Vox Charta » Galactic

Evidence for top-heavy stellar initial mass functions with increasing density and decreasing metallicity

astro-ph — Thu, 23 Feb 2012 02:02:21 +0000

Residual-gas expulsion after cluster formation has recently been shown to leave an imprint in the low-mass present-day stellar mass function (PDMF) which allowed the estimation of birth conditions of some Galactic globular clusters (GCs) such as mass, radius and star formation efficiency. We show that in order to explain their characteristics (masses, radii, metallicity, PDMF) their stellar initial mass function (IMF) must have been top-heavy. It is found that the IMF is required to become more top-heavy the lower the cluster metallicity and the larger the pre-GC cloud-core density are. The deduced trends are in qualitative agreement with theoretical expectation. The results are consistent with estimates of the shape of the high-mass end of the IMF in the Arches cluster, Westerlund 1, R136 and NGC 3603, as well as with the IMF independently constrained for ultra-compact dwarf galaxies (UCDs). The latter suggests that GCs and UCDs might have formed along the same channel or that UCDs formed via mergers of GCs. A fundamental plane is found which describes the variation of the IMF with density and metallicity of the pre-GC cloud-cores simultaneously. The implications for the evolution of galaxies and chemical enrichment over cosmological times are expected to be major.

Measurement of Galactic Logarithmic Spiral Arm Pitch Angle Using Two-Dimensional Fast Fourier Transform Decomposition

astro-ph — Thu, 23 Feb 2012 01:58:45 +0000

A logarithmic spiral is a prominent feature appearing in a majority of observed galaxies. This feature has long been associated with the traditional Hubble classification scheme, but historical quotes of pitch angle of spiral galaxies have been almost exclusively qualitative. We have developed a methodology, utilizing two-dimensional fast Fourier transformations of images of spiral galaxies, in order to isolate and measure the pitch angles of their spiral arms. Our technique provides a quantitative way to measure this morphological feature. This will allow comparison of spiral galaxy pitch angle to other galactic parameters and test spiral arm genesis theories. In this work, we detail our image processing and analysis of spiral galaxy images and discuss the robustness of our analysis techniques.

Identification of Ambient Molecular Clouds Associated with Galactic Supernova Remnant IC443

astro-ph — Thu, 23 Feb 2012 01:57:15 +0000

The Galactic supernova remnant (SNR) IC443 is one of the most studied core-collapse SNRs for its interaction with molecular clouds. However, the ambient molecular clouds with which IC443 is interacting have not been thoroughly studied and remain poorly understood. Using Five College Radio Astronomy Observatory 14m telescope, we obtained fully sampled maps of ~ 1{\deg} \times 1{\deg} region toward IC443 in the 12CO J=1-0 and HCO+ J=1-0 lines. In addition to the previously known molecular clouds in the velocity range v_lsr = -6 to -1 km/s (-3 km/s clouds), our observations reveal two new ambient molecular cloud components: small (~ 1′) bright clouds in v_lsr = -8 to -3 km/s (SCs), and diffuse clouds in v_lsr = +3 to +10 km/s (+5 km/s clouds). Our data also reveal the detailed kinematics of the shocked molecular gas in IC443, however the focus of this paper is the physical relationship between the shocked clumps and the ambient cloud components. We find strong evidence that the SCs are associated with the shocked clumps. This is supported by the positional coincidence of the SCs with shocked clumps and other tracers of shocks. Furthermore, the kinematic features of some shocked clumps suggest that these are the ablated material from the SCs upon the impact of the SNR shock. The SCs are interpreted as dense cores of parental molecular clouds that survived the destruction by the pre-supernova evolution of the progenitor star or its nearby stars. We propose that the expanding SNR shock is now impacting some of the remaining cores and the gas is being ablated and accelerated producing the shocked molecular gas. The morphology of the +5 km/s clouds suggests an association with IC443. On the other hand, the -3 km/s clouds show no evidence for interaction.

Abundances of PNe in the Outer Disk of M31

astro-ph — Thu, 23 Feb 2012 01:44:54 +0000

We present spectroscopic observations and chemical abundances of 16 planetary nebulae (PNe) in the outer disk of M31. The [O III] 4363 line is detected in all objects, allowing a direct measurement of the nebular temperature essential for accurate abundance determinations. Our results show that the abundances in these M31 PNe display the same correlations and general behaviors as Type II PNe in the Milky Way Galaxy. We also calculate photoionization models to derive estimates of central star properties. From these we infer that our sample PNe, all near the peak of the Planetary Nebula Luminosity Function, originated from stars near 2 M_sun. Finally, under the assumption that these PNe are located in M31’s disk, we plot the oxygen abundance gradient, which appears shallower than the gradient in the Milky Way.

The Gas Properties of the W3 GMC: A HARP study

astro-ph — Thu, 23 Feb 2012 01:40:05 +0000

We present 12CO, 13CO and C18O J=3-2 maps of the W3 GMC made at the James Clerk Maxwell Telescope. We combine these observations with Five Colleges Radio Astronomy Observatory CO J=1-0 data to produce the first map of molecular-gas temperatures across a GMC and the most accurate determination of the mass distribution in W3 yet obtained. We measure excitation temperatures in the part of the cloud dominated by triggered star formation (the High Density Layer, HDL) of 15-30 K, while in the rest of the cloud, which is relatively unaffected by triggering (Low Density Layer, LDL), the excitation temperature is generally less than 12 K. We identify a temperature gradient in the HDL which we associate with an age sequence in the embedded massive star-forming regions. We measure the mass of the cloud to be 4.4+/-0.4 x 10^5 solar masses, in agreement with previous estimates. Existing sub-mm continuum data are used to derive the fraction of gas mass in dense clumps as a function of position in the cloud. This fraction, which we interpret as a Clump Formation Efficiency (CFE), is significantly enhanced across the HDL, probably due to the triggering. Finally, we measure the 3D rms Mach Number as a function of position and find a correlation between the Mach number and the CFE within the HDL only. This correlation is interpreted as due to feedback from the newly-formed stars and a change in its slope between the three main star-forming regions is construed as another evolutionary effect. We conclude that triggering has affected the star-formation process in the W3 GMC primarily by creating additional dense structures that can collapse into stars. Any traces of changes in CFE due to additional turbulence have since been overruled by the feedback effects of the star-forming process itself.

Chandra Survey of Nearby Edge-on Disk Galaxies I: Luminosities of Galactic Coronae [Replacement]

astro-ph — Thu, 23 Feb 2012 01:36:22 +0000

X-ray-emitting coronae of nearby galaxies are expected to be produced either by accretion from the intergalactic medium and/or by various galactic feedback. We herein present a systematical analysis of the Chandra observations of 53 nearby edge-on disk galaxies over a range of 3 orders of magnitude in SFR. Various coronal properties, such as the luminosity, vertical/horizontal extent, and other inferred parameters, are characterized for all the sample galaxies. For galaxies with high enough counting statistics, we also examine the thermal and chemical states of the coronal gas. Here we concentrate on the coronal luminosity (Lx), estimated in 0.5-2keV and within 5 times the diffuse X-ray vertical scale height. We find Lx strongly correlates with the SFR for the whole sample. But the inclusion of Ia SNe in the total energy input (E_SN) gives an even tighter correlation, which may be characterized with a linear relation, Lx=0.5%E_SN, and with a dispersion of 0.45dex. Moreover, the coronal radiation efficiency (\eta=Lx/E_SN) shows little correlation with either the stellar mass or the gravitational mass (M_TF, inferred from the rotation velocity), but is significantly correlated with their ratio (M_TF/M_*), which may be expressed as a linear scaling relation \eta=0.35%M_TF/M_* for the entire ranges of galaxy parameters. This joint scaling relation suggests that the coronae are self-regulated by the combination of gravitational confinement and feedback. But SN appears to be the primary heating source, because about half of our galaxies are not massive enough to allow for the accretion to play a major role. The commonly low \eta further suggests that the bulk of the SN energy likely flows out into large-scale galactic halos for essentially all the galaxies. Such ubiquitous outflows could have profound implications for understanding the ecosystem, hence the evolution of galaxies.

