models are employed in order to check the accuracy of the well known parabolic approximation of the equation of state for asymmetric nuclear matter in the determination of the transition density $n_t$ and transition pressure $P_t$. The results corroborate the statement that the error due to the assumption that a priori the equation of state is parabolic may introduce a large error in the determination of related properties of a neutron star as the crustal fraction of the moment of inertia.

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.

Mergers and ejections of black holes in globular clusters

astro-ph — Wed, 22 Feb 2012 01:43:44 +0000

The negative effective magnetic pressure in stratified forced turbulence [Replacement]

astro-ph — Wed, 22 Feb 2012 01:40:52 +0000

To understand the basic mechanism of the formation of magnetic flux concentrations, we determine by direct numerical simulations the turbulence contributions to the mean magnetic pressure in a strongly stratified isothermal layer with large plasma beta, where a weak uniform horizontal mean magnetic field is applied. The negative contribution of turbulence to the effective mean magnetic pressure is determined for strongly stratified forced turbulence over a range of values of magnetic Reynolds and Prandtl numbers. Small-scale dynamo action is shown to reduce the negative effect of turbulence on the effective mean magnetic pressure. However, the turbulence coefficients describing the negative effective magnetic pressure phenomenon are found to be converged for magnetic Reynolds numbers between 60 and 600, which is the largest value considered here. In all these models the turbulent intensity is arranged to be nearly independent of height, so the kinetic energy density decreases with height due to the decrease in density. In a second series of numerical experiments, the turbulent intensity increases with height such that the turbulent kinetic energy density is nearly independent of height. Turbulent magnetic diffusivity and turbulent pumping velocity are determined with the test-field method for both cases. The vertical profile of the turbulent magnetic diffusivity is found to agree with what is expected based on simple mixing length expressions. Turbulent pumping is shown to be down the gradient of turbulent magnetic diffusivity, but it is twice as large as expected. Corresponding numerical mean-field models are used to show that a large-scale instability can occur in both cases, provided the degree of scale separation is large enough and hence the turbulent magnetic diffusivity small enough.

Phase diagram of dilute cosmic matter [Replacement]

astro-ph — Wed, 22 Feb 2012 01:40:29 +0000

Enhancement of nuclear pasta formation due to multi-nucleus simultaneous collision is presented based on time-dependent density functional calculations with periodic boundary condition. This calculation corresponds to the situation with density lower than the known low-density existence limit of the nuclear pasta phase. In order to evaluate the contribution from three-nucleus simultaneous collisions inside the cosmic matter, the possibility of multi-nucleus simultaneous collisions is examined by a systematic Monte-Carlo calculation, and the mean free path of a nucleus is obtained. Consequently the low-density existence limit of the nuclear pasta phase is formed to be lower than believed up to now.

Investigation of the Progenitors of Nova Explosions [Replacement]

astro-ph — Wed, 22 Feb 2012 01:37:51 +0000

Recurrent novae (RNe) play an important role as one of the suspected progenitor systems of Type Ia supernovae (SNe) which are used as primary distance indicators in cosmology. Thus, it is important to investigate the nature of their central binary systems to determine the relation between the parameters of the central system and outburst type, and finally ascertain the population of novae that might be available to give rise to the progenitors of Type Ia SNe. The details of the preliminary investigation looking for characteristics that may distinguish recurrent and classical novae (CNe) systems, the selection of initial targets for detailed study, and preliminary results are presented. We identified 10 suspected RNe among the Galactic CNe for investigation with the Liverpool Telescope population and our initial photometric observations of their quiescent systems suggest 2 may belong to the RS Oph type and 2 to the U Sco type RNe. Spectroscopic follow-up is now underway.

The Galactic WC stars: Stellar parameters from spectral analyses indicate a new evolutionary sequence [Replacement]

astro-ph — Wed, 22 Feb 2012 01:35:32 +0000

[Abridged] [...] AIMS: We aim to establish the stellar parameters and mass-loss rates of the Galactic WC stars. These data provide the empirical basis of studies of (i) the role of WC stars in the evolution of massive stars, (ii) the wind-driving mechanisms, and (iii) the feedback of WC stars as input to models of the chemical and dynamical evolution of galaxies. Methods: We analyze the nearly complete sample of un-obscured Galactic WC stars, using optical spectra as well as ultraviolet spectra when available. The observations are fitted with theoretical spectra, using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. A large grid of line-blanked models has been established for the range of WC subtypes WC4 – WC8, and smaller grids for the WC9 parameter domain. Both WO stars and WN/WC transit types are also analyzed using special models. Results: Stellar and atmospheric parameters are derived for more than 50 GalacticWC and two WO stars, covering almost the whole GalacticWC population as far as the stars are single, and un-obscured in the visual. In the Hertzsprung-Russell diagram, theWC stars reside between the hydrogen and the helium zero-age main sequences, having luminosities L from 10^4.9 to 10^5.6 Lsun. The mass-loss rates scale very tightly with L^0.8. The two WO stars in our sample turn out to be outstandingly hot (\approx200 kK) and do not fit into the WC scheme. Conclusions: By comparing the empirical WC positions in the Hertzsprung-Russell diagram with evolutionary models, and from recent supernova statistics, we conclude that WC stars have evolved from initial masses between 20 solar masses and 45 Msun. In contrast to previous assumptions, it seems that WC stars in general do not descend from the most massive stars. Only the WO stars might stem from progenitors that have been initially more massive than 45 Msun.

The Spitzer Spectroscopic Survey of S-type Stars [Replacement]

astro-ph — Wed, 22 Feb 2012 01:35:17 +0000

S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because of their peculiar chemical composition, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust. In this paper, we present a large sample of 87 intrinsic galactic S-type AGB stars, observed at infrared wavelengths with the Spitzer Space Telescope, and supplemented with ground-based optical data. On the one hand, we derive the stellar parameters from the optical spectroscopy and photometry, using a grid of model atmospheres. On the other, we decompose the infrared spectra to quantify the flux-contributions from the different dust species. Finally, we compare the independently determined stellar parameters and dust properties. For the stars without significant dust emission, we detect a strict relation between the presence of SiS absorption in the Spitzer spectra and the C/O ratio of the stellar atmosphere. These absorption bands can thus be used as an additional diagnostic for the C/O ratio. For stars with significant dust emission, we define three groups, based on the relative contribution of certain dust species to the infrared flux. We find a strong link between group-membership and C/O ratio. We show that these groups can be explained by assuming that the dust-condensation can be cut short before silicates are produced, while the remaining free atoms and molecules can then form the observed magnesium sulfides or the carriers of the unidentified 13 and 20 micron features. Finally, we present the detection of emission features attributed to molecules and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons and magnesium sulfide grains. We show that we often detect magnesium sulfides together with molecular SiS and we propose that it is formed by a reaction of SiS molecules with Mg.

Turbulent convection model in the overshooting region: II. Theoretical analysis [Replacement]

astro-ph — Wed, 22 Feb 2012 01:32:28 +0000

Turbulent convection models are thought to be good tools to deal with the convective overshooting in the stellar interior. However, they are too complex to be applied in calculations of stellar structure and evolution. In order to understand the physical processes of the convective overshooting and to simplify the application of turbulent convection models, a semi-analytic solution is necessary. We obtain the approximate solution and asymptotic solution of the turbulent convection model in the overshooting region, and find some important properties of the convective overshooting: I. The overshooting region can be partitioned into three parts: a thin region just outside the convective boundary with high efficiency of turbulent heat transfer, a power law dissipation region of turbulent kinetic energy in the middle, and a thermal dissipation area with rapidly decreasing turbulent kinetic energy. The decaying indices of the turbulent correlations $k$, $\bar{u_{r}’T'}$, and $\bar{T’T'}$ are only determined by the parameters of the TCM, and there is an equilibrium value of the anisotropic degree $\omega$. II. The overshooting length of the turbulent heat flux $\bar{u_{r}’T'}$ is about $1H_k$($H_k=|\frac{dr}{dlnk}|$). III. The value of the turbulent kinetic energy at the convective boundary $k_C$ can be estimated by a method called \textsl{the maximum of diffusion}. Turbulent correlations in the overshooting region can be estimated by using $k_C$ and exponentially decreasing functions with the decaying indices.

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.

Recent Advances in the Exploration of the Small-Scale Structure of the Quiet Solar Atmosphere: Vortex Flows, the Horizontal Magnetic Field, and the Stokes-V Line-Ratio Method

astro-ph — Tue, 21 Feb 2012 02:06:42 +0000

We review (i) observations and numerical simulations of vortical flows in the solar atmosphere and (ii) measurements of the horizontal magnetic field in quiet Sun regions. First, we discuss various manifestations of vortical flows and emphasize the role of magnetic fields in mediating swirling motion created near the solar surface to the higher layers of the photosphere and to the chromosphere. We reexamine existing simulation runs of solar surface magnetoconvection with regard to vortical flows and compare to previously obtained results. Second, we review contradictory results and problems associated with measuring the angular distribution of the magnetic field in quiet Sun regions. Furthermore, we review the Stokes-V-amplitude ratio method for the lines Fe I 630.15 and 630.25 nm. We come to the conclusion that the recently discovered two distinct populations in scatter plots of this ratio must not bee interpreted in terms of ‘uncollapsed’ and ‘collapsed’ fields but they stem from weak granular magnetic fields and weak canopy fields located at the boundaries between granules and the intergranular space. Based on new simulation runs, we reaffirm earlier findings of a predominance of the horizontal field components over the vertical one, particularly in the upper photosphere and at the base of the chromosphere.

On the nature of the EXor accretion events: an unfrequent manifestation of a common phenomenology ?

astro-ph — Tue, 21 Feb 2012 02:03:59 +0000

We present the results of a comparison between classical and newly identified EXor based on literature data and aimed at recognizing possible differences or similarities of both categories. Optical and near-IR two-color diagrams, modalities of fluctuations, and derived values of the mass accretion rates are indicative of strong similarities between the two samples. We demonstrate how the difference between the outburst and the quiescence spectral energy distribution of all the EXor can be well fitted with a single blackbody, as if an additional thermal component appears during the outbursting phase. Temperatures of this additional component span between 1000 and 4500 K, while the radii of the emitting regions (assumed to be a uniform disk) span between 0.01 and 0.1 AU, sizes typical of the inner portions of the circumstellar disk. Spots persisting up to 50% of the outburst duration, not exceeding the 10% of the stellar surface, and with temperatures compatible with the EXor mass accretion rates, are able to account for both the appearance of the additional thermal component and the dust sublimation in the inner structures of the disk. We also compare the EXor events with the most significant color and magnitude fluctuations of active T Tauri stars finding that (i} burst accretion phenomena should also be important for this latter class; (ii} EXor events could be more frequent then those accidentally discovered. Remarkable is the case of the source V2493 Cyg, a T Tauri star recently identified as a strong outbursting object: new optical and near-IR photometric and spectroscopic data are presented trying to clarify its EXor or FUor nature.

First Science Results From SOFIA/FORCAST: Super-Resolution Imaging of the S140 Cluster at 37\micron

astro-ph — Tue, 21 Feb 2012 02:02:46 +0000

We present 37\micron\ imaging of the S140 complex of infrared sources centered on IRS1 made with the FORCAST camera on SOFIA. These observations are the longest wavelength imaging to resolve clearly the three main sources seen at shorter wavelengths, IRS 1, 2 and 3, and are nearly at the diffraction limit of the 2.5-m telescope. We also obtained a small number of images at 11 and 31\micron\ that are useful for flux measurement. Our images cover the area of several strong sub-mm sources seen in the area — SMM 1, 2, and 3 — that are not coincident with any mid-infrared sources and are not visible in our longer wavelength imaging either. Our new observations confirm previous estimates of the relative dust optical depth and source luminosity for the components in this likely cluster of early B stars. We also investigate the use of super-resolution to go beyond the basic diffraction limit in imaging on SOFIA and find that the van Cittert algorithm, together with the “multi-resolution” technique, provides excellent results.

Non-relativistic Extended Gravity and its applications across different astrophysical scales

astro-ph — Tue, 21 Feb 2012 02:01:36 +0000

Using dimensional analysis techniques we present an extension of Newton’s gravitational theory built under the assumption that Milgrom’s acceleration constant is a fundamental quantity of nature. The gravitational force converges to Newton’s gravity and to a MOND-like description in two different mass and length regimes. It is shown that a modification on the force sector (and not in the dynamical one as MOND does) is more convenient and can reproduce and predict different phenomena usually ascribed to dark matter at the non-relativistic level.