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.

The Lesser Role of Shear in Star Formation: Insight from the Galactic Ring Survey [Replacement]

astro-ph — Thu, 23 Feb 2012 01:31:38 +0000

We analyse the role played by shear in regulating star formation in the Galaxy on the scale of individual molecular clouds. The clouds are selected from the $^{13}$CO $J=1-0$ line of the Galactic Ring Survey. We estimate the shear parameter which is the ratio of a critical surface density for the clouds to be disrupted by shear to their actual surface density. We find that for almost all molecular clouds considered in the sample, there is no evidence that shear is playing a significant role in opposing the effects of self-gravity. Furthermore, we find that the shear parameter of the clouds does not depend on their position in the Galaxy, which implies that shear can not explain the radial profiles of the Galactic star formation rates. We also find that for gravitationally bound clouds, higher shear parameters do not imply lower masses nor that the shear parameter correlates with the clouds level of fragmentation. Our results suggest that shear is playing only a minor role in affecting the rates at which gravitationally bound molecular clouds convert their gas into dense cores and thereafter into stars.

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.

The Resolved Structure and Dynamics of an Isolated Dwarf Galaxy: A VLT and Keck Spectroscopic Survey of WLM

astro-ph — Wed, 22 Feb 2012 02:03:17 +0000

We present spectroscopic data for 180 red giant branch stars in the isolated dwarf irregular galaxy WLM. Observations of the Calcium II triplet lines in spectra of RGB stars covering the entire galaxy were obtained with FORS2 at the VLT and DEIMOS on Keck II allowing us to derive velocities, metallicities, and ages for the stars. With accompanying photometric and radio data we have measured the structural parameters of the stellar and gaseous populations over the full galaxy. The stellar populations show an intrinsically thick configuration with $0.39 \leq q_{0} \leq 0.57$. The stellar rotation in WLM is measured to be $17 \pm 1$ km s$^{-1}$, however the ratio of rotation to pressure support for the stars is $V/\sigma \sim 1$, in contrast to the gas whose ratio is seven times larger. This, along with the structural data and alignment of the kinematic and photometric axes, suggests we are viewing WLM as a highly inclined oblate spheroid. Stellar rotation curves, corrected for asymmetric drift, are used to compute a dynamical mass of $4.3\pm 0.3\times10^{8} $M$_{\odot}$ at the half light radius ($r_{h} = 1656 \pm 49$ pc). The stellar velocity dispersion increases with stellar age in a manner consistent with giant molecular cloud and substructure interactions producing the heating in WLM. Coupled with WLM’s isolation, this suggests that the extended vertical structure of its stellar and gaseous components and increase in stellar velocity dispersion with age are due to internal feedback, rather than tidally driven evolution. These represent some of the first observational results from an isolated Local Group dwarf galaxy which can offer important constraints on how strongly internal feedback and secular processes modulate SF and dynamical evolution in low mass isolated objects.

First Science Observations with SOFIA/FORCAST: 6 to 37 micron Imaging of the Central Orion Nebula

astro-ph — Wed, 22 Feb 2012 02:03:01 +0000

We present new mid-infrared images of the central region of the Orion Nebula using the newly commissioned SOFIA airborne telescope and its 5 — 40 micron camera FORCAST. The 37.1 micron images represent the highest resolution observations (<4") ever obtained of this region at these wavelengths. After BN/KL (which is described in a separate letter in this issue), the dominant source at all wavelengths except 37.1 micron is the Ney-Allen Nebula, a crescent-shaped extended source associated with theta 1D. The morphology of the Ney-Allen nebula in our images is consistent with the interpretation that it is ambient dust swept up by the stellar wind from theta 1D, as suggested by Smith et al. (2005). Our observations also reveal emission from two "proplyds" (proto-planetary disks), and a few embedded young stellar objects (YSOs; IRc9, and OMC1S IRS1, 2, and 10). The spectral energy distribution for IRc9 is presented and fitted with standard YSO models from Robitaille et al. (2007) to constrain the total luminosity, disk size, and envelope size. The diffuse, nebular emission we observe at all FORCAST wavelengths is most likely from the background photodissociation region (PDR) and shows structure that coincides roughly with H_alpha and [N II] emission. We conclude that the spatial variations in the diffuse emission are likely due to undulations in the surface of the background PDR.

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.

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.

Observations and Analysis of High-Resolution Magnetic Field Structures in Molecular Clouds

astro-ph — Wed, 22 Feb 2012 01:48:06 +0000

Recent high-angular-resolution (up to 0.7″) dust polarization observations toward star forming regions are summarized. With the Sub-Millimeter Array, the emission from the dense structures is traced and resolved. The detected magnetic field morphologies vary from hourglass-like structures to isolated patches depending on the evolutionary stage of the source. These observed features have also served as a testbed to develop new analysis methods, with a particular focus on quantifying the role of the magnetic field in the star formation process.

Mergers and ejections of black holes in globular clusters

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

The Lesser Role of Shear in Star Formation: Insight from the Galactic Ring Survey

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

Fermi-LAT Observations of the Diffuse Gamma-Ray Emission: Implications for Cosmic Rays and the Interstellar Medium [Replacement]

astro-ph — Wed, 22 Feb 2012 01:33:25 +0000

The gamma-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi-LAT mission and compare with models of the diffuse gamma-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the Xco-factor, the ratio between integrated CO-line intensity and molecular hydrogen column density, the fluxes and spectra of the gamma-ray point sources from the first Fermi-LAT catalogue, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as gamma rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but under-predict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point source populations and spectral variations of cosmic rays throughout the Galaxy. [Abridged]

Fermi-LAT Observations of the Diffuse Gamma-Ray Emission: Implications for Cosmic Rays and the Interstellar Medium

astro-ph — Tue, 21 Feb 2012 02:07:03 +0000

Gaia: a Window to Large Scale Flows

astro-ph — Tue, 21 Feb 2012 02:03:36 +0000

Using redshifts as proxy for galaxy distances, estimates of the 2D transverse peculiar velocities of distant galaxies ($cz\ltsim 2\times 10^4 \kms$) can be obtained from Gaia’s measurements of proper motions. Owing to the large number of galaxies expected to be observed by Gaia, these transverse velocities are likely to supersede traditional probes of the large scale velocity field based on current and future distance indicator measurements. This Gaia probe of large scale motions is completely independent of any intrinsic relations between galaxy properties, hence it is essentially free of selection biases. It is also free from {homogeneous and} inhomogeneous Malmquist biases that typically plague distance indicator catalogs. Further, it provides additional information to traditional probes which yield line-of-sight peculiar velocities.

No role for muons in the DAMA annual modulation results

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

Since long time it has been pointed out by the DAMA collaboration that muons surviving the Gran Sasso mountain cannot mimic the Dark Matter annual modulation signature exploited by the DAMA/NaI and DAMA/LIBRA experiments. In the present paper, these and further arguments are gathered together in order to enable a wider community to suitably approach this point.

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.

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.

3 to 12 millimetre studies of dense gas towards the western rim of supernova remnant RX J1713.7-3946

astro-ph — Tue, 21 Feb 2012 01:58:13 +0000

The young X-ray and gamma-ray-bright supernova remnant RXJ1713.7-3946 (SNR G347.3-0.5) is believed to be associated with molecular cores that lie within regions of the most intense TeV emission. Using the Mopra telescope, four of the densest cores were observed using high-critical density tracers such as CS(J=1-0,J=2-1) and its isotopologue counterparts, NH3(1,1) and (2,2) inversion transitions and N2H+(J=1-0) emission, confirming the presence of dense gas >10^4cm^-3 in the region. The mass estimates for Core C range from 40M_{\odot} (from CS(J=1-0)) to 80M_{\odot} (from NH3 and N2H+), an order of magnitude smaller than published mass estimates from CO(J=1-0) observations. We also modelled the energy-dependent diffusion of cosmic-ray protons accelerated by RXJ1713.7-3946 into Core C, approximating the core with average density and magnetic field values. We find that for considerably suppressed diffusion coefficients (factors \chi=10^{-3} down to 10^{-5} the galactic average), low energy cosmic-rays can be prevented from entering the inner core region. Such an effect could lead to characteristic spectral behaviour in the GeV to TeV gamma-ray and multi-keV X-ray fluxes across the core. These features may be measurable with future gamma-ray and multi-keV telescopes offering arcminute or better angular resolution, and can be a novel way to understand the level of cosmic-ray acceleration in RXJ1713.7-3946 and the transport properties of cosmic-rays in the dense molecular cores.

Estimation of xi parameter on the Moffat Gravity

astro-ph — Tue, 21 Feb 2012 01:55:11 +0000

Scalar Tensor Vector Gravity(STVG) is one of modified gravity theories developed by John Moffat(2005). MOG is abbreviated name for this theory.It can explain a galactic rotation curve and the structure formation without dark matter. It can also explain acceleration universe without dark energy.But,they obtaion only a spherically symmetric, static vacuum solution about MOG. On this theory,the gravitational field produced by two point sources is not simply the sum of their respective spherically symmetric static vacuum solutions. However,in arXiv:0805.4774, the method to adapt MOG to extended distribution of matter is described by phenomenalism. A new parameter “xi” is introduced in this phenomenalical description.This paper shows estimation of MOG’s xi parameter. In conclusion,”xi” should be less than O(10^2) to reproduce “flat” rotation curves observed.

Virial Sequences for Thick Discs and Haloes: Flattening and Global Anisotropy [Replacement]

astro-ph — Tue, 21 Feb 2012 01:34:13 +0000

The virial theorem prescribes the ratio of the globally-averaged equatorial to vertical velocity dispersion of a tracer population in spherical and flattened dark haloes. This gives sequences of physical models in the plane of global anisotropy and flattening. The tracer may have any density, though there are particularly simple results for power-laws and exponentials. We prove the flattening theorem: for a spheroidally stratified tracer density with axis ratio q in a dark density potential with axis ratio g, the ratio of globally averaged equatorial to vertical velocity dispersion depends only on q/g. As the stellar halo density and velocity dispersion of the Milky Way are accessible to observations, this provides a new method for measuring the flattening of the dark matter. If the kinematics of the local halo subdwarfs are representative, then the Milky Way’s dark halo is oblate with a flattening in the potential of g ~ 0.85, corresponding to a flattening in the dark matter density of ~ 0.7. The fractional pressure excess for power-law populations is roughly proportional to both the ellipticity and the fall-off exponent. Given the same pressure excess, if the density profile of one stellar population declines more quickly than that of another, then it must be rounder. This implies that the dual halo structure claimed by Carollo et al. (2007) for the Galaxy, a flatter inner halo and a rounder outer halo, is inconsistent with the virial theorem. For the thick disc, we provide formulae for the virial sequences of double-exponential discs in logarithmic and Navarro-Frenk-White (NFW) haloes. There are good matches to the observational data on the flattening and anisotropy of the thick disc if the thin disc is exponential with a short scalelength ~ 2.6 kpc and normalisation of 56 solar masses per square parsec, together with a logarithmic dark halo.

A Relic Star Cluster in the Sextans Dwarf Spheroidal Galaxy – Implications for Early Star and Galaxy Formation

astro-ph — Mon, 20 Feb 2012 01:48:36 +0000

We present tentative evidence for the existence of a dissolved star cluster in the Sextans dwarf spheroidal galaxy. In a sample of six stars, we identify three (possibly four) stars around [Fe/H] =-2.7 that are highly clustered in a multi-dimensional chemical abundance space. The estimated initial stellar mass of the cluster is M*,init = 1.9^+1.5_-0.9 (1.6^+1.2_-0.8)*10^5 Msol assuming a Salpeter (Kroupa) initial mass function (IMF). If corroborated by follow-up spectroscopy, this ancient star cluster at [Fe/H] =-2.7 is the most metal-poor system identified to date. Inspired by this finding, we also present a new way to interpret the cumulative metallicity functions of dwarf galaxies. From available observational data, we speculate that the ultra-faint dwarf galaxy population, or a significant fraction thereof, and the more luminous, classical dwarf spheroidal population were formed in different environments and would thus be distinct in origin.

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.

Evidence for Multiple Pathways to Deuterium Enhancements in Protoplanetary Disks

astro-ph — Mon, 20 Feb 2012 01:42:26 +0000

The distributions of deuterated molecules in protoplanetary disks are expected to depend on the molecular formation pathways. We use observations of spatially resolved DCN emission from the disk around TW Hya, acquired during ALMA Science verification with a ~3″ synthesized beam, together with comparable DCO+ observations from the Submillimeter Array, to investigate differences in the radial distributions of these species and hence differences in their formation chemistry. In contrast to DCO+, which shows an increasing column density with radius, DCN is better fit by a model that is centrally peaked. We infer that DCN forms at a smaller radii and thus at higher temperatures than DCO+. This is consistent with chemical network model predictions of DCO+ formation from H2D+ at T<30 K and DCN formation from additional pathways involving CH2D+ at higher temperatures. We estimate a DCN/HCN abundance ratio of ~0.017, similar to the DCO+/HCO+ abundance ratio. Deuterium fractionation appears to be efficient at a range of temperatures in this protoplanetary disk. These results suggest caution in interpreting the range of deuterium fractions observed in Solar System bodies, as multiple formation pathways should be taken into account.

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.

Hypervelocity Planets and Transits Around Hypervelocity Stars [Replacement]

astro-ph — Mon, 20 Feb 2012 01:34:29 +0000

The disruption of a binary star system by the massive black hole at the Galactic Centre, SgrA*, can lead to the capture of one star around SgrA* and the ejection of its companion as a hypervelocity star (HVS). We consider the possibility that these stars may have planets and study the dynamics of these planets. Using a direct $N$-body integration code, we simulated a large number of different binary orbits around SgrA*. For some orbital parameters, a planet is ejected at a high speed. In other instances, a HVS is ejected with one or more planets orbiting around it. In these cases, it may be possible to observe the planet as it transits the face of the star. A planet may also collide with its host star. In such cases the atmosphere of the star will be enriched with metals. In other cases, a planet is tidally disrupted by SgrA*, leading to a bright flare.

The embedded cluster or association Trumpler 37 in IC1396: a search for evolutionary constraints [Replacement]

astro-ph — Mon, 20 Feb 2012 01:34:06 +0000

It is currently widely accepted that open star clusters and stellar associations result from the evolution of embedded star clusters. Parameters such star formation efficiency, time-scale of gas removal and velocity dispersion can be determinants of their future as bound or unbound systems. Finding objects at an intermediate evolution state can provide constraints to model the embedded cluster evolution. In the HII region IC1396, Trumpler 37 is an extended young cluster that presents characteristics of an association. We employed the Two Micron All Sky Survey (2MASS) photometry to analysing its structure and stellar content, and determining its astrophysical parameters. We also analysed 11 bright-rimmed clouds in IC1396 in order to search for young infrared star clusters, and the background open star cluster Teutsch 74, to verify whether it has any contribution to the observed stellar density profile of Trumpler 37. The derived parameters and comparison with template objects from other studies lead us to conclude that Trumpler 37, rather than as a star cluster, will probably emerge from its molecular cloud as an OB association.