The Na 8200 Angstrom Doublet as an Age Indicator in Low-Mass Stars

astro-ph — Tue, 21 Feb 2012 02:00:58 +0000

We investigate the use of the gravity sensitive neutral sodium (NaI) doublet at 8183 Angstroms 8195 Angstroms (Na 8200 Angstrom doublet) as an age indicator for M dwarfs. We measured the Na doublet equivalent width (EW) in giants, old dwarfs, young dwarfs, and candidate members of the Beta Pic moving group using medium resolution spectra. Our Na 8200 Angstrom doublet EW analysis shows that the feature is useful as an approximate age indicator in M-type dwarfs with (V-K_s) >= 5.0, reliably distinguishing stars older and younger than 100 Myr. A simple derivation of the dependence of the Na EW on temperature and gravity supports the observational results. An analysis of the effects of metallicity show that this youth indicator is best used on samples with similar metallicity. The age estimation technique presented here becomes useful in a mass regime where traditional youth indicators are increasingly less reliable, is applicable to other alkali lines, and will help identify new-low mass members in other young clusters and associations.

A spatio-temporal description of the abrupt changes in the photospheric magnetic and Lorentz-force vectors during the 2011 February 15 X2.2 flare

astro-ph — Tue, 21 Feb 2012 02:00:06 +0000

The active region NOAA 11158 produced the first X-class flare of Solar Cycle 24, an X2.2 flare at 01:44 UT on 2011 February 15. Here we analyze SDO/HMI magnetograms covering a 12-hour interval centered at the time of this flare. We describe the spatial distributions of the photospheric magnetic changes associated with this flare, including the abrupt changes in the field vector, vertical electric current and Lorentz force vector. We also trace these parameters’ temporal evolution. The abrupt magnetic changes were concentrated near the neutral line and in two neighboring sunspots. Near the neutral line, the field vectors became more horizontal during the flare and the shear increased. This was due to an increase in strength of the horizontal field components near the neutral line, most significant in the horizontal component parallel to the neutral line but the perpendicular component also increased in strength. The vertical component did not show a significant, permanent overall change at the neutral line. The increase in total flux at the neutral line was accompanied by a compensating flux decrease in the surrounding volume. In both of the sunspots near the neutral line the azimuthal flux abruptly decreased during the flare but this change was permanent in only one of the spots. There was a large, abrupt, downward vertical Lorentz force change during the flare, consistent with results of past analyses and recent theoretical work. The horizontal Lorentz force acted in opposite directions on each side of neutral line, with the two sunspots at each end subject to abrupt torsional forces. The shearing forces were consistent with a decrease of shear near the neutral line, whereas the field itself became more sheared as a result of the flux collapsing towards the neutral line from the surrounding volume.

Two New Tidally Distorted White Dwarfs

astro-ph — Tue, 21 Feb 2012 01:59:42 +0000

We identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both stars are extremely low mass (ELM, < 0.2 Msun) WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 Msun, has one of the strongest Doppler beaming signals ever observed in a binary system (0.59 \pm 0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074 Rsun. For J2119, the radius exceeds 0.10 Rsun. These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23 \pm 0.06% reflection effect, and we discuss possible sources for this excess heating. Both J1741 and J2119 are strong gravitational wave sources, and the time-of-minimum of the ellipsoidal variations can be used to detect the orbital period decay. This may be possible on a timescale of a decade or less.

A Search for Infrared Emission from Core-Collapse Supernovae at the Transitional Phase

astro-ph — Tue, 21 Feb 2012 01:59:22 +0000

Most of the observational studies of supernova (SN) explosions are limited to early phases ( 100 yr) in our Galaxy or very nearby galaxies. SNe at the epoch between these two, which we call “transitional” phase, have not been explored in detail except for several extragalactic SNe including SN 1987A in the Large Magellanic Cloud. We present theoretical predictions for the infrared (IR) dust emissions by several mechanisms; emission from dust formed in the SN ejecta, light echo by circumstellar and interstellar dust, and emission from shocked circumstellar dust. We search for IR emission from 6 core-collapse SNe at the transitional phase in the nearby galaxies NGC 1313, NGC 6946, and M101 by using the data taken with the AKARI satellite and Spitzer. Among 6 targets, we detect the emission from SN 1978K in NGC 1313. SN 1978K is associated with 1.3 x 10^{-3} Msun of silicate dust. We show that, among several mechanisms, the shocked circumstellar dust is the most probable emission source to explain the IR emission observed for CSM-rich SN 1978K. IR emission from the other 5 objects is not detected. Our current observations are sensitive to IR luminosity of > 10^{38} erg s^{-1}, and the non-detection of SN 1962M excludes the existence of the shocked circumstellar dust for a high gas mass-loss rate of sim 10^{-4} Msun yr^{-1}. Observations of SNe at the transitional phase with future IR satellites will fill the gap of IR observations of SNe with the age of 10-100 years, and give a new opportunity to study the circumstellar and interstellar environments of the progenitor, and possibly dust formation in SNe.

Turbulent convection model in the overshooting region: II. Theoretical analysis

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

Multi-Moment Advection scheme for Vlasov simulations [Cross-Listing]

astro-ph — Tue, 21 Feb 2012 01:57:37 +0000

We present a new numerical scheme for solving the advection equation and its application to the Vlasov simulation. 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. We have developed one- and two-dimensional schemes and show that they provide quite accurate solutions compared to other existing schemes with the same memory usage. The two-dimensional scheme can solve the solid body rotation problem of a gaussian profile with little numerical diffusion. This is a very important property for Vlasov simulations of magnetized plasma. The application of the scheme to the electromagnetic Vlasov simulation of collisionless shock waves is presented as a benchmark test.

Yellow and Red Supergiants in the Large Magellanic Cloud

astro-ph — Tue, 21 Feb 2012 01:57:15 +0000

Due to their transitionary nature, yellow supergiants provide a critical challenge for evolutionary modeling. Previous studies within M31 and the SMC show that the Geneva evolutionary models do a poor job at predicting the lifetimes of these short-lived stars. Here we extend this study to the LMC while also investigating the galaxy’s red supergiant content. This task is complicated by contamination by Galactic foreground stars that color and magnitude criteria alone cannot weed out. Therefore, we use proper motions and the LMC’s large systemic radial velocity (\sim278 km/s) to separate out these foreground dwarfs. After observing nearly 2,000 stars, we identified 317 probable yellow supergiants, 6 possible yellow supergiants and 505 probable red supergiants. Foreground contamination of our yellow supergiant sample was \sim80%, while that of the the red supergiant sample was only 3%. By placing the yellow supergiants on the H-R diagram and comparing them against the evolutionary tracks, we find that new Geneva evolutionary models do an exemplary job at predicting both the locations and the lifetimes of these transitory objects.

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

Kelvin-Helmholtz instability in partially ionized compressible plasmas

astro-ph — Tue, 21 Feb 2012 01:54:20 +0000

The Kelvin-Helmholtz Instability (KHI) has been observed in the solar atmosphere. Ion-neutral collisions may play a relevant role for the growth rate and evolution of the KHI in solar partially ionized plasmas as in, e.g., solar prominences. Here, we investigate the linear phase of the KHI at an interface between two partially ionized magnetized plasmas in the presence of a shear flow. The effects of ion-neutral collisions and compressibility are included in the analysis. We obtain the dispersion relation of the linear modes and perform parametric studies of the unstable solutions. We find that in the incompressible case the KHI is present for any velocity shear regardless the value of the collision frequency. In the compressible case, the domain of instability depends strongly on the plasma parameters, specially the collision frequency and the density contrast. For high collision frequencies and low density contrasts the KHI is present for super-Alfvenic velocity shear only. For high density contrasts the threshold velocity shear can be reduced to sub-Alfvenic values. For the particular case of turbulent plumes in prominences, we conclude that sub-Alfvenic flow velocities can trigger the KHI thanks to the ion-neutral coupling.

The GREGOR solar telescope on Tenerife

astro-ph — Tue, 21 Feb 2012 01:52:13 +0000

2011 was a successful year for the GREGOR project. The telescope was finally completed in May with the installation of the 1.5-meter primary mirror. The installation of the first-light focal plane instruments was completed by the end of the year. At the same time, the preparations for the installation of the high-order adaptive optics were finished, its integration to the telescope is scheduled for early 2012. This paper describes the telescope and its instrumentation in their present first light configuration, and provides a brief overview of the science goals of GREGOR.

Probing the envelopes of massive young stellar objects with diffraction limited mid-infrared imaging

astro-ph — Tue, 21 Feb 2012 01:49:16 +0000

Massive stars form whilst they are still embedded in dense envelopes. As a result, the roles of rotation, mass loss and accretion in massive star formation are not well understood. This study evaluates the source of the Q-band, lambda=19.5 microns, emission of massive young stellar objects (MYSOs). This allows us to determine the relative importance of rotation and outflow activity in shaping the circumstellar environments of MYSOs on 1000 AU scales. We obtained diffraction limited mid-infrared images of a sample of 20 MYSOs using the VLT/VISIR and Subaru/COMICS instruments. For these 8 m class telescopes and the sample selected, the diffraction limit, ~0.6″, corresponds to approximately 1000 AU. We compare the images and the spectral energy distributions (SEDs) observed to a 2D, axis-symmetric dust radiative transfer model that reproduces VLTI/MIDI observations of the MYSO W33A. We vary the inclination, mass infall rate, and outflow opening angle to simultaneously recreate the behaviour of the sample of MYSOs in the spatial and spectral domains. The mid-IR emission of 70 percent of the MYSOs is spatially resolved. In the majority of cases, the spatial extent of their emission and their SEDs can be reproduced by the W33A model featuring an in-falling, rotating dusty envelope with outflow cavities. There is independent evidence that most of the sources which are not fit by the model are associated with ultracompact HII regions and are thus more evolved. We find that, in general, the diverse 20 micron morphology of MYSOs can be attributed to warm dust in the walls of outflow cavities seen at different inclinations. This implies that the warm dust in the outflow cavity walls dominates the Q-band emission of MYSOs. In turn, this emphasises that outflows are an ubiquitous feature of massive star formation.

Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars

astro-ph — Tue, 21 Feb 2012 01:48:59 +0000

Based on the turbulent convection model (TCM), we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediate-mass stars, focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell (HR) diagram. A diffusive mixing model is adopted during the Red Giant Branch (RGB) phase. The properties of the blue loop are changed by modification of the element profiles above the H-burning shell, which results from the incomplete mixing in the bottom overshooting region when stellar model evolves up along the RGB. Such modification of the element profiles will lead to an increase of the opacity in the region just above the H-burning shell and a decrease of the opacity in the outer homogeneous convection zone, which will result in a quick decrease of the H-shell nuclear luminosity $L_{H}$ when the stellar model evolves from the RGB tip to its bottom and, finally, a much weaker and smaller convection zone will be obtained in the stellar envelope. This helps to form a longer blue loop. The extension of the blue loop is very sensitive to the parameters $(C_{X}$ and $\alpha_{TCM})$ of the diffusive mixing model and of the TCM. The results mainly show that: 1) comparing to the results of the classical model with the mixing-length theory, the lengths of the obtained blue loops with different combinations of the values of $C_{X}$ and $\alpha_{TCM}$ are all increased and the length of the blue loop increases with the values of parameters $C_{X}$ and $\alpha_{TCM}$; 2) the diffusive mixing model can significantly extend the time of stellar models lingering on the blue side of the HR diagram, even though the length of the blue loop for the 7$M_{\odot}$ star has a less prominent difference between the classical and diffusive mixing model;

Supersonic Magnetic Flows in the Quiet Sun

astro-ph — Tue, 21 Feb 2012 01:48:32 +0000

In this contribution we describe some recent observations of high-speed magnetized flows in the quiet Sun granulation. These observations were carried out with the Imaging Magnetograph eXperiment (IMaX) onboard the stratospheric balloon {\sc Sunrise}, and possess an unprecedented spatial resolution and temporal cadence. These flows were identified as highly shifted circular polarization (Stokes $V$) signals. We estimate the LOS velocity responsible for these shifts to be larger than 6 km s$^{-1}$, and therefore we refer to them as {\it supersonic magnetic flows}. The average lifetime of the detected events is 81.3 s and they occupy an average area of about 23\,000 km$^2$. Most of the events occur within granular cells and correspond therefore to upflows. However some others occur in intergranular lanes or bear no clear relation to the convective velocity pattern. We analyze a number of representative examples and discuss them in terms of magnetic loops, reconnection events, and convective collapse.