A large, multi-epoch H{\alpha} survey at z=2.23, 1.47, 0.84 & 0.40: the 11 Gyr evolution of star-forming galaxies from HiZELS

astro-ph — Fri, 17 Feb 2012 01:52:44 +0000

This paper presents new deep and wide narrow-band surveys undertaken with UKIRT, Subaru and the VLT; a unique combined effort to select large, robust samples of H-alpha (Ha) emitters at z=0.40, 0.84, 1.47 and 2.23 (corresponding to look-back times of 4.2, 7.0, 9.2 and 10.6 Gyrs) in a uniform manner over ~2 deg^2 in the COSMOS and UDS fields. The deep multi-epoch Ha surveys reach ~3M_sun/yr out to z=2.2 for the first time, while the wide area and the coverage over two independent fields allow to greatly overcome cosmic variance. A total of 1742, 637, 515 and 556 Ha emitters are homogeneously selected at z=0.40, 0.84, 1.47 and 2.23, respectively, and used to determine the Ha luminosity function and its evolution. The faint-end slope is found to be -1.60+-0.08 over z=0-2.23, showing no evolution. The characteristic luminosity of SF galaxies, L*, evolves significantly as log[L*(z)]=0.45z+log[L*(z=0)]. This is the first time Ha has been used to trace SF activity with a single homogeneous survey at z=0.4-2.23. Overall, the evolution seen in Ha is in good agreement with the evolution seen using inhomogeneous compilations of other tracers of star formation, such as FIR and UV, jointly pointing towards the bulk of the evolution in the last 11 Gyrs being driven by a strong luminosity increase from z~0 to z~2.2. Our uniform analysis allows to derive the Ha star formation history of the Universe, for which the simple parametrisation log(SFRD)=-2.1/(1+z) is a good approximation for z<2.23. Both the shape and normalisation of the Ha star formation history are consistent with the measurements of the stellar mass density growth, confirming that our Ha analysis traces the bulk of the formation of stars in the Universe up to z~2.2. The star formation activity over the last ~11Gyrs is responsible for producing ~95% of the total stellar mass density observed locally today.

Residual Energy in Weak and Strong MHD Turbulence

astro-ph — Fri, 17 Feb 2012 01:49:26 +0000

Recent numerical and observational studies revealed that spectra of magnetic and velocity fluctuations in MHD turbulence have different scaling indexes. This intriguing feature has been recently explained in the case of weak MHD turbulence, that is, turbulence consisting of weakly interacting Alfven waves. However, astrophysical turbulence is strong in majority of cases. In the present work, we propose a unifying picture that allows one to address weak and strong MHD turbulence on the same footing. We argue that magnetic and kinetic energies are different in both weak and strong MHD turbulence. Their difference, the so-called residual energy, is spontaneously generated by turbulence, it has the Fourier spectrum E_r(k)=E_v(k)-E_b(k) \propto -f_w(k_||/k_perp) k_perp^{-2} in weak turbulence, and E_r(k) \propto -f_s(k_||/k_perp) k_perp^{-3} in strong turbulence. Here f_w,s(x) are functions declining fast for x>C_w,s and not significantly varying for $x

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.

First science results from SOFIA/FORCAST: The mid-infrared view of the compact HII region W3A

astro-ph — Fri, 17 Feb 2012 01:44:20 +0000

The massive star forming region W3 was observed with the faint object infrared camera for the SOFIA telescope (FORCAST) as part of the Short Science program. The 6.4, 6.6, 7.7, 19.7, 24.2, 31.5 and 37.1 \um bandpasses were used to observe the emission of Polycyclic Aromatic Hydrocarbon (PAH) molecules, Very Small Grains and Big Grains. Optical depth and color temperature maps of W3A show that IRS2 has blown a bubble devoid of gas and dust of $\sim$0.05 pc radius. It is embedded in a dusty shell of ionized gas that contributes 40% of the total 24 \um emission of W3A. This dust component is mostly heated by far ultraviolet, rather than trapped Ly$\alpha$ photons. This shell is itself surrounded by a thin ($\sim$0.01 pc) photodissociation region where PAHs show intense emission. The infrared spectral energy distribution (SED) of three different zones located at 8, 20 and 25\arcsec from IRS2, show that the peak of the SED shifts towards longer wavelengths, when moving away from the star. Adopting the stellar radiation field for these three positions, DUSTEM model fits to these SEDs yield a dust-to-gas mass ratio in the ionized gas similar to that in the diffuse ISM. However, the ratio of the IR-to-UV opacity of the dust in the ionized shell is increased by a factor $\simeq$3 compared to the diffuse ISM.

MBM 12 and MBM 16 distances

astro-ph — Fri, 17 Feb 2012 01:43:27 +0000

Among the multitude of intrinsic SDSS index vs. index diagrams the $(g-r) \ vs. \ (r-i)$ diagram is characterized by showing only minor $(g-r)$ variation for the M dwarfs. The $(g-r) \ vs. \ (r-i)$ reddening vector has a slope almost identical to the slope of the main sequence earlier than $\approx$M2, meaning that dwarfs later than $\sim$M2 are not contaminated by reddened dwarfs of earlier type. Chemical composition, stellar activity and evolution have only minor effects on the location of the M2$-$M7 dwarfs in the $(g-r) \ vs. \ (r-i)$ diagram implying that reddening may be isolated in a rather unique way. From $r$, $M_{r,(r-i)_0}$ and $E_{g-r}$ we may construct distance vs. $A_r$ diagrams. This purely photometric method is applied on SDSS DR8 data in the MBM 12 region. We derive individual stellar distances with a precision $\approx20-26$%. For extinctions in the $r-band$ the estimate is better than 0.2 mag for $\approx 67%$ and between 0.3 and 0.4 for the remaining $\approx 33%$. The extinction discontinuities noticed in the distance vs. $A_r$ diagrams suggest that MBM 12 is at $\approx$160 pc and MBM 16 at a somewhat smaller distance $\approx$100 pc. The distance for which $\Delta (A_r)/\sigma (\Delta(A_r))$ = 3, where $\Delta (A_r)$ refers to $\bar{A_{r, on}}-\bar{A_{r, off}}$, may possibly be used as an indicator for the cloud distance as well: For MBM 12 and 16 these distance estimates equal 160 and 100 pc, respectively

Unveiling hidden properties of young star clusters: differential reddening, star-formation spread and binary fraction

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

Usually, important parameters of young, low-mass star clusters are very difficult to obtain by means of photometry, especially when differential reddening and/or binaries occur in large amounts. We present a semi-analytical approach that, applied to the Hess diagram of a young star cluster, is able to retrieve the values of mass, age, star-formation spread, distance modulus, foreground and differential reddening, and binary fraction. The global optimisation method known as adaptive simulated annealing (ASA) is used to minimise the residuals between the observed and simulated Hess diagrams of a star cluster. The simulations are realistic and take the most relevant parameters of young clusters into account. Important features of the simulations are: a normal (Gaussian) differential reddening distribution, a time-decreasing star-formation rate, the unresolved binaries, and the smearing effect produced by photometric uncertainties on Hess diagrams. Free parameters are: cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening, and binary fraction.

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.

Building the cosmic distance scale: from Hipparcos to Gaia

astro-ph — Fri, 17 Feb 2012 01:40:03 +0000

Hipparcos, the first ever experiment of global astrometry, was launched by ESA in 1989 and its results published in 1997 (Perryman et al., Astron. Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho catalogues, ESA SP-1200, 1997). A new reduction was later performed using an improved satellite attitude reconstruction leading to an improved accuracy for stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys. 439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007). The Hipparcos Catalogue provided an extended dataset of very accurate astrometric data (positions, trigonometric parallaxes and proper motions), enlarging by two orders of magnitude the quantity and quality of distance determinations and luminosity calibrations. The availability of more than 20000 stars with a trigonometric parallax known to better than 10% opened the way to a drastic revision of our 3-D knowledge of the solar neighbourhood and to a renewal of the calibration of many distance indicators and age estimations. The prospects opened by Gaia, the next ESA cornerstone, planned for launch in June 2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more dramatic: a billion objects with systematic and quasi simultaneous astrometric, spectrophotometric and spectroscopic observations, about 150 million stars with expected distances to better than 10%, all over the Galaxy. All stellar distance indicators, in very large numbers, will be directly measured, providing a direct calibration of their luminosity and making possible detailed studies of the impacts of various effects linked to chemical element abundances, age or cluster membership. With the help of simulations of the data expected from Gaia, obtained from the mission simulator developed by DPAC, we will illustrate what Gaia can provide with some selected examples.

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

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

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

What Shapes the Structure of Molecular Clouds: Turbulence or Gravity? [Replacement]

astro-ph — Fri, 17 Feb 2012 01:34:23 +0000

We revisit the origin of Larson’s scaling relations, which describe the structure and kinematics of molecular clouds, based on recent observations and large-scale simulations of supersonic turbulence. Using dimensional analysis, we first show that both linewidth-size and mass-size correlations observed on scales 0.1-50 pc can be explained by a simple conceptual theory of compressible turbulence without resorting to the often assumed virial equilibrium or detailed energy balance condition. The scaling laws can be consistently interpreted as a signature of supersonic turbulence with no need to invoke gravity. We then show how self-similarity of structure established by the turbulence breaks in star-forming clouds through development of gravitational instabilities in the vicinity of the sonic scale, l_s~0.1 pc, leading to the formation of prestellar cores.

A study of deuterated water in the low-mass protostar IRAS16293-2422 [Replacement]

astro-ph — Fri, 17 Feb 2012 01:33:59 +0000

The HDO/H2O ratio is a powerful diagnostic to understand the evolution of water from the first stages of star formation to the formation of planets and comets. Our aim is to determine precisely the abundance distribution of HDO towards the low-mass protostar IRAS16293-2422 and learn more about the water formation mechanisms by determining the HDO/H2O abundance ratio. A spectral survey of the source IRAS16293-2422 was carried out in the framework of the CHESS Herschel Key program with the HIFI instrument, allowing the detection of numerous HDO lines. Other transitions have been observed previously with ground-based telescopes. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of HDO by assuming an abundance jump. To determine the H2O abundance throughout the envelope, a similar study was made of the H2-18O observed lines, as the H2O main isotope lines are contaminated by the outflows. We derive an inner HDO abundance of 1.7e-7 and an outer HDO abundance of 8e-11. To reproduce the HDO absorption lines, it is necessary to add an absorbing layer in front of the envelope. It may correspond to a water-rich layer created by the photodesorption of the ices at the edges of the molecular cloud. The HDO/H2O ratio is ~1.4-5.8% in the hot corino whereas it is ~0.2-2.2% in the outer envelope. It is estimated at ~4.8% in the added absorbing layer. Although it is clearly higher than the cosmic D/H abundance, the HDO/H2O ratio remains lower than the D/H ratio derived for other deuterated molecules observed in the same source. The similarity of the ratios derived in the hot corino and in the added absorbing layer suggests that water formed before the gravitational collapse of the protostar, contrary to formaldehyde and methanol, which formed later once the CO molecules had depleted on the grains.

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.

The fundamental plane for radio magnetars [Replacement]

astro-ph — Fri, 17 Feb 2012 01:31:41 +0000

High magnetic fields are a distinguishing feature of neutron stars and the existence of sources (the soft gamma repeaters and the anomalous X-ray pulsars) hosting an ultra-magnetized neutron star (or magnetar) has been recognized in the past few decades. Magnetars are believed to be powered by magnetic energy and not by rotation, as with normal radio pulsars. Until recently, the radio quietness and magnetic fields typically above the quantum critical value (Bq~4.4×10^{13} G), were among the characterizing properties of magnetars. The recent discovery of radio pulsed emission from a few of them, and of a low dipolar magnetic field soft gamma repeater, weakened further the idea of a clean separation between normal pulsars and magnetars. In this Letter we show that radio emission from magnetars might be powered by rotational energy, similarly to what occurs in normal radio pulsars. The peculiar characteristics of magnetars radio emission should be traced in the complex magnetic geometry of these sources. Furthermore, we propose that magnetar radio activity or inactivity can be predicted from the knowledge of the star’s rotational period, its time derivative and the quiescent X-ray luminosity.

User Guide for the Discrete Dipole Approximation Code DDSCAT 7.2 [Replacement]

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

DDSCAT 7.2 is a freely available open-source Fortran-90 software package applying the discrete dipole approximation (DDA) to calculate scattering and absorption of electromagnetic waves by targets with arbitrary geometries and complex refractive index. The targets may be isolated entities (e.g., dust particles), but may also be 1-d or 2-d periodic arrays of “target unit cells”, which can be used to study absorption, scattering, and electric fields around arrays of nanostructures. The DDA approximates the target by an array of polarizable points. The theory of the DDA and its implementation in DDSCAT is presented in Draine (1988) and Draine & Flatau (1994), and its extension to periodic structures in Draine & Flatau (2008). Efficient near-field calculations are enabled following Flatau & Draine (2012). DDSCAT 7.2 allows accurate calculations of electromagnetic scattering from targets with size parameters 2*pi*aeff/lambda < 25 provided the refractive index m is not large compared to unity (|m-1| < 2). DDSCAT 7.2 includes support for MPI, OpenMP, and the Intel Math Kernel Library (MKL). DDSCAT 7.2 supports calculations for a variety of target geometries (e.g., ellipsoids, regular tetrahedra, rectangular solids, finite cylinders, hexagonal prisms, etc.). Target materials may be both inhomogeneous and anisotropic. It is straightforward for the user to import new target geometries into the code. DDSCAT 7.2 calculates total cross sections for absorption and scattering and selected elements of the Mueller scattering intensity matrix for specified orientation of the target relative to the incident wave, and for specified scattering directions. DDSCAT 7.2 calculates E throughout a user-specified rectangular volume containing the target. A Fortran-90 code READNF to read E and P from files created by DDSCAT 7.2 is included in the distribution.

Supermassive black hole ancestors

astro-ph — Thu, 16 Feb 2012 01:59:09 +0000

We study a model in which supermassive black holes (SMBHs) can grow by the combined action of gas accretion on heavy seeds and mergers of both heavy (m_s^h=10^5 Msol) and light (m_s^l = 10^2 Msol) seeds. The former result from the direct collapse of gas in T_s^h >1.5×10^4K, H_2-free halos; the latter are the endproduct of a standard H_2-based star formation process. The H_2-free condition is attained by exposing halos to a strong (J_21 > 10^3) Lyman-Werner UV background produced by both accreting BHs and stars, thus establishing a self-regulated growth regime. We find that this condition is met already at z close to 18 in the highly biased regions in which quasars are born. The key parameter allowing the formation of SMBHs by z=6-7 is the fraction of halos that can form heavy seeds: the minimum requirement is that f_heavy>0.001; SMBH as large as 2×10^10 Msol can be obtained when f_heavy approaches unity. Independently of f_heavy, the model produces a high-z stellar bulge-black hole mass relation which is steeper than the local one, implying that SMBHs formed before their bulge was in place. The formation of heavy seeds, allowed by the Lyman-Werner radiative feedback in the quasar-forming environment, is crucial to achieve a fast growth of the SMBH by merger events in the early phases of its evolution, i.e. z>7. The UV photon production is largely dominated by stars in galaxies, i.e. black hole accretion radiation is sub-dominant. Interestingly, we find that the final mass of light BHs and of the SMBH in the quasar is roughly equal by z=6; by the same time only 19% of the initial baryon content has been converted into stars. The SMBH growth is dominated at all epochs z > 7.2 by mergers (exceeding accretion by a factor 2-50); at later times accretion becomes by far the most important growth channel. We finally discuss possible shortcomings of the model.