Kinematic Properties of Globally-Propagating Waves in the Solar Corona

astro-ph — Tue, 21 Feb 2012 01:47:46 +0000

Globally-propagating coronal bright fronts (CBFs) in the solar corona are among the most dramatic manifestations of solar activity, but are not well understood despite strong links with both solar flares and coronal mass ejections. Extreme UltraViolet (EUV) observations from the STEREO and SDO spacecraft are used here to study their kinematics and morphology. The first STEREO observations of a CBF are presented, with the pulse observed in all available EUV passbands (171, 195, 284 and for the first time, 304A). The pulse displayed similar kinematics in all passbands, although the derived pulse velocity and acceleration were found to be strongly influenced by the observing cadence, implying that previous kinematics may have been underestimated. Different techniques for identifying CBFs and deriving their true kinematics were tested, with traditional techniques shown to be prone to undefined user-dependent errors. This was overcome through the development of a statistically rigorous, semi-automated identification algorithm, which was then used to determine the kinematics of four CBF events observed by STEREO. All of the events studied exhibited clear deceleration as well as increases in both spatial and temporal pulse width, indicating that the CBFs are dispersive. A CBF pulse observed by both STEREO and SDO was also studied using the algorithm, with lower initial velocity and weaker deceleration noted in STEREO observations compared to SDO, reaffirming the effects of image cadence on the derived kinematics. The kinematics obtained using SDO were highly passband dependent, suggesting a compressive nature. Significant pulse broadening was also noted in observations from both spacecraft, allowing the dispersion rate of the pulse to be determined. These results indicate that coronal bright fronts are best interpreted as fast-mode magnetoacoustic waves propagating in an inhomogeneous medium.

CRIRES-POP: A library of high resolution spectra in the near-infrared

astro-ph — Tue, 21 Feb 2012 01:45:46 +0000

New instrumental capabilities and the wealth of astrophysical information extractable from the near-infrared wavelength region have led to a growing interest in the field of high resolution spectroscopy at 1-5 mu. We aim to provide a library of observed high-resolution and high signal-to-noise-ratio near-infrared spectra of stars of various types throughout the Hertzsprung-Russell diagram. This is needed for the exploration of spectral features in this wavelength range and for comparison of reference targets with observations and models. High quality spectra were obtained using the CRIRES near-infrared spectrograph at ESO’s VLT covering the range from 0.97 to 5.3 mu at high spectral resolution. Accurate wavelength calibration and correction for of telluric lines were performed by fitting synthetic transmission spectra for the Earth’s atmosphere to each spectrum individually. We describe the observational strategy and the current status and content of the library which includes 13 objects. The first examples of finally reduced spectra are presented. This publication will serve as a reference paper to introduce the library to the community and explore the extensive amount of material.

Photometric study of the Blazhko star RZ Lyr

astro-ph — Tue, 21 Feb 2012 01:45:12 +0000

The analysis of recent, extended multicolour CCD and archive photoelectric, photographic and visual observations has revealed several important properties of RZ Lyr, an RRab-type variable exhibiting large-amplitude Blazhko modulation. On the time-base of \sim110 yr, a strict anticorrelation between the pulsation and modulation period changes is established. The light curve of RZ Lyr shows a remarkable bump on the descending branch in the small-amplitude phase of the modulation, similarly to the light curves of bump Cepheids. We speculate that the stellar structure temporally suits a 4:1 resonance between the periods of the fundamental and one of the higher-order radial modes in this modulation phase. The light-curve variation of RZ Lyr can be correctly fitted with a two-modulation-component solution; the 121 d period of the main modulation is nearly but not exactly four times longer than the period of the secondary modulation component. Using the inverse photometric method, the variations in the pulsation-averaged values of the physical parameters in different phases of both modulation components are determined.

On the nature of candidate luminous blue variables in M33

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

Luminous blue variables (LBVs) are expected to play an important role in massive stellar evolution as well as being the progenitors of some of the most luminous supernovae known. In this paper we provide a multiwavelength study of the population of (candidate) LBVs identified within M33. New spectra provide an observational baseline of >4yr with respect to published data, which is well suited to identifying LBV outbursts. Multi-epoch optical and mid-IR surveys of M33 further constrain the variability of the sample and permit a search for dusty circumstellar ejecta. Spectroscopic and photometric variability appears common amongst the sample, although in many cases further observations will be required to determine its physical origin. Nevertheless, we report a new outburst of M33 Var C, while the transition of the WNLh star B517 to a cooler B supergiant phase between 1993-2010 confirms an LBV classification. Proof-of-concept quantitative analysis is provided for Romano’s star; the results being consistent with the finding that its bolometric luminosity varies during its LBV excursions. The combination of the temperature and luminosity of two stars, the B hypergiant [HS80] 110A and the cool hypergiant B324, appears to be in violation of the empirical Humphreys-Davidson limit. Mid-IR observations demonstrate that a number of candidates appear associated with hot circumstellar dust, although no objects as extreme as Eta Carinae are identified. The combined multiwavelength dataset suggests that the population of LBVs studied is contaminated by stars demonstrating the B[e] phenomenon. Of these, a subset of optically faint, low luminosity stars associated with hot dust are of particular interest since they appear similar to the likely progenitors of SN 2008S and the 2008 NGC300 transient, albeit suffering less intrinsic extinction. [ABRIDGED]

First Science Observations with SOFIA/FORCAST: Properties of Intermediate-Luminosity Protostars and Circumstellar Disks in OMC-2

astro-ph — Tue, 21 Feb 2012 01:42:37 +0000

We examine eight young stellar objects in the OMC-2 star forming region based on observations from the SOFIA/FORCAST early science phase, the Spitzer Space Telescope, the Herschel Space Observatory, 2MASS, APEX, and other results in the literature. We show the spectral energy distributions of these objects from near-infrared to millimeter wavelengths, and compare the SEDs with those of sheet collapse models of protostars and circumstellar disks. Four of the objects can be modelled as protostars with infalling envelopes, two as young stars surrounded by disks, and the remaining two objects have double-peaked SEDs. We model the double-peaked sources as binaries containing a young star with a disk and a protostar. The six most luminous sources are found in a dense group within a 0.15 x 0.25 pc region; these sources have luminosities ranging from 300 L_sun to 20 L_sun. The most embedded source (OMC-2 FIR 4) can be fit by a class 0 protostar model having a luminosity of ~50 L_sun and mass infall rate of ~10^-4 solar masses per year.

Properties of the negative effective magnetic pressure instability [Replacement]

astro-ph — Tue, 21 Feb 2012 01:40:45 +0000

As was demonstrated in earlier studies, turbulence can result in a negative contribution to the effective mean magnetic pressure, which, in turn, can cause a large-scale instability. In this study, hydromagnetic mean-field modelling is performed for an isothermally stratified layer in the presence of a horizontal magnetic field. The negative effective magnetic pressure instability (NEMPI) is comprehensively investigated. It is shown that, if the effect of turbulence on the mean magnetic tension force vanishes, which is consistent with results from direct numerical simulations of forced turbulence, the fastest growing eigenmodes of NEMPI are two-dimensional. The growth rate is found to depend on a parameter beta_* characterizing the turbulent contribution of the effective mean magnetic pressure for moderately strong mean magnetic fields. A fit formula is proposed that gives the growth rate as a function of turbulent kinematic viscosity, turbulent magnetic diffusivity, the density scale height, and the parameter beta_*. The strength of the imposed magnetic field does not explicitly enter provided the location of the vertical boundaries are chosen such that the maximum of the eigenmode of NEMPI fits into the domain. The formation of sunspots and solar active regions is discussed as possible applications of NEMPI.

Suppression of Self-Induced Flavor Conversion in the Supernova Accretion Phase [Replacement]

astro-ph — Tue, 21 Feb 2012 01:38:04 +0000

Self-induced flavor conversions of supernova (SN) neutrinos can strongly modify the flavor dependent fluxes. We perform a linearized flavor stability analysis with accretion-phase matter profiles of a 15 M_sun spherically symmetric model and corresponding neutrino fluxes. We use realistic energy and angle distributions, the latter deviating strongly from quasi-isotropic emission, thus accounting for both multi-angle and multi-energy effects. For our matter and neutrino density profile we always find stable conditions: flavor conversions are limited to the usual MSW effect. In this case one may distinguish the neutrino mass hierarchy in a SN neutrino signal if the mixing angle theta_13 is as large as suggested by recent experiments.

Search for and Characterization of Galactic Open Clusters with 2MASS [Replacement]

astro-ph — Tue, 21 Feb 2012 01:35:19 +0000

We have developed a star-counting algorithm and tested it on the 2MASS star catalog to search for density enhancements significantly above the field in Galactic latitude $|b|<50^{\circ}$. Nearly 500 open clusters are "rediscovered", along with 89 globular clusters, 35 galaxies, 55 galaxy clusters, 11 H II regions, and 4 regions contaminated by nearby bright stars. Fifty-two density enhancement regions remain unaccounted for. Here we present one such candidate HDG\,01, $(\ell, b)=(144.9038, 0.4338)$ which has an angular size of 3', a distance of 1.5 kpc, hence a physical size of 3.7 pc. Due to some nebulous shape, this star cluster should be an young-aged ($\le 10$ Myr).

Spin-Down of Radio Millisecond Pulsars at Genesis [Replacement]

astro-ph — Tue, 21 Feb 2012 01:33:40 +0000

Millisecond pulsars are old neutron stars that have been spun up to high rotational frequencies via accretion of mass from a binary companion star. An important issue for understanding the physics of the early spin evolution of millisecond pulsars is the impact of the expanding magnetosphere during the terminal stages of the mass-transfer process. Here I report binary stellar evolution calculations that show that the braking torque acting on a neutron star, when the companion star decouples from its Roche-lobe, is able to dissipate >50% of the rotational energy of the pulsar. This effect may explain the apparent difference in observed spin distributions between x-ray and radio millisecond pulsars and help account for the noticeable age discrepancy with their young white dwarf companions.

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.

Skyrme Interaction and Nuclear Matter Constraints [Cross-Listing]

astro-ph — Mon, 20 Feb 2012 01:44:54 +0000

This paper presents a detailed assessment of the ability of the 240 Skyrme interaction parameter sets in the literature to satisfy a series of criteria derived from macroscopic properties of nuclear matter in the vicinity of nuclear saturation density at zero temperature and their density dependence, derived by the liquid drop model, experiments with giant resonances and heavy-ion collisions. The objective is to identify those parameterizations which best satisfy the current understanding of the physics of nuclear matter over a wide range of applications. Out of the 240 models, only 16 are shown to satisfy all these constraints. Additional, more microscopic, constraints on density dependence of the neutron and proton effective mass beta-equilibrium matter, Landau parameters of symmetric and pure neutron nuclear matter, and observational data on high- and low-mass cold neutron stars further reduce this number to 5, a very small group of recommended Skyrme parameterizations to be used in future applications of the Skyrme interaction of nuclear matter related observables. Full information on partial fulfillment of individual constraints by all Skyrme models considered is given. The results are discussed in terms of the physical interpretation of the Skyrme interaction and the validity of its use in mean-field models. Future work on application of the Skyrme forces, selected on the basis of variables of nuclear matter, in Hartree-Fock calculation of properties of finite nuclei, is outlined.

Investigation of the new cataclysmic variable 1RXS J180834.7+101041

astro-ph — Mon, 20 Feb 2012 01:44:31 +0000

We present the results of our photometric and spectroscopic studies of the new eclipsing cataclysmic variable star 1RXS J180834.7+101041. Its spectrum exhibits double-peaked hydrogen and helium emission lines. The Doppler maps constructed from hydrogen lines show a nonuniform distribution of emission in the disk similar to that observed in IP Peg. This suggests that the object can be a cataclysmic variable with tidal density waves in the disk. We have determined the component masses (M_WD =0.8 \pm 0.22 M_sun and M_RD =0.14 \pm 0.02 M_sun) and the binary inclination (i =78 \pm 1.5 deg) based on well-known relations between parameters for cataclysmic variable stars. We have modeled the binary light curves and showed that the model of a disk with two spots is capable of explaining the main observed features of the light curves.