Discovery of interstellar mercapto radicals (SH) with the GREAT instrument on SOFIA

astro-ph — Thu, 16 Feb 2012 01:58:46 +0000

We report the first detection of interstellar mercapto radicals, obtained along the sight-line to the submillimeter continuum source W49N. We have used the GREAT instrument on SOFIA to observe the 1383 GHz Doublet Pi 3/2 J = 5/2 – 3/2 lambda doublet in the upper sideband of the L1 receiver. The resultant spectrum reveals SH absorption in material local to W49N, as well as in foreground gas, unassociated with W49N, that is located along the sight-line. For the foreground material at velocities in the range 37 – 44 km/s with respect to the local standard of rest, we infer a total SH column density ~ 2.6 E+12 cm-2, corresponding to an abundance of ~ 7 E-9 relative to H2, and yielding an SH/H2S abundance ratio ~ 0.13. The observed SH/H2S abundance ratio is much smaller than that predicted by standard models for the production of SH and H2S in turbulent dissipation regions and shocks, and suggests that the endothermic neutral-neutral reaction SH + H2 -> H2S + H must be enhanced along with the ion-neutral reactions believed to produce CH+ and SH+ in diffuse molecular clouds.

A Chandra Survey of Supermassive Black Holes with Dynamical Mass Measurements

astro-ph — Thu, 16 Feb 2012 01:57:42 +0000

We present Chandra observations of 12 galaxies that contain supermassive black holes with dynamical mass measurements. Each galaxy was observed for 30 ksec and resulted in a total of 68 point source detections in the target galaxies including supermassive black hole sources, ultraluminous X-ray sources, and extragalactic X-ray binaries. Based on our fits of the X-ray spectra, we report fluxes, luminosities, Eddington ratios, and slope of the power-law spectrum. Normalized to the Eddington luminosity, the 2–10 keV band X-ray luminosities of the SMBH sources range from $10^{-8}$ to $10^{-6}$, and the power-law slopes are centered at $\sim2$ with a slight trend towards steeper (softer) slopes at smaller Eddington fractions, implying a change in the physical processes responsible for their emission at low accretion rates. We find 20 ULX candidates, of which six are likely ($>90%$ chance) to be true ULXs. The most promising ULX candidate has an isotropic luminosity in the 0.3–10 keV band of $1.0_{-0.3}^{+0.6} \times 10^{40}$ erg/s.

Interstellar Sodium and Calcium Absorption toward SN 2011dh in M51

astro-ph — Thu, 16 Feb 2012 01:56:39 +0000

We present high-resolution echelle observations of SN 2011dh, which exploded in the nearby, nearly face-on spiral galaxy M51. Our data, acquired on three nights when the supernova was near maximum brightness, reveal multiple absorption components in Na I D and Ca II H and K, which we identify with gaseous material in the Galactic disk or low halo and in the disk and halo of M51. The M51 components span a velocity range of over 140 km s^-1, extending well beyond the range exhibited by H I 21 cm emission at the position of the supernova. Since none of the prominent Na I or Ca II components appear to coincide with the peak in H I emission, the supernova may lie just in front of the bulk of the H I disk. The Na I/Ca II ratios for the components with the most extreme positive and negative velocities relative to the disk are ~1.0, similar to those for more quiescent components, suggesting that the absorption originates in relatively cool gas. Production scenarios involving a galactic fountain and/or tidal interactions between M51 and its companion would be consistent with these results. The overall weakness of Na I D absorption in the direction of SN 2011dh confirms a low foreground and host galaxy extinction for the supernova.

Supernova-driven outflows and chemical evolution of dwarf spheroidal galaxies

astro-ph — Thu, 16 Feb 2012 01:54:04 +0000

We present a general phenomenological model for the metallicity distribution (MD) in terms of [Fe/H] for dwarf spheroidal galaxies (dSphs). These galaxies appear to have stopped accreting gas from the intergalactic medium and are fossilized systems with their stars undergoing slow internal evolution. For a wide variety of infall histories of unprocessed baryonic matter to feed star formation, most of the observed MDs can be well described by our model. The key requirement is that the fraction of the gas mass lost by supernova-driven outflows is close to unity. This model also predicts a relationship between the total stellar mass and the mean metallicity for dSphs in accord with properties of their dark matter halos. The model further predicts as a natural consequence that the abundance ratios [E/Fe] for elements such as O, Mg, and Si decrease for stellar populations at the higher end of the [Fe/H] range in a dSph. We show that for infall rates far below the net rate of gas loss to star formation and outflows, the MD in our model is very sharply peaked at one [Fe/H] value, similar to what is observed in most globular clusters. This suggests that globular clusters may be end members of the same family as dSphs.

Jet rotation investigated in the near-ultraviolet with HST/STIS

astro-ph — Thu, 16 Feb 2012 01:51:47 +0000

We present results of the second phase of our near-ultraviolet investigation into protostellar jet rotation using HST/STIS. We obtain long-slit spectra at the base of five T Tauri jets to determine if there is a difference in radial velocity between the jet borders which may be interpreted as a rotation signature. These observations are extremely challenging and push the limits of current instrumentation, but have the potential to provide long-awaited observational support for the magneto-centrifugal mechanism of jet launching in which jets remove angular momentum from protostellar systems. We successfully detect all five jet targets (from RW Aur, HN Tau, DP Tau and CW Tau) in several near-ultraviolet emission lines, including the strong Mg II doublet. However, only RW Aur’s bipolar jet presents sufficient signal-to-noise for analysis. The approaching jet lobe shows a difference of 10 km/s in a direction which agrees with the disk rotation sense, but is opposite to previously published optical measurements for the receding jet. The near-ultraviolet difference is not found six months later, nor is it found in the fainter receding jet. Overall, in the case of RW Aur, differences are not consistent with a simple jet rotation interpretation. Indeed, given the renowned complexity and variability of this system, it now seems likely that any rotation signature is confused by other influences, with the inevitable conclusion that RW Aur is not suited to a jet rotation study.

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.

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

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

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

Origin of strong magnetic fields in Milky-Way like galactic haloes

astro-ph — Thu, 16 Feb 2012 01:45:55 +0000

An analytical model predicting the growth rates, the absolute growth times and the saturation values of the magnetic field strength within galactic haloes is presented. The analytical results are compared to cosmological MHD simulations of Milky-Way like galactic halo formation performed with the N-body / \textsc{Spmhd} code \textsc{Gadget}. The halo has a mass of $\approx{}3\cdot{}10^{12}$ $M_{\odot}$ and a virial radius of $\approx{}$270 kpc. The simulations in a $\Lambda$CDM cosmology also include radiative cooling, star formation, supernova feedback and the description of non-ideal MHD. A primordial magnetic seed field ranging from $10^{-10}$ to $10^{-34}$ G in strength agglomerates together with the gas within filaments and protohaloes. There, it is amplified within a couple of hundred million years up to equipartition with the corresponding turbulent energy. The magnetic field strength increases by turbulent small-scale dynamo action. The turbulence is generated by the gravitational collapse and by supernova feedback. Subsequently, a series of halo mergers leads to shock waves and amplification processes magnetizing the surrounding gas within a few billion years. At first, the magnetic energy grows on small scales and then self-organizes to larger scales. Magnetic field strengths of $\approx{}10^{-6}$ G are reached in the center of the halo and drop to $\approx{}10^{-9}$ G in the IGM. Analyzing the saturation levels and growth rates, the model is able to describe the process of magnetic amplification notably well and confirms the results of the simulations.

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 Broadband Study of the Emission from the Composite Supernova Remnant MSH 11-62

astro-ph — Thu, 16 Feb 2012 01:44:14 +0000

MSH 11-62 (G291.1-0.9) is a composite supernova remnant for which radio and X-ray observations have identified the remnant shell as well as its central pulsar wind nebula. The observations suggest a relatively young system expanding into a low density region. Here we present a study of MSH 11-62 using observations with the Chandra, XMM-Newton, and Fermi observatories, along with radio observations from the Australia Telescope Compact Array (ATCA). We identify a compact X-ray source that appears to be the putative pulsar that powers the nebula, and show that the X-ray spectrum of the nebula bears the signature of synchrotron losses as particles diffuse into the outer nebula. Using data from the Fermi LAT, we identify gamma-ray emission originating from MSH 11-62. With density constraints from the new X-ray measurements of the remnant, we model the evolution of the composite system in order to constrain the properties of the underlying pulsar and the origin of the gamma-ray emission.

Dynamics of secular evolution

astro-ph — Thu, 16 Feb 2012 01:42:07 +0000

The text of lectures to the 2011 Tenerife Winter School. The School’s theme was “Secular Evolution of Galaxies” and my task was to present the underlying stellar-dynamical theory. Other lecturers were speaking on the role of bars and chemical evolution, so these topics are avoided here. We start with an account of the connections between isolating integrals, quasiperiodicity and angle-action variables – these variables played a unifying role throughout the lectures. This leads on to the phenomenon of resonant trapping and how this can lead to chaos in cuspy potentials and phase-space mixing in slowly evolving potentials. Surfaces of section and frequency analysis are introduced as diagnostics of phase-space structure. Real galactic potentials include a fluctuating part that drives the system towards unattainable thermal equilibrium. Two-body encounters are only one source of fluctuations, and all fluctuations will drive similar evolution. We derive the orbit-averaged Fokker-Planck equation and relations that hold between the second-order diffusion coefficients and both the power spectrum of the fluctuations and the first-order diffusion coefficients. From the observed heating of the solar neighbourhood we show that the second-order diffusion coefficients must scale as J^{1/2}. We show that periodic spiral structure shifts angular momentum outwards, heating at the Lindblad resonances and mixing at corotation. The equation that would yield the normal modes of a stellar disc is first derived and then used to discuss the propagation of tightly-wound spiral waves. The winding up of such waves is explains why cool stellar discs are responsive systems that amplify ambient noise. An explanation is offered of why the Lin-Shu-Kalnajs dispersion relation and even global normal-mode calculations provide a very incomplete understanding of the dynamics of stellar discs.

The star formation history and metal content of the "Green Peas". New detailed GTC-OSIRIS spectrophotometry of three galaxies

astro-ph — Thu, 16 Feb 2012 01:41:52 +0000

We present deep broad-band imaging and long-slit spectroscopy of three compact, low-mass starburst galaxies at redshift z\sim0.2-0.3, also referred to as Green Peas (GP). We measure physical properties of the ionized gas and derive abundances for several species with high precision. We find that the three GPs display relatively low extinction, low oxygen abundances, and remarkably high N/O ratios We also report on the detection of clear signatures of Wolf-Rayet (WR) stars in these galaxies. We carry out a pilot spectral synthesis study using a combination of both population and evolutionary synthesis models. Their outputs are in qualitative agreement, strongly suggesting a formation history dominated by starbursts. In agreement with the presence of WR stars, these models show that these GPs currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However, as models imply, they are old galaxies having had formed most of their stellar mass several Gyr ago. The presence of old stars has been spectroscopically verified in one of the galaxies by the detection of Mg I 5167, 5173 absorption line. Additionally, we perform a surface photometry study based on HST data, that indicates that the three galaxies posses an exponential low-surface brightness envelope. If due to stellar emission, the latter is structurally compatible to the evolved hosts of luminous BCD/HII galaxies, suggesting that GPs are identifiable with major episodes in the assembly history of local BCDs. These conclusions highlight the importance of these objects as laboratories for studying galaxy evolution at late cosmic epochs.

User Guide for the Discrete Dipole Approximation Code DDSCAT 7.2 [Cross-Listing]

astro-ph — Thu, 16 Feb 2012 01:41:34 +0000

Homogeneous Metallicities and Radial Velocities for Galactic Globular Clusters [Replacement]

astro-ph — Thu, 16 Feb 2012 01:34:03 +0000

Well determined radial velocities and abundances are essential for analyzing the properties of the Globular Cluster system of the Milky Way. However more than 50% of these clusters have no spectroscopic measure of their metallicity. In this context, this work provides new radial velocities and abundances for twenty Milky Way globular clusters which lack or have poorly known values for these quantities. The radial velocities and abundances are derived from spectra obtained at the Ca II triplet using the FORS2 imager and spectrograph at the VLT, calibrated with spectra of red giants in a number of clusters with well determined abundances. For about half of the clusters in our sample we present significant revisions of the existing velocities or abundances, or both. We also confirm the existence of a sizable abundance spread in the globular cluster M54, which lies at the center of the Sagittarius dwarf galaxy. In addition evidence is provided for the existence of a small intrinsic internal abundance spread (sigma [Fe/H](int) ~ 0.11-0.14 dex, similar to that of M54) in the luminous distant globular cluster NGC 5824. This cluster thus joins the small number of Galactic globular clusters known to possess internal metallicity ([Fe/H]) spreads.

Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies [Replacement]

astro-ph — Thu, 16 Feb 2012 01:32:50 +0000

We report preliminary results concerning the detailed chemical composition of metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter UfDSphs). The abundances have been determined thanks to spectra obtained with X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios slightly lower to what is measured in field halo star of the same metallicity.We did not find extreme abundances in our Hercules stars as the one found by Koch for his 2 Hercules stars. The synthesis of the neutron capture elements Ba and Sr seems to originate from the same nucleosynthetic process in operation during the early stages of the galactic evolution.

Formaldehyde and methanol deuteration in protostars: fossiles from a past fast high density pre-collapse phase [Replacement]

astro-ph — Thu, 16 Feb 2012 01:32:23 +0000

Extremely high deuteration of several molecules have been observed around low mass protostars since a decade. Among them, formaldehyde and methanol present particularly high deuteration, with observations of abundant doubly and triply deuterated forms. Both species are thought to be mainly formed on interstellar grains during the low temperature and dense pre-collapse phase by H and D atom additions on the iced CO. We present here a theoretical study of the formaldehyde and methanol deuteration obtained with our gas-grain model, GRAINOBLE. This model takes into account the multilayer nature of the mantle and explores the robustness of the results against the uncertainties of poorly constrained chemical and surface model parameters. The comparison of the model predictions with the observations leads to two major results: i) the observed high deuteration is obtained during the last phase of the pre-collapse stage, when the density reaches 5 10^6 cm^-3, and this phase is fast, lasting only several thousands years. ii) D and H abstraction and substitution reactions are crucial in making up the observed deuteration ratios; This work shows the power of chemical composition as a tool to reconstruct the past history of protostars.

Collisional excitation of far-infrared line emissions from warm interstellar carbon monoxide (CO)

astro-ph — Wed, 15 Feb 2012 02:05:12 +0000

Motivated by recent observations with Herschel/PACS, and the availability of new rate coefficients for the collisional excitation of CO (Yang et al. 2010), the excitation of warm astrophysical CO is revisited with the use of numerical and analytic methods. For the case of an isothermal medium, results have been obtained for a wide range of gas temperatures (100 to 5000 K) and H2 densities (1E+3 to 1E+9 cm-3), and presented in the form of rotational diagrams, in which the logarithm of the column density per magnetic substate, log (N[J]/g[J]), is plotted for each state, as a function of its energy, E[J]. For rotational transitions in the wavelength range accessible to Herschel/PACS, such diagrams are nearly linear when n(H2) > 1E+8 cm-3. When log10(n[H2]) = 6.8 to 8, they exhibit significant negative curvature, whereas when log10(n[H2]) < 4.8 the curvature is uniformly positive throughout the PACS-accessible range. Thus, the observation of a positively-curved CO rotational diagram does not NECESSARILY require the presence of multiple temperature components. Indeed, for some sources observed with Herschel/PACS, the CO rotational diagrams show a modest positive curvature that can be explained by a single isothermal component. Typically, the required physical parameters are H2 densities in the 1E+4 to 1E+5 cm-3 range and temperatures, T, close to the maximum at which CO can survive. Other sources exhibit rotational diagrams with more curvature than can be accounted for by a single temperature component. For the case of a medium with a power-law distribution of gas temperatures, with dN/dT proportional to T to the power -b, results have been obtained for H2 densities 1E+3 to 1E+9 cm-3 and power-law indices, b, in the range 1 to 5; such a medium can account for a CO rotational diagram that is more positively curved than any resulting from an isothermal medium.

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.