A Bayesian Approach to Calibrating Period-Luminosity Relations of RR Lyrae Stars in the Mid-Infrared

astro-ph — Mon, 20 Feb 2012 01:43:32 +0000

A Bayesian approach to calibrating period-luminosity (PL) relations has substantial benefits over generic least-squares fits. In particular, the Bayesian approach takes into account the full prior distribution of the model parameters, such as the a priori distances, and refits these parameters as part of the process of settling on the most highly-constrained final fit. Additionally, the Bayesian approach can naturally ingest data from multiple wavebands and simultaneously fit the parameters of PL relations for each waveband in a procedure that constrains the parameter posterior distributions so as to minimize the scatter of the final fits appropriately in all wavebands. Here we describe the generalized approach to Bayesian model fitting and then specialize to a detailed description of applying Bayesian linear model fitting to the mid-infrared PL relations of RR Lyrae variable stars. For this example application we quantify the improvement afforded by using a Bayesian model fit. We also compare distances previously predicted in our example application to recently published parallax distances measured with the Hubble Space Telescope and find their agreement to be a vindication of our methodology. Our intent with this article is to spread awareness of the benefits and applicability of this Bayesian approach and encourage future PL relation investigations to consider employing this powerful analysis method.

On the nature of the galactic early-B hypergiants

astro-ph — Mon, 20 Feb 2012 01:43:19 +0000

Despite their importance to a number of astrophysical fields, the lifecycles of very massive stars are still poorly defined. In order to address this shortcoming, we present a detailed quantitative study of the physical properties of four early-B hypergiants (BHGs); Cyg OB2 #12, zeta Sco, HD190603 and BP Cru. These are combined with an analysis of their long-term spectroscopic and photometric behaviour in order to determine their evolutionary status. The long-term datasets revealed that they are remarkably stable over long periods (>40yr), with the possible exception of zeta Sco prior to the 20th century, in contrast to the typical excursions that characterise luminous blue variables (LBVs). Zeta Sco, HD190603 and BP Cru possess physical properties intermediate between B supergiants and LBVs; we therefore suggest that BHGs are the immediate descendants and progenitors (respectively) of such stars (for initial masses in the range ~30-60Msun). In contrast, while the wind properties of Cyg OB2 #12 are consistent with this hypothesis, the combination of extreme luminosity and spectroscopic mass (~110Msun) and comparatively low temperature means it cannot be accommodated in such a scheme. Likewise, despite its co-location with several LBVs above the Humphreys-Davidson (HD) limit, the lack of long term variability and its unevolved chemistry apparently excludes such an identification. Since such massive stars are not expected to evolve to such cool temperatures, the properties of Cyg OB2 #12 are difficult to understand under current evolutionary paradigms. [ABRIDGED]

The Detonation Mechanism of the Pulsationally-Assisted Gravitationally-Confined Detonation Model of Type Ia Supernovae

astro-ph — Mon, 20 Feb 2012 01:41:53 +0000

We describe the detonation mechanism comprising the “Pulsationally Assisted” Gravitationally Confined Detonation (GCD) model of Type Ia supernovae (SNe Ia). This model is analogous to the previous GCD model reported in Jordan (2008); however, the chosen initial conditions produce a substantively different detonation mechanism, resulting from a larger energy release during the deflagration phase. The resulting final energy releases and nickel-56 yields conform better to observational values than is the case for the “classical” GCD models. In the present class of models, the ignition of a deflagration phase leads to a rising, burning plume of ash. The ash breaks out of the surface of the white dwarf, flows laterally around the star, and converges on the collision region at the antipodal point from where it broke out. The amount of energy released during the deflagration phase is enough to cause the star to rapidly expand, so that when the ash reaches the antipodal point, the surface density is too low to initiate a detonation. Instead, as the ash flows into the collision region (while mixing with surface fuel) the star reaches its maximally expanded state and then contracts. The stellar contraction acts to increase the density of the star, including the density in the collision region. This both raises the temperature and density of the fuel-ash mixture in the collision region and ultimately leads to thermodynamic conditions that produce a detonation. We demonstrate this mechanism with three 3-dimensional (3D), full star simulations of this model using the FLASH code, varying the initial offset of the ignition points for each model. The simulations are characterized by nuclear energy releases ranging from 38% to 78% of the binding energy of the white dwarf during the deflagration phase. We show that the conditions for detonation are achieved in all three of the models.

Evidence for Widespread Cooling in an Active Region Observed with the SDO Atmospheric Imaging Assembly

astro-ph — Mon, 20 Feb 2012 01:39:59 +0000

A well known behavior of EUV light curves of discrete coronal loops is that the peak intensities of cooler channels or spectral lines are reached at progressively later times than hotter channels. This time lag is understood to be the result of hot coronal loop plasma cooling through these lower respective temperatures. However, loops typically comprise only a minority of the total emission in active regions. Is this cooling pattern a common property of active region coronal plasma, or does it only occur in unique circumstances, locations, and times? The new SDO/AIA data provide a wonderful opportunity to answer this question systematically for an entire active region. We measure the time lag between pairs of SDO/AIA EUV channels using 24 hours of images of AR 11082 observed on 19 June 2010. We find that there is a time-lag signal consistent with cooling plasma, just as is usually found for loops, throughout the active region including the diffuse emission between loops for the entire 24 hour duration. The pattern persists consistently for all channel pairs and choice of window length within the 24 hour time period, giving us confidence that the plasma is cooling from temperatures of greater than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower than ~ 0.8 MK. This suggests that the bulk of the emitting coronal plasma in this active region is not steady; rather, it is dynamic and constantly evolving. These measurements provide crucial constraints on any model which seeks to describe coronal heating.

The Kinematics and Morphology of Solar Coronal Mass Ejections

astro-ph — Mon, 20 Feb 2012 01:39:47 +0000

Solar coronal mass ejections (CMEs) are large-scale eruptions of plasma and magnetic field from the Sun into the corona and interplanetary space. They are the most significant drivers of adverse space weather at Earth and other locations in the heliosphere, so it is important to understand the physics governing their eruption and propagation. However the diffuse morphology and transient nature of CMEs makes them difficult to identify and track using traditional image processing techniques. In this thesis the implementation of multiscale image processing techniques to identify and track the CME front through coronagraph images is detailed. An ellipse characterisation of the CME front is used to determine the CME kinematics and morphology with increased precision as compared to techniques used in current CME catalogues, and efforts are underway to automate this procedure for applying to a large number of CME observations for future analysis. It was found that CMEs do not simply undergo constant acceleration, but rather tend to show a higher acceleration early in their propagation. The angular width of CMEs was also found to change as they propagate, normally increasing with height from the Sun. However these results were derived from plane-of-sky measurements with no correction for how the true CME geometry and direction affect the kinematics and morphology observed. With the advent of the unique dual perspectives of the STEREO spacecraft, the multiscale methods were extended to an elliptical tie-pointing technique in order reconstruct the front of a CME in three-dimensions. Applying this technique to the Earth-directed CME of 12 December 2008 allowed an accurate determination of its true kinematics and morphology, and the CME was found to undergo early acceleration, non-radial motion, angular width expansion, and aerodynamic drag in the solar wind as it propagated towards Earth.

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.

The Measurement of Solar Diameter and Limb Darkening Function with the Eclipse Observations [Replacement]

astro-ph — Mon, 20 Feb 2012 01:38:23 +0000

The Total Solar Irradiance varies over a solar cycle of 11 years and maybe over cycles with longer period. Is the solar diameter variable over time too? We introduce a new method to perform high resolution astrometry of the solar diameter from the ground, through the observations of eclipses by reconsidering the definition of the solar edge. A discussion of the solar diameter and its variations must be linked to the Limb Darkening Function (LDF) using the luminosity evolution of a Baily’s Bead and the profile of the lunar limb available from satellite data. This approach unifies the definition of solar edge with LDF inflection point for eclipses and drift-scan or heliometric methods. The method proposed is applied for the videos of the eclipse in 15 January 2010 recorded in Uganda and in India. The result shows light at least 0.85 arcsec beyond the inflection point, and this suggests to reconsider the evaluations of the historical eclipses made with naked eye.

Evidence for a non-universal stellar initial mass function in low-redshift high-density early-type galaxies [Replacement]

astro-ph — Mon, 20 Feb 2012 01:37:47 +0000

We determine an absolute calibration of stellar mass-to-light ratios for the densest \simeq 3% of early-type galaxies in the local universe (redshift z\simeq 0.08) from SDSS DR7. This sample of \sim 4000 galaxies has, assuming a Chabrier IMF, effective stellar surface densities, Sigma_e > 2500 M_sun/pc^2, stellar population synthesis (SPS) stellar masses log_10(M_sps/M_sun)<10.8, and aperture velocity dispersions of sigma_ap=168^{+37}_{-34} km/s (68% range). In contrast to typical early-type galaxies, we show that these dense early-type galaxies follow the virial fundamental plane, which suggests that mass-follows-light. With the additional assumption that any dark matter does not follow the light, the dynamical masses of dense galaxies provide a direct measurement of stellar masses. Our dynamical masses (M_dyn), obtained from the spherical Jeans equations, are only weakly sensitive to the choice of anisotropy (\beta) due to the relatively large aperture of the SDSS fiber for these galaxies: R_ap \simeq 1.5 R_e. Assuming isotropic orbits (\beta=0) we find a median log_{10} (M_dyn/M_sps) = 0.233 \pm 0.003, consistent with a Salpeter IMF, while more bottom heavy IMFs and standard Milky-Way IMFs are strongly disfavored. Our results are consistent with, but do not require, a dependence of the IMF on dynamical mass or velocity dispersion. We find evidence for a color dependence to the IMF such that redder galaxies have heavier IMFs with M_dyn/M_sps \propto (g-r)^{1.13\pm0.09}. This may reflect a more fundamental dependence of the IMF on the age or metallicity of a stellar population, or the density at which the stars formed.

Kinetic cascade beyond magnetohydrodynamics of solar wind turbulence in two-dimensional hybrid simulations [Replacement]

astro-ph — Mon, 20 Feb 2012 01:33:52 +0000

The nature of solar wind turbulence in the dissipation range at scales much smaller than the large MHD scales remains under debate. Here a two-dimensional model based on the hybrid code abbreviated as A.I.K.E.F. is presented, which treats massive ions as particles obeying the kinetic Vlasov equation and massless electrons as a neutralizing fluid. Up to a certain wavenumber in the MHD regime, the numerical system is initialized by assuming a superposition of isotropic Alfv\’en waves with amplitudes that follow the empirically confirmed spectral law of Kolmogorov. Then turbulence develops and energy cascades into the dispersive spectral range, where also dissipative effects occur. Under typical solar wind conditions, weak turbulence develops as a superposition of normal modes in the kinetic regime. Spectral analysis in the direction parallel to the background magnetic field reveals a cascade of left-handed Alfv\’en/ion-cyclotron waves up to wave vectors where their resonant absorption sets in, as well as a continuing cascade of right-handed fast-mode and whistler waves. Perpendicular to the background field, a broad turbulent spectrum is found to be built up of fluctuations having a strong compressive component. Ion-Bernstein waves seem to be possible normal modes in this propagation direction for lower driving amplitudes. Also signatures of short-scale pressure-balanced structures (very oblique slow-mode waves) are found.

The proper motion of PSR J1550-5418 measured with VLBI: a second magnetar velocity measurement [Replacement]

astro-ph — Mon, 20 Feb 2012 01:33:41 +0000

The formation mechanism of neutron stars with extremely large magnetic field strengths (magnetars) remains unclear. Some formation scenarios predict that magnetars should be born with extremely high space velocities, >1000 km/s. Using the Long Baseline Array in Australia, we have measured the proper motion of the intermittently radio-bright magnetar J1550-5418 (1E 1547.0-5408) to be 9.2 +/- 0.6 mas/yr. For a likely distance of 6 +/- 2 kpc, the implied transverse velocity is 280 + 130 – 120 km/s after correcting for Galactic rotation. Along with the ~200 km/s transverse velocity measured for the magnetar XTE J1810-197, this result suggests that formation pathways producing large magnetic fields do not require very large birth kicks.

Some CoRoT highlights – A grip on stellar physics and beyond [Replacement]

astro-ph — Mon, 20 Feb 2012 01:33:30 +0000

About 2 years ago, back in 2009, the first CoRoT Symposium was the occasion to present and discuss unprecedented data revealing the behaviour of stars at the micromagnitude level. Since then, the observations have been going on, the target sample has enriched and the work of analysis of these data keeps producing first rank results. These analyses are providing the material to address open questions of stellar structure and evolution and to test the so many physical processes at work in stars. Based on this material, an increasing number of interpretation studies is being published, addressing various key aspects: the extension of mixed cores, the structure of near surface convective zones, magnetic activity, mass loss, … Definitive conclusions will require cross-comparison of results on a larger ground (still being built), but it is already possible at the time of this Second CoRoT Symposium, to show how the various existing results take place in a general framework and contribute to complete our initial scientific objectives. A few results already reveal the potential interest in considering stars and planets globally, as it is stressed in several talks at this symposium. It is also appealing to consider the fast progress in the domain of Red Giants and see how they illustrate the promising potential of space photometry beyond the field of stellar physics, in connex fields like Galactic dynamics and evolution.

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).

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.

Numerical simulations of strong incompressible magnetohydrodynamic turbulence [Cross-Listing]

astro-ph — Fri, 17 Feb 2012 01:48:10 +0000

Magnetised plasma turbulence pervades the universe and is likely to play an important role in a variety of astrophysical settings. Magnetohydrodynamics (MHD) provides the simplest theoretical framework in which phenomenological models for the turbulent dynamics can be built. Numerical simulations of MHD turbulence are widely used to guide and test the theoretical predictions; however, simulating MHD turbulence and accurately measuring its scaling properties is far from straightforward. Computational power limits the calculations to moderate Reynolds numbers and often simplifying assumptions are made in order that a wider range of scales can be accessed. After describing the theoretical predictions and the numerical approaches that are often employed in studying strong incompressible MHD turbulence, we present the findings of a series of high-resolution direct numerical simulations. We discuss the effects that insufficiencies in the computational approach can have on the solution and its physical interpretation.

The Photometric Period and Variability of the Cataclysmic Variable V849 Herculis (PG 1633+115)

astro-ph — Fri, 17 Feb 2012 01:46:21 +0000

We report time-resolved photometry of the cataclysmic variable V849 Her, and measure a period of 0.1414 \pm 0.0030 days (3.394 \pm 0.072 hours). We also present photometry taken over several weeks in 2010 and 2011, as well as light curves from 1995 to 2011 by the American Association of Variable Star Observers. The spectra, absolute magnitude derived from infrared magnitudes, and variability all suggest that V849 Her is a nova-like variable. The shallow (0.5-magnitude) low states we observe resemble the erratic low states of the VY Sculptoris stars, although they may recur quasi-periodically over an average cycle of 12.462 \pm 0.074 days.

The spectroscopic observations of CoRoT asteroseismic targets with HARPS

astro-ph — Fri, 17 Feb 2012 01:45:41 +0000

CoRoT photometric measurements of asteroseismic targets need complementary ground-based spectroscopic observations. We are using the planet-hunter HARPS spectrograph attached to the 3.6m-ESO telescope in the framework of two consecutive Large Programmes. We discuss its use to study line-profile variations and we report on a specific result obtained for the Delta Sct star HD 170699.

Evolutionary changes in the periods of galactic RRab stars

astro-ph — Fri, 17 Feb 2012 01:45:26 +0000

Le Borgne et al. (2007) report on the determination of evolutionary changes in the periods of field RR Lyr stars. Thanks to the extension of the GEOS database, we could analyze a sample twice larger than the previous one. We obtained a different picture of the period changes, with a number of stars showing an increasing period greater than that of stars showing a decreasing period.

Reconstruction of total solar irradiance 1974-2009

astro-ph — Fri, 17 Feb 2012 01:45:07 +0000

Context: The study of variations in total solar irradiance (TSI) is important for understanding how the Sun affects the Earth’s climate. Aims: Full-disk continuum images and magnetograms are now available for three full solar cycles. We investigate how modelled TSI compares with direct observations by building a consistent modelled TSI dataset. The model, based only on changes in the photospheric magnetic flux can then be tested on rotational, cyclical and secular timescales. Methods: We use Kitt Peak and SoHO/MDI continuum images and magnetograms in the SATIRE-S model to reconstruct TSI over cycles 21-23. To maximise independence from TSI composites, SORCE/TIM TSI data are used to fix the one free parameter of the model. We compare and combine the separate data sources for the model to estimate an uncertainty on the reconstruction and prevent any additional free parameters entering the model. Results: The reconstruction supports the PMOD composite as being the best historical record of TSI observations, although on timescales of the solar rotation the IRMB composite provides somewhat better agreement. Further to this, the model is able to account for 92% of TSI variations from 1978 to 2009 in the PMOD composite and over 96% during cycle 23. The reconstruction also displays an inter-cycle, secular decline of 0.20 (+0.12 / -0.09) Wm-2 between cycle 23 minima, in agreement with the PMOD composite. Conclusions: SATIRE-S is able to recreate TSI observations on all timescales of a day and longer over 31 years from 1978. This is strong evidence that changes in photospheric magnetic flux alone are responsible for almost all solar irradiance variations over the last three solar cycles.

How to optimize nonlinear force-free coronal magnetic field extrapolations from SDO/HMI vector magnetograms?

astro-ph — Fri, 17 Feb 2012 01:43:07 +0000

The SDO/HMI instruments provide photospheric vector magnetograms with a high spatial and temporal resolution. Our intention is to model the coronal magnetic field above active regions with the help of a nonlinear force-free extrapolation code. Our code is based on an optimization principle and has been tested extensively with semi-analytic and numeric equilibria and been applied before to vector magnetograms from Hinode and ground based observations. Recently we implemented a new version which takes measurement errors in photospheric vector magnetograms into account. Photospheric field measurements are often due to measurement errors and finite nonmagnetic forces inconsistent as a boundary for a force-free field in the corona. In order to deal with these uncertainties, we developed two improvements: 1.) Preprocessing of the surface measurements in order to make them compatible with a force-free field 2.) The new code keeps a balance between the force-free constraint and deviation from the photospheric field measurements. Both methods contain free parameters, which have to be optimized for use with data from SDO/HMI. Within this work we describe the corresponding analysis method and evaluate the force-free equilibria by means of how well force-freeness and solenoidal conditions are fulfilled, the angle between magnetic field and electric current and by comparing projections of magnetic field lines with coronal images from SDO/AIA. We also compute the available free magnetic energy and discuss the potential influence of control parameters.

RR Lyrae type stars, ST Boo and RR Leo: 2007 Observations and the preliminary results of the frequency analaysis

astro-ph — Fri, 17 Feb 2012 01:42:41 +0000

We present BVR light curves of pulsating stars, ST Boo and RR Leo, obtained between March and September 2007 at the Ankara University Observatory (AUG) and the T\”{U}B{\.I}TAK National Observatory (TUG). Although these observational data are insufficient to obtain the reliable results for a frequency analysis of ST Boo and RR Leo stars, in this study, we tried to investigate the pulsation phenomena of these two stars, as an overview, using the Period04 software package. As preliminary results, we present the possible frequencies for ST Boo and RR Leo.

How to Correctly Stitch Together {\it Kepler} Data of a Blazhko Star

astro-ph — Fri, 17 Feb 2012 01:42:30 +0000

One of the most challenging difficulties that precedes the frequency analysis of {\it Kepler} data for a Blazhko star is stitching together the data from different seasons (quarters). We discuss the preliminary steps in the stitching, detrending and rescaling process using the data for long-term Blazhko stars. We present the process on {\it Kepler} data of a Blazhko star with a variable Blazhko cycle and some first results of our analysis.

Planet-Induced Emission Enhancements in HD 179949: Results from McDonald Observations

astro-ph — Fri, 17 Feb 2012 01:42:01 +0000

We monitored the Ca II H and K lines of HD 179949, a notable star in the southern hemisphere, to observe and confirm previously identified planet induced emission (PIE) as an effect of star-planet interaction. We obtained high resolution spectra (R ~ 53,000) with a signal-to-noise ratio S/N >~ 50 in the Ca II H and K cores during 10 nights of observation at the McDonald Observatory. Wide band echelle spectra were taken using the 2.7 m telescope. Detailed statistical analysis of Ca II K revealed fluctuations in the Ca II K core attributable to planet induced chromospheric emission. This result is consistent with previous studies by Shkolnik et al. (2003). Additionally, we were able to confirm the reality and temporal evolution of the phase shift of the maximum of star-planet interaction previously found. However, no identifiable fluctuations were detected in the Ca II H core. The Al I lambda 3944 A line was also monitored to gauge if the expected activity enhancements are confined to the chromospheric layer. Our observations revealed some variability, which is apparently unassociated with planet induced activity.

Jet-driving protostars identified from infrared observations of the Carina Nebula complex

astro-ph — Fri, 17 Feb 2012 01:40:16 +0000

Aims: Jets are excellent signposts for very young embedded protostars, so we want to identify jet-driving protostars as a tracer of the currently forming generation of stars in the Carina Nebula, which is one of the most massive galactic star-forming regions and which is characterised by particularly high levels of massive-star feedback on the surrounding clouds. Methods: We used archive data to construct large (> 2 deg x 2 deg) Spitzer IRAC mosaics of the Carina Nebula and performed a spatially complete search for objects with excesses in the 4.5 micron band, typical of shock-excited molecular hydrogen emission. We also identified the mid-infrared point sources that are the likely drivers of previously discovered Herbig-Haro jets and molecular hydrogen emission line objects. We combined the Spitzer photometry with our recent Herschel far-infrared data to construct the spectral energy distributions, and used the Robitaille radiative-transfer modelling tool to infer the properties of the objects. Results: The radiative-transfer modelling suggests that the jet sources are protostars with masses between ~1 M_sol and ~10 M_sol that are surrounded by circumstellar disks and embedded in circumstellar envelopes. Conclusions: The estimated protostar masses < 10 M_sol suggest that the current star-formation activity in the Carina Nebula is restricted to low- and intermediate-mass stars. More optical than infrared jets can be observed, indicating that star formation predominantly takes place close to the surfaces of clouds.

Nonlinear fast magnetoacoustic wave interaction with 2D magnetic X-points in the ion cyclotron range of frequencies

astro-ph — Fri, 17 Feb 2012 01:39:45 +0000

Context. This paper investigates the role of the Hall term in the propagation and dissipation of waves which interact with 2D magnetic X-points and considers the effect of the Hall term on the nature of the resulting reconnection. Aims. The goal is to determine how the evolution of a nonlinear fast magnetoacoustic wave pulse, and the behaviour of the oscillatory reconnection which results from the interaction of the pulse with a line-tied 2D magnetic X-point, is affected by the Hall term in the generalised Ohm’s law. Methods. A Lagrangian remap shock-capturing code (Lare2d) is used to study the evolution of an initial fast magnetoacoustic wave annulus for a range of values of the ion skin depth (di) in resistive Hall MHD. A magnetic null-point finding algorithm is also used to locate and track the evolution of the multiple null-points that are formed in the system. Results. In general, the fast wave is coupled to a shear wave and, for finite di, to whistler and ion cyclotron waves. Dispersive whistler effects cause rapid oscillations of the X-point, which (in combination with the arrival of the main body of the pulse) leads to the creation of magnetic islands and multiple null points under the influence of the Hall term. At later times, competition of local Lorentz and gas pressure forces return the system to a near-equilibrium state. The rate of oscillatory reconnection recovered during this latter phase appears to be unaffected by the value of di.

Seismic evidence for non-synchronization in two close sdB+dM binaries from Kepler photometry

astro-ph — Fri, 17 Feb 2012 01:39:21 +0000

We report on extended photometry of two pulsating sdB stars in close binaries. For both cases, we use rotational splitting of the pulsation frequencies to show that the sdB component rotates much too slowly to be in synchronous rotation. We use a theory of tidal interaction in binary stars to place limits on the mass ratios that are independent of estimates based on the radial velocity curves. The companions have masses below 0.26 M\odot. The pulsation spectra show the signature of high-overtone g-mode pulsation. One star, KIC 11179657, has a clear sequence of g-modes with equal period spacings as well as several periodicities that depart from that trend. KIC 02991403 shows a similar sequence, but has many more modes that do not fit the simple pattern.