Special and General Relativistic Effects in Galactic Rotation Curves

astro-ph — Wed, 15 Feb 2012 02:03:54 +0000

The observed flat rotation curves of galaxies require either the presence of dark matter in Newtonian gravitational potentials or a significant modification to the theory of gravity at galactic scales. Detecting relativistic Doppler shifts and gravitational effects in the rotation curves offers a tool for distinguishing between predictions of gravity theories that modify the inertia of particles and those that modify the field equations. These higher-order effects also allow us in principle, to test whether dark matter particles obey the equivalence principle. We calculate here the magnitudes of the relativistic Doppler and gravitational shifts expected in realistic models of galaxies in a general metric theory of gravity. We identify a number of observable quantities that measure independently the special- and general-relativistic effects in each galaxy and suggest that both effects might be detected in a statistical sense by combining appropriately the rotation curves of a large number of galaxies.

Gas-grain models for interstellar anion chemistry

astro-ph — Wed, 15 Feb 2012 01:56:55 +0000

Long-chain hydrocarbon anions CnH- (n=4, 6, 8) have recently been found to be abundant in a variety of interstellar clouds. In order to explain their large abundances in the denser (prestellar/protostellar) environments, new chemical models are constructed that include gas-grain interactions. Models including accretion of gas-phase species onto dust grains and cosmic-ray-induced desorption of atoms are able to reproduce the observed anion-to-neutral ratios, as well as the absolute abundances of anionic and neutral carbon chains, with a reasonable degree of accuracy. Due to their destructive effects, the depletion of oxygen atoms onto dust results in substantially greater polyyne and anion abundances in high-density gas (with n_{H_2} >~ 10^5 cm^{-3}). The large abundances of carbon-chain-bearing species observed in the envelopes of protostars such as L1527 can thus be explained without the need for warm carbon-chain chemistry. The C6H- anion-to-neutral ratio is found to be most sensitive to the atomic O and H abundances and the electron density. Therefore, as a core evolves, falling atomic abundances and rising electron densities are found to result in increasing anion-to-neutral ratios. Inclusion of cosmic-ray desorption of atoms in high-density models delays freeze-out, which results in a more temporally-stable anion-to-neutral ratio, in better agreement with observations. Our models include reactions between oxygen atoms and carbon-chain anions to produce carbon-chain-oxide species C6O, C7O, HC6O and HC7O, the abundances of which depend on the assumed branching ratios for associative electron detachment.

Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies

astro-ph — Wed, 15 Feb 2012 01:54:16 +0000

The origin of dust in galaxies revisited: the mechanism determining dust content

astro-ph — Wed, 15 Feb 2012 01:52:50 +0000

The origin of cosmic dust is a fundamental issue in planetary science. This paper revisits the origin of dust in galaxies, in particular, in the Milky Way, by using a chemical evolution model of a galaxy composed of stars, interstellar medium, metals (elements heavier than helium), and dust. We start from a review of time-evolutionary equations of the four components, and then, we present simple recipes for the stellar remnant mass and yields of metal and dust based on models of stellar nucleosynthesis and dust formation. After calibrating some model parameters with the data from the solar neighborhood, we have confirmed a shortage of the stellar dust production rate relative to the dust destruction rate by supernovae if the destruction efficiency suggested by theoretical works is correct. If the dust mass growth by material accretion in molecular clouds is active, the observed dust amount in the solar neighborhood is reproduced. We present a clear analytic explanation of the mechanism for determining dust content in galaxies after the activation of accretion growth: a balance between accretion growth and supernova destruction. Thus, the dust content is independent of the uncertainty of the stellar dust yield after the growth activation. The timing of the activation is determined by a critical metal mass fraction which depends on the growth and destruction efficiencies. The solar system formation seems to have occurred well after the activation and plenty of dust would have existed in the proto-solar nebula.

The kinetic temperature in a damped Lyman-alpha absorption system in Q2206-199 – an example of the warm neutral medium

astro-ph — Wed, 15 Feb 2012 01:48:17 +0000

By comparing the widths of absorption lines from OI, SiII and FeII in the redshift z=2.076 single-component damped Lyman alpha absorption system in the spectrum of Q2206-199 we establish that these absorption lines arise in Warm Neutral Medium gas at ~12000 +/- 3000K. This is consistent with thermal equilibrium model estimates of ~ 8000K for the Warm Neutral Medium in galaxies, but not with the presence of a significant cold component. It is also consistent with, but not required by, the absence of CII* fine structure absorption in this system. Some possible implications concerning abundance estimates in narrow-line WNM absorbers are discussed.

A molecular line study of the filamentary infrared dark cloud G304.74+01.32

astro-ph — Wed, 15 Feb 2012 01:45:53 +0000

The aim of this study is to better understand the physical and chemical properties of the filamentary IRDC G304.74+01.32. In particular, we aim to investigate the kinematics and dynamical state of the cloud and clumps within it, and the amount of CO depletion. All the submillimetre peak positions in the cloud identified from our previous LABOCA 870-micron map were observed in C17O(2-1) with APEX. Selected positions were also observed in the 13CO(2-1), SiO(5-4), and CH3OH(5_k-4_k) transitions at ~1 mm wavelength. The C17O lines were detected towards all target positions at similar radial velocities, indicating that G304.74 is a coherent filamentary structure. CO does not appear to be significantly depleted in the clumps. Two- to three methanol 5_k-4_k lines near ~241.8 GHz were detected towards all selected target positions, whereas SiO(5-4) was seen in only one of these positions. The 13CO(2-1) lines show blue asymmetric profiles, indicating large-scale infall motions. The clumps show trans- to supersonic non-thermal motions, and virial-parameter analysis suggests that most of them are gravitationally bound. The external pressure may also play a non-negligible role in the dynamics. This is qualitatively consistent with our earlier suggestion that the filament was formed by converging supersonic turbulent flows. The analysis suggests that the fragmentation of the filament into clumps is caused by “sausage”-type instability, in agreement with results from other IRDCs. The star-formation activity in the cloud, such as outflows, is likely responsible in releasing CO from the icy grain mantles back into the gas phase. Shocks related to outflows may have also injected CH3OH, SiO, and DCN into the gas-phase in SMM 3.

The fundamental plane for radio magnetars

astro-ph — Wed, 15 Feb 2012 01:45:19 +0000

Formaldehyde and methanol deuteration in protostars: fossiles from a past fast high density pre-collapse phase

astro-ph — Wed, 15 Feb 2012 01:45:03 +0000

The extinction curve in the visible and the value of Rv [Replacement]

astro-ph — Wed, 15 Feb 2012 01:39:25 +0000

This article discusses the interstellar extinction curve in the visible and the value of Rv. It is concluded that the visible extinction curve is likely to be linear in the visible, and that indirect estimates of Rv from tentative determinations of Av, infrared, or UV observations are questionable. There is currently no evidence of any variation of Rv with direction. If Rv is close to 3, as it has been inferred from mid-infrared data, starlight in the visible is extinguished by a factor F/F_0=(2.5exp{-2micron/lambda})^{E(B-V)} in the visible. But if the visible wavelength range alone is considered, 4 appears as its most natural and probable value, and F/F_0= exp{-2E(B-V)/lambda}.

Pulsed Radio Emission from the Fermi-LAT Pulsar J1732-3131: Search and A Possible Detection at 34.5 MHz [Replacement]

astro-ph — Wed, 15 Feb 2012 01:39:10 +0000

We report our search for and a possible detection of periodic radio pulses at 34.5 MHz from the Fermi-LAT pulsar J1732-3131. The candidate detection has been possible in only one of the many sessions of observations made with the low-frequency array at Gauribidanur, India, when the otherwise radio weak pulsar may have apparently brightened many folds. The candidate dispersion measure along the sight-line, based on the broad periodic profiles from about 20 minutes of data, is estimated to be 15.44 +/-0.32 pc/cc. We present the details of our periodic & single-pulse search, and discuss the results and their implications relevant to both, the pulsar and the intervening medium.

A unified supernova catalogue