Search for the time dependence of the 137Cs decay constant [Cross-Listing]

astro-ph — Fri, 17 Feb 2012 01:38:06 +0000

Starting from June 2011, the activity of a 137Cs source has been measured by means of a HPGe detector installed deep underground in the Gran Sasso Laboratory. In total about 5100 energy spectra, one hour measuring time each, have been collected. These data allowed the search for time variations of the decay constant with periods from a few hours to 1 year. No signal with amplitude larger than 9.6{\cdot}10-5 at 95% C.L. has been detected. These limits are more than one order of magnitude lower than the values on the oscillation amplitude reported in literature. In particular, for 1 year period an oscillation amplitude larger than 8.5{\cdot}10-5 has been excluded at 95% C.L., independently of the phase. The same data give a value of 29.96{\pm}0.08 years for the 137Cs half life, in good agreement with the world mean value of 30.05{\pm}0.08 years.

Metal Abundance Calibration of the Ca II Triplet Lines in RR Lyrae Stars [Replacement]

astro-ph — Fri, 17 Feb 2012 01:36:49 +0000

The GAIA satellite is likely to observe thousands of RR Lyrae stars within a small spectral window, between 8470A and 8750A, at a resolution of 11,500. In order to derive the metallicity of RR Lyrae stars from Gaia, we have obtained numerous spectra of RR Lyrae stars at a resolution of 35,000 with the Apache Point Observatory 3.5 m echelle spectrograph. We have correlated the Ca II triplet line strengths with metallicity as derived from Fe II abundances, analogous to Preston’s (1959) use of the Ca II K line to estimate the metallicity of RR Lyrae stars. The Ca II line at 8498A is the least blended with neighboring Paschen lines and thus provides the best correlation.

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.

UV Properties of Galactic Globular Clusters with GALEX I. The Color-Magnitude Diagrams [Replacement]

astro-ph — Fri, 17 Feb 2012 01:33:34 +0000

We present GALEX data for 44 Galactic globular clusters obtained during 3 GALEX observing cycles between 2004 and 2008. This is the largest homogeneous data set on the UV photometric properties of Galactic globular clusters ever collected. The sample selection and photometric analysis are discussed, and color-magnitude diagrams are presented. The blue and intermediate-blue horizontal branch is the dominant feature of the UV color-magnitude diagrams of old Galactic globular clusters. Our sample is large enough to display the remarkable variety of horizontal branch shapes found in old stellar populations. Other stellar types that are obviously detected are blue stragglers and post core-He burning stars. The main features of UV color-magnitude diagrams of Galactic globular clusters are briefly discussed. We establish the locus of post-core He burning stars in the UV color-magnitude diagram and present a catalog of candidate AGB-manqu \’e, post early-AGB, and post-AGB stars within our cluster sample.

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.

Variability in X-ray line ratios in helium-like ions of massive stars: the radiation-driven case

astro-ph — Thu, 16 Feb 2012 01:55:35 +0000

Line ratios in “fir” triplets of helium-like ions have proven to be a powerful diagnostic of conditions in X-ray emitting gas surrounding massive stars. Recent observations indicate that these ratios can be variable with time. The possible causes of variation in line ratios are limited: changes in the radiation field or changes in density, and changes in mass-loss or geometry. In this paper, we investigate the ability of changes in the radiation field to induce variability in the ratio R=f/i. To isolate the radiative effect, we use a heuristic model of temperature and radius changes in variable stars in the B and O range with low-density, steady-state winds. We model the changes in emissivity of X-ray emitting gas close to the star due to differences in level-pumping from available UV photons at the location of the gas. We find that under these conditions, variability in R is dominated by the stellar temperature. Although the relative amplitude of variability is roughly comparable for most lines at most temperatures, detectable variations are limited to a few lines for each spectral type. We predict that variable values in R due to stellar variability must follow predictable trends found in our simulations. Our model uses radial pulsations as a mode of stellar variability that maximizes the amplitude of variation in R. This model is robust enough to show which ions will provide the best opportunity for observing variability in the f/i ratio at different stellar temperatures, and the correlation of that variability with other observable parameters. In real systems, the effects would be more complex than in our model, with differences in phase and suppressed amplitude in the presence of non-radial pulsations. This suggests that changes in R across many lines concurrently are not likely to be produced by a variable radiation field.

Magnetic field amplification during gravitational collapse – Influence of initial conditions on dynamo evolution and saturation

astro-ph — Thu, 16 Feb 2012 01:53:02 +0000

We study the influence of initial conditions on the magnetic field amplification during the collapse of a magnetised gas cloud. We focus on the dependence of the growth and saturation level of the dynamo generated field on the turbulent properties of the collapsing cloud. In particular, we explore the effect of varying the initial strength and injection scale of turbulence and the initial uniform rotation of the collapsing magnetised cloud. In order to follow the evolution of the magnetic field in both the kinematic and the nonlinear regime, we choose an initial field strength of $\simeq 1\,\mkG$ with the magnetic to kinetic energy ratio, $E_{\rm m}/E_{\rm k} \sim 10^{-4}$. Both gravitational compression and the small-scale dynamo initially amplify the magnetic field. Further into the evolution, the dynamo-generated magnetic field saturates but the total magnetic field continues to grow because of compression. The saturation of the small-scale dynamo is marked by a change in the slope of $B/\rho^{2/3}$ and by a shift in the peak of the magnetic energy spectrum from small scales to larger scales. For the range of initial Mach numbers explored in this study, the dynamo growth rate increases as the Mach number increases from $v_{\rm rms}/c_{\rm s}\sim 0.2$ to 0.4 and then starts decreasing from $v_{\rm rms}/c_{\rm s}\sim 1.0$. We obtain saturation values of $E_{\rm m}/E_{\rm k} = 0.2 – 0.3$ for these runs. Simulations with different initial injection scales of turbulence also show saturation at similar levels. For runs with different initial rotation of the cloud, the magnetic energy saturates at $E_{\rm m}/E_{\rm k}\sim 0.2 – 0.4$ of the equipartition value. (Abridged)

Study of candidate Be stars in the Magellanic Clouds using NIR photometry and optical spectroscopy

astro-ph — Thu, 16 Feb 2012 01:52:26 +0000

Mennickent et al.and Sabogal et al.identified a large number of Classical Be (CBe) candidates in the L&SMC based on their photometric variability using the OGLEII database. They classified these stars into four different groups based on the appearance of their variability. We studied the infrared properties of the sample as well as the spectroscopic properties of a subsample. We cross-correlated the optical sample with the IRSF catalog to obtain the J, H, Ks magnitudes of all the four types of stars in the L&SMC. Spectra of 120 stars belonging to the types 1, 2 and 3 were analysed to study their spectral properties. Among the four types, the type 4 stars is the dominant group. The NIR colour-colour diagrams suggest that the type 4 stars in the LMC have a subclass, which is not found in our Galaxy or in the SMC. The main type 4 sample which is \sim 49% of the total sample has NIR properties similar to the Galactic CBe stars and the SMC type 4 stars. Though the new subclass of type 4 stars have high E(B – V) \sim 0.75, they are not located close to regions with high reddening. The type 3 stars (\sim 6% & 7.3% in the L&SMC) are found to have large H{\alpha} EW in the SMC and some are found to have large NIR excess. This small fraction of stars are unlikely to be CBe stars. The type 2 stars are found in larger fraction in the SMC, when compared to the LMC. The spectroscopic and the NIR properties suggest that these could be CBe stars. The spectroscopic sample of type 1 stars which show H{\alpha} in emission and confirmed as CBe stars are more abundant in the SMC by a factor of 2.6. If the effect of metallicity is to cause more CBe stars in the SMC, when compared to the LMC, then type 1, type 2 and type 4 stars follow this rule, with an enhancement of 2.6, 2.4 and 1.3 respectively.

Kramers rate theory of ionization and dissociation of bound states [Cross-Listing]

astro-ph — Thu, 16 Feb 2012 01:48:43 +0000

Calculating the microscopic dissociation rate of a bound state, such as a classical diatomic molecule, has been difficult so far. The problem was that standard theories require an energy barrier over which the bound particle (or state) escapes into the preferred low-energy state. This is not the case when the long-range repulsion responsible for the barrier is either absent or screened (as in Cooper pairs, ionized plasma, or biomolecular complexes). We solve this classical problem by accounting for entropic memory at the microscopic level. The theory predicts dissociation rates for arbitrary potentials and is successfully tested on the example of plasma, where it yields an estimate of ionization in the core of Sun in excellent agreement with experiments. In biology, the new theory accounts for crowding in receptor-ligand kinetics and protein aggregation.

A systematic variation of the stellar initial mass function in early-type galaxies

astro-ph — Thu, 16 Feb 2012 01:48:17 +0000

Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars. It depends on the stellar initial mass function (IMF) describing the distribution of stellar masses when the population formed. Consequently knowledge of the IMF is critical to virtually every aspect of galaxy evolution. More than half a century after the first IMF determination, no consensus has emerged on whether it is universal in different galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot be both universal, but they could not break the degeneracy between the two effects. Only recently indications were found that massive elliptical galaxies may not have the same IMF as our Milky Way. Here we report unambiguous evidence for a strong systematic variation of the IMF in early-type galaxies as a function of their stellar mass-to-light ratio, producing differences up to a factor of three in mass. This was inferred from detailed dynamical models of the two-dimensional stellar kinematics for the large Atlas3D representative sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass. Our finding indicates that the IMF depends intimately on a galaxy’s formation history.

Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon

astro-ph — Thu, 16 Feb 2012 01:47:44 +0000

Aims: We demonstrate the universal character of the quiet-Sun chromosphere among inactive stars (solar-type and giants). By assessing the main physical processes, we shed new light on some common observational phenomena. Methods: We discuss measurements of the solar Mt. Wilson S-index, obtained by the Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the established chromospheric basal Ca II K line flux to the Mt. Wilson S-index data of inactive (“flat activity”) stars, including giants. Results: During the unusually deep and extended activity minimum of 2009, the Sun reached S-index values considerably deeper than in any of its previously observed minima. In several brief periods, the Sun coincided exactly with the S-indices of inactive (“flat”, presumed Maunder Minimum-type) solar analogues of the Mt. Wilson sample; at the same time, the solar visible surface was also free of any plages or remaining weak activity regions. The corresponding minimum Ca II K flux of the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson sample are found to be identical to the corresponding Ca II K chromospheric basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a universal phenomenon among inactive stars. Its mixed-polarity magnetic field, generated by a local, “fast” turbulent dynamo finally provides a natural explanation for the minimal soft X-ray emission observed for inactive stars. Given such a local dynamo also works for giant chromospheres, albeit on larger length scales, i.e., l ~ R/g, with R and g as stellar radius and surface gravity, respectively, the existence of giant spicular phenomena and the guidance of mechanical energy toward the acceleration zone of cool stellar winds along flux-tubes have now become traceable.

Elemental and isotopic abundances and chemical evolution of galaxies

astro-ph — Thu, 16 Feb 2012 01:45:12 +0000

Elemental and isotopic abundances are the fossils of galactic archaeology. The observed [X/Fe]-[Fe/H] relations in the Galactic bulge and disk and the mass-metallicity relation of galaxies are roughly reproduced with chemodynamical simulations of galaxies under the standard \Lambda-CDM picture and standard stellar physics. The isotopic ratios such as ^{17,18}O and ^{25,26}Mg may require a refinement of modelling of supernova and asymptotic giant branch stars. The recent observation of the Carbon-rich damped Lyman \alpha system can be reproduced only with faint core-collapse supernovae. This suggests that chemical enrichment by the first stars in the first galaxies is driven not by pair-instability supernovae but by core-collapse supernovae (\sim 20-50M_\odot). The observed F abundances can be reproduced with the neutrino processes of core-collapse supernovae. As in F, the observations of elemental abundances in small systems may requires further complications of chemical enrichment. In globular clusters the relative contribution from low-mass supernovae is likely to be smaller than in the field, while the contribution from massive supernovae seems smaller in dwarf spheroidal galaxies than in the solar neighbourhood.

A Mismatch in the Ultraviolet Spectra between Low-Redshift and Intermediate-Redshift Type Ia Supernovae as a Possible Systematic Uncertainty for Supernova Cosmology [Replacement]

astro-ph — Thu, 16 Feb 2012 01:40:08 +0000

We present Keck high-quality rest-frame ultraviolet (UV) through optical spectra of 21 Type Ia supernovae (SNe Ia) in the redshift range 0.11 < z < 0.37 and a mean redshift of 0.22 that were discovered during the Sloan Digital Sky Survey-II (SDSS-II) SN Survey. Using the broad-band photometry of the SDSS survey, we are able to reconstruct the SN host-galaxy spectral energy distributions (SEDs), allowing for a correction for the host-galaxy contamination in the SN Ia spectra. Comparison of composite spectra constructed from a subsample of 17 high-quality spectra to those created from a low-redshift sample with otherwise similar properties shows that the Keck/SDSS SNe Ia have, on average, extremely similar rest-frame optical spectra but show a UV flux excess. This observation is confirmed by comparing synthesized broad-band colors of the individual spectra, showing a difference in mean colors at the 2.4 – 4.4 sigma level for various UV colors. We further see a slight difference in the UV spectral shape between SNe with low-mass and high-mass host galaxies. Additionally, we detect a relationship between the flux ratio at 2770 and 2900 A and peak luminosity that differs from that observed at low redshift. We find that changing the UV SED of an SN Ia within the observed dispersion can change the inferred distance moduli by ~0.1 mag. This effect only occurs when the data probe the rest-frame UV. We suggest that this discrepancy could be due to differences in the host-galaxy population of the two SN samples or to small-sample statistics.

Spatio-kinematic modelling: Testing the link between planetary nebulae and close binaries [Replacement]

astro-ph — Thu, 16 Feb 2012 01:38:55 +0000

It is widely believed that central star binarity plays an important role in the formation and evolution of aspherical planetary nebulae, however observational support for this hypothesis is lacking. Here, we present the most recent results of a continuing programme to model the morphologies of all planetary nebulae known to host a close binary central star. Initially, this programme allows us to compare the inclination of the nebular symmetry axis to that of the binary plane, testing the theoretical expectation that they will lie perpendicular – to date, all have satisfied this expectation, indicating that each nebula has been shaped by its central binary star. As a greater sample of nebulae are modelled, it will be possible to search for trends connecting the parameters of both nebula and central binary, strengthening our understanding of the processes at work in these objects. I will discuss some of the more obvious comparisons, and their current statuses, as well as the obvious links to common envelope evolution.

Hydrodynamics of Core-Collapse Supernovae at the Transition to Explosion. I. Spherical Symmetry [Replacement]

astro-ph — Thu, 16 Feb 2012 01:37:48 +0000

We study the transition to runaway expansion of an initially stalled core-collapse supernova shock. The neutrino luminosity, mass accretion rate, and neutrinospheric radius are all treated as free parameters. In spherical symmetry, this transition is mediated by a global non-adiabatic instability that develops on the advection time and reaches non-linear amplitude. Here we perform high-resolution, time-dependent hydrodynamic simulations of stalled supernova shocks with realistic microphysics to analyze this transition. We find that radial instability is a sufficient condition for runaway expansion if the neutrinospheric parameters do not vary with time and if heating by the accretion luminosity is neglected. For a given unstable mode, transition to runaway occurs when fluid in the gain region reaches positive specific energy. We find approximate instability criteria that accurately describe the behavior of the system over a wide region of parameter space. The threshold neutrino luminosities are in general different than the limiting value for a steady-state solution. We hypothesize that multidimensional explosions arise from the excitation of unstable large-scale modes of the turbulent background flow, at threshold luminosities that are lower than in the laminar case.

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.

Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval and reanalyzed FORS1/2 observations [Replacement]

astro-ph — Thu, 16 Feb 2012 01:33:22 +0000

Recent spectropolarimetric studies of 7 SPB and $\beta$ Cep stars have suggested that photospheric magnetic fields are more common in B-type pulsators than in the general population of B stars, suggesting a significant connection between magnetic and pulsational phenomena. We present an analysis of new and previously published spectropolarimetric observations of these stars. New Stokes $V$ observations obtained with the high-resolution ESPaDOnS and Narval instruments confirm the presence of a magnetic field in one of the stars ($\epsilon$ Lup), but find no evidence of magnetism in 5 others. A re-analysis of the published longitudinal field measurements obtained with the low-resolution FORS1/2 spectropolarimeters finds that the measurements of all stars show more scatter from zero than can be attributed to Gaussian noise, suggesting the presence of a signal and/or systematic under-estimation of error bars. Re-reduction and re-measurement of the FORS1/2 spectra from the ESO archive demonstrates that small changes in reduction procedure lead to substantial changes in the inferred longitudinal field, and substantially reduces the number of field detections at the 3$\sigma$ level. Furthermore, we find that the published periods are not unique solutions to the time series of either the original or the revised FORS1/2 data. We conclude that the reported field detections, proposed periods and field geometry models for $\alpha$ Pyx, 15 CMa, 33 Eri and V1449 Aql are artefacts of the data analysis and reduction procedures, and that magnetic fields at the reported strength are no more common in SPB/$\beta$ Cep stars than in the general population of B stars.

Thirty years of SN 1980K: Evidence for light echoes [Replacement]

astro-ph — Thu, 16 Feb 2012 01:31:56 +0000

We report optical and mid-infrared photometry of SN 1980K between 2004 and 2010, which show slow monotonic fading consistent with previous spectroscopic and photometric observations made 8 to 17 years after outburst. The slow rate-of-change over two decades suggests that this evolution may result from scattered and thermal light echoes off of extended circumstellar material. We present a semi- analytic dust radiative-transfer model that uses an empirically corrected effective optical depth to provide a fast and robust alternative to full Monte-Carlo radiative transfer modeling for homogenous dust at low to intermediate optical depths. We find that unresolved echoes from a thin circumstellar shell 14-15 lt-yr from the progenitor, and containing about 0.02 Msun of carbon-rich dust, can explain the broadband spectral and temporal evolution. The size, mass and dust composition are in good agreement with the contact discontinuity observed in scattered echoes around SN 1987A. The origin of slowly-changing high-velocity [O I] and Halpha lines is also considered. We propose an origin in shocked high-velocity metal-rich clumps of ejecta, rather than arising in the impact of ejecta on slowly-moving circumstellar material, as is the case with hot spots in SN 1987A.

A study of the kinematics and binary-induced shaping of the planetary nebula HaTr 4 [Replacement]

astro-ph — Thu, 16 Feb 2012 01:31:28 +0000

We present the first detailed spatio-kinematical analysis and modelling of the planetary nebula HaTr 4, one of few known to contain a post-common-envelope central star system. Common envelope evolution is believed to play an important role in the shaping of planetary nebulae, but the exact nature of this role is yet to be understood. High spatial- and spectral- resolution spectroscopy of the [OIII]5007 nebular line obtained with VLT-UVES are presented alongside deep narrowband Ha+[NII]6584 imagery obtained using EMMI-NTT, and together the two are used to derive the three-dimensional morphology of HaTr 4. The nebula is found to display an extended ovoid morphology with an enhanced equatorial region consistent with a toroidal waist – a feature believed to be typical amongst planetary nebulae with post-common-envelope central stars. The nebular symmetry axis is found to lie perpendicular to the orbital plane of the central binary, concordant with the idea that the formation and evolution of HaTr 4 has been strongly influenced by its central binary.

Quantifying the Universality of the Stellar Initial Mass Function in Old Star Clusters

astro-ph — Wed, 15 Feb 2012 02:04:44 +0000

We present a new technique to quantify cluster-to-cluster variations in the observed present-day stellar mass functions of a large sample of star clusters. Our method quantifies these differences as a function of both the stellar mass and the total cluster mass, and offers the advantage that it is insensitive to the precise functional form of the mass function. We applied our technique to data taken from the ACS Survey for Globular Clusters, from which we obtained completeness-corrected stellar mass functions in the mass range 0.25-0.75 M$_{\odot}$ for a sample of 27 clusters. The results of our observational analysis were then compared to Monte Carlo simulations for globular cluster evolution spanning a range of initial mass functions, total numbers of stars, concentrations, and virial radii. We show that the present-day mass functions of the clusters in our sample can be reproduced by assuming an universal initial mass function for all clusters, and that the cluster-to-cluster differences are consistent with what is expected from two-body relaxation. A more complete exploration of the initial cluster conditions will be needed in future studies to better constrain the precise functional form of the initial mass function. This study is a first step toward using our technique to constrain the dynamical histories of a large sample of old Galactic star clusters and, by extension, star formation in the early Universe.

The Gemini NICI Planet-Finding Campaign: Discovery of a Multiple System Orbiting the Young A Star HD 1160

astro-ph — Wed, 15 Feb 2012 02:03:36 +0000

We report the discovery by the Gemini NICI Planet-Finding Campaign of two low-mass companions to the young A0V star HD 1160 at projected separations of 81 +/- 5 AU (HD 1160 B) and 533 +/- 25 AU (HD 1160 C). VLT images of the system taken over a decade for the purpose of using HD 1160 A as a photometric calibrator confirm that both companions are physically associated. By comparing the system to members of young moving groups and open clusters with well-established ages, we estimate an age of 50 (+50,-40) Myr for HD 1160 ABC. While the UVW motion of the system does not match any known moving group, the small magnitude of the space velocity is consistent with youth. Near-IR spectroscopy shows HD 1160 C to be an M3.5 +/- 0.5 star with an estimated mass of 0.22 (+0.03,-0.04) M_Sun, while NIR photometry of HD 1160 B suggests a brown dwarf with a mass of 33 (+12,-9) M_Jup. The very small mass ratio (0.014) between the A and B components of the system is rare for A star binaries, and would represent a planetary-mass companion were HD 1160 A to be slightly less massive then the Sun.

On the evolutionary and pulsation mass of Classical Cepheids: III. the case of the eclipsing binary Cepheid CEP0227 in the Large Magellanic Cloud

astro-ph — Wed, 15 Feb 2012 02:02:47 +0000

We present a new Bayesian approach to constrain the intrinsic parameters (stellar mass, age) of the eclipsing binary system CEP0227 in the LMC. We computed evolutionary models covering a broad range in chemical compositions and in stellar mass. Independent sets of models were constructed either by neglecting or by including a moderate convective core overshooting (beta=0.2) during central H-burning phases. Models were also constructed either by neglecting or by assuming a canonical (eta=0.4,0.8) or an enhanced (eta=4) mass loss rate. The solutions were computed in three different planes: luminosity-temperature, mass-radius and gravity-temperature. By using the Bayes Factor, we found that the most probable solutions were obtained in the gravity-temperature plane with a Gaussian mass prior distribution. The evolutionary models constructed by assuming a moderate convective core overshooting (beta=0.2) and a canonical mass loss rate (eta=0.4) give stellar masses for the primary Cepheid M=4.14^{+0.04}_{-0.05} M_sun and for the secondary M=4.15^{+0.04}_{-0.05} M_sun that agree at the 1% level with dynamical measurements. Moreover, we found ages for the two components and for the combined system t=151^{+4}_{-3} Myr that agree at the 5% level. The solutions based on evolutionary models that neglect the mass loss attain similar parameters, while those ones based on models that either account for an enhanced mass loss or neglect convective core overshooting have lower Bayes Factors and larger confidence intervals. The dependence on the mass loss rate might be the consequence of the crude approximation we use to mimic this phenomenon. By using the isochrone of the most probable solution and a Gaussian prior on the LMC distance, we found a distance modulus 18.53^{+0.02}_{-0.02} mag and a reddening value E(B-V)= 0.142^{+0.005}_{-0.010} mag that agree well with literature estimates.

The Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters

astro-ph — Wed, 15 Feb 2012 01:59:35 +0000

It is now widely accepted that globular cluster red giant branch stars owe their strange abundance patterns to a combination of pollution from progenitor stars and in situ extra mixing. In this hybrid theory a first generation of stars imprint abundance patterns into the gas from which a second generation forms. The hybrid theory suggests that extra mixing is operating in both populations and we use the variation of [C/Fe] with luminosity to examine how efficient this mixing is. We investigate the observed red giant branches of M3, M13, M92, M15 and NGC 5466 as a means to test a theory of thermohaline mixing. The second parameter pair M3 and M13 are of intermediate metallicity and our models are able to account for the evolution of carbon along the RGB in both clusters. Although, in order to fit the most carbon-depleted main-sequence stars in M13 we require a model whose initial [C/Fe] abundance leads to a carbon abundance lower than is observed. Furthermore our results suggest that stars in M13 formed with some primary nitrogen (higher C+N+O than stars in M3). In the metal-poor regime only NGC 5466 can be tentatively explained by thermohaline mixing operating in multiple populations. We find thermohaline mixing unable to model the depletion of [C/Fe] with magnitude in M92 and M15. It appears as if extra mixing is occurring before the luminosity function bump in these clusters. To reconcile the data with the models would require first dredge-up to be deeper than found in extant models.

Approximations for radiative cooling and heating in the solar chromosphere

astro-ph — Wed, 15 Feb 2012 01:49:39 +0000

Context. The radiative energy balance in the solar chromosphere is dominated by strong spectral lines that are formed out of LTE. It is computationally prohibitive to solve the full equations of radiative transfer and statistical equilibrium in 3D time dependent MHD simulations. Aims. To find simple recipes to compute the radiative energy balance in the dominant lines under solar chromospheric conditions. Methods. We use detailed calculations in time-dependent and 2D MHD snapshots to derive empirical formulae for the radiative cooling and heating. Results. The radiative cooling in neutral hydrogen lines and the Lyman continuum, the H and K and intrared triplet lines of singly ionized calcium and the h and k lines of singly ionized magnesium can be written as a product of an optically thin emission (dependent on temperature), an escape probability (dependent on column mass) and an ionization fraction (dependent on temperature). In the cool pockets of the chromosphere the same transitions contribute to the heating of the gas and similar formulae can be derived for these processes. We finally derive a simple recipe for the radiative heating of the chromosphere from incoming coronal radiation. We compare our recipes with the detailed results and comment on the accuracy and applicability of the recipes.

Effects of cyclic fluctuations in meridional circulation using a low order dynamo model

astro-ph — Wed, 15 Feb 2012 01:48:02 +0000

We develop and subsequently explore the solution space of a simple flux transport dynamo model that incorporates a time dependent large scale meridional circulation. Based on recent observations we prescribed an analytical form for the amplitude of this circulation and study its impact in the evolution of the magnetic field. We find that cyclic variations in the amplitude and frequency of the meridional flow affect the strength of the solar cycle. Variations in the amplitude of the fluctuations influence the shape of the solar cycle but are only relevant to the cycle’s strength variations when they occur at a frequency different from or out of phase of the solar cycle’s.

The pygmy dipole strength, the neutron radius of ${}^{208}$Pb and the symmetry energy [Cross-Listing]

astro-ph — Wed, 15 Feb 2012 01:47:31 +0000

The accurate characterization of the nuclear symmetry energy and its density dependence is one of the outstanding open problems in nuclear physics. A promising nuclear observable in order to constrain the density dependence of the symmetry energy at saturation is the neutron skin thickness of medium and heavy nuclei. Recently, a low-energy peak in the isovector dipole response of neutron-rich nuclei has been discovered that may be correlated with the neutron skin thickness. The existence of this correlation is currently under debate due to our limited experimental knowledge on the microscopic structure of such a peak. We present a detailed analysis of Skyrme Hartree-Fock (HF) plus random phase approximation (RPA) predictions for the dipole response in several neutron-rich nuclei and try to elucidate whether models of common use in nuclear physics confirm or dismiss its possible connection with the neutron skin thickness. Finally, we briefly present theoretical results for parity violating electron scattering on ${}^{208}$Pb at the conditions of the PREx experiment and discuss the implications for the neutron skin thickness of ${}^{208}$Pb and the slope of the symmetry energy.

Formation of a disk-structure and jets in the symbiotic prototype Z And during its 2006-2010 active phase

astro-ph — Wed, 15 Feb 2012 01:46:36 +0000

We present an analysis of spectrophotometric observations of the latest cycle of activity of the symbiotic binary Z And from 2006 to 2010. We estimate the temperature of the hot component of Z And to be \approx 150000 – 170000 K at minimum brightness, decreasing to \approx 90000 K at the brightness maximum. Our estimate of the electron density in the gaseous nebula is N_{e}=10^{10}-10^{12} cm^{-3} in the region of formation of lines of neutral helium and 10^6-10^7 cm^{-3} in the region of formation of the [OIII] and [NeIII] nebular lines. A trend for the gas density derived from helium lines to increase and the gas density derived from [OIII] and [NeIII] lines to simultaneously decrease with increasing brightness of the system was observed. Our estimates show that the ratios of the theoretical and observed fluxes in the [OIII] and [NeIII] lines agree best when the O/Ne ratio is similar to its value for planetary nebulae. The model spectral energy distribution showed that, in addition to a cool component and gaseous nebula, a relatively cool pseudophotosphere (5250-11 500 K) is present in the system. The simultaneous presence of a relatively cool pseudophotosphere and high-ionization spectral lines is probably related to a disk-like structure of the pseudophotosphere. The pseudophotosphere formed very rapidly, over several weeks, during a period of increasing brightness of Z And. We infer that in 2009, as in 2006, the activity of the system was accompanied by a collimated bipolar ejection of matter. In contrast to the situation in 2006, the jets were detected even before the system reached its maximum brightness. Moreover, components with velocities close to 1200 km/s disappeared at the maximum, while those with velocities close to 1800 km/s appeared.

Thirty years of SN 1980K: Evidence for light echoes

astro-ph — Wed, 15 Feb 2012 01:44:48 +0000

The M Dwarf Problem in the Galaxy

astro-ph — Wed, 15 Feb 2012 01:44:35 +0000

We present evidence that there is an M dwarf problem similar to the previously identified G dwarf and K dwarf problems: the number of low-metallicity M dwarfs is not sufficient to match simple closed-box models of local Galactic chemical evolution. We estimated the metallicity of 4141 M dwarf stars with spectra from the Sloan Digital Sky Survey (SDSS) using a molecular band strength versus metallicity calibration developed using high resolution spectra of nearby M dwarfs. Using a sample of M dwarfs with measured magnitudes, parallaxes, and metallicities, we derived a relation that describes the absolute magnitude variation as a function of metallicity. When we examined the metallicity distribution of SDSS stars, after correcting for the different volumes sampled by the magnitude-limited survey, we found that there is an M dwarf problem, with the number of M dwarfs at [Fe/H] ~ -0.5 less than 1% the number at [Fe/H] = 0, where a simple model of Galactic chemical evolution predicts a more gradual drop in star numbers with decreasing metallicity.

A BVRcIc Survey of W Ursae Majoris Binaries

astro-ph — Wed, 15 Feb 2012 01:42:56 +0000

We report on a BVRcIc survey of field W Ursae Majoris binary stars and present accurate colors for 606 systems that have been observed on at least three photometric nights from a robotic observatory in southern Arizona. Comparison with earlier photometry for a subset of the systems shows good agreement. We investigate two independent methods of determining the interstellar reddening, although both have limitations that can render them less effective than desired. A subset of 101 systems shows good agreement between the two reddening methods.

A study of the kinematics and binary-induced shaping of the planetary nebula HaTr 4

astro-ph — Wed, 15 Feb 2012 01:42:44 +0000

The properties of dynamically ejected runaway and hyper-runaway stars

astro-ph — Tue, 14 Feb 2012 02:19:52 +0000

Runaway stars are stars observed to have large peculiar velocities. Two mechanisms are thought to contribute to the ejection of runaway stars, both involve binarity (or higher multiplicity). In the binary supernova scenario a runaway star receives its velocity when its binary massive companion explodes as a supernova (SN). In the alternative dynamical ejection scenario, runaway stars are formed through gravitational interactions between stars and binaries in dense, compact clusters or cluster cores. Here we study the ejection scenario. We make use of extensive N-body simulations of massive clusters, as well as analytic arguments, in order to to characterize the expected ejection velocity distribution of runaways stars. We find the ejection velocity distribution of the fastest runaways (\sim80 km s^-1) depends on the binary distribution in the cluster, consistent with our analytic toy model, whereas the distribution of lower velocity runaways appears independent of the binaries properties. For a realistic log constant distribution of binary separations, we find the velocity distribution to follow a simple power law; Gamma(v)\simv^(-8/3) for the high velocity runaways and v^(-3/2) for the low velocity ones. We calculate the total expected ejection rates of runaway stars from our simulated massive clusters and explore their mass function and their binarity. The mass function of runaway stars is biased towards high masses, and depends strongly on their velocity. The binarity of runaways is a decreasing function of their ejection velocity, with no binaries expected to be ejected with v>150 km s^-1. We also find that hyper-runaways with velocities of hundreds of km s^-1 can be dynamically ejected from stellar clusters, but only at very low rates, which cannot account for a significant fraction of the observed population of hyper-velocity stars in the Galactic halo.

Critical Conditions for Core-Collapse Supernovae

astro-ph — Tue, 14 Feb 2012 02:17:33 +0000

The explosion of a core collapse supernova is approximated by the breakdown of the steady-state solution for accretion onto a proto-neutron-star (PNS). We analytically show that as the neutrino luminosity exceeds a critical value L_c, the neutrinosphere pressure exceeds the hydrostatic limit even for an optimal shock radius R. This yields L_c \propto M^2 T^2 (with logarithmic corrections) and R \propto M/T, in agreement with numerical results, where M, T are the PNS mass, neutrino temperature. The near-critical flow can be approximated as a ballistic shell on top of an isothermal layer.

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 2010 August 01 type II burst: A CME-CME Interaction, and its radio and white-light manifestations

astro-ph — Tue, 14 Feb 2012 02:12:41 +0000

We present observational results of a type II burst associated with a CME-CME interaction observed in the radio and white-light wavelength range. We applied radio direction-finding techniques to observations from the STEREO and Wind spacecraft, the results of which were interpreted using white-light coronagraphic measurements for context. The results of the multiple radio-direction finding techniques applied were found to be consistent both with each other and with those derived from the white-light observations of coronal mass ejections (CMEs). The results suggest that the Type II burst radio emission is causally related to the CMEs interaction.

Detection of the Second r-process Peak Element Tellurium in Metal-Poor Stars

astro-ph — Tue, 14 Feb 2012 02:11:19 +0000

Using near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope, we detect neutral tellurium in three metal-poor stars enriched by products of r-process nucleosynthesis, BD+17 3248, HD 108317, and HD 128279. Tellurium (Te, Z=52) is found at the second r-process peak (A=130) associated with the N=82 neutron shell closure, and it has not been detected previously in Galactic halo stars. The derived tellurium abundances match the scaled solar system r-process distribution within the uncertainties, confirming the predicted second peak r-process residuals. These results suggest that tellurium is predominantly produced in the main component of the r-process, along with the rare earth elements.

Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval and reanalyzed FORS1/2 observations

astro-ph — Tue, 14 Feb 2012 02:09:35 +0000

An extensive photometric study of the recently discovered intermediate polar V515 And (XSS J00564+4548)

astro-ph — Tue, 14 Feb 2012 02:05:20 +0000

We report results of photometry of the intermediate polar V515 And. The observations were obtained over 33 nights in 2008 and 2009. The total duration of the observations was 233 h. We clearly detected two oscillations with periods of 465.48493\pm0.00007$ and 488.61822\pm0.00009 s, which may be the white dwarf spin period and the orbital sideband. The semi-amplitudes of the oscillations are 25 and 20 mmag, accordingly. The oscillation with a period of 465.48493 s has a stable smooth asymmetric pulse profile whereas the pulse profile of the oscillation with a period of 488.61822 s reveals significant changes from a quasi-sinusoidal shape to a shape somewhat resembling a light-curve of an eclipsing binary. Two detected oscillations imply an orbital period of 2.73 h. V515 And is one of the most rapidly spinning intermediate polars with orbital periods less than 3 h and may be not in spin equilibrium. This can be proved by future observations. For this purpose we obtained oscillation ephemerises with a formal shelf life of about 100 yr. (a 1 sigma confidence level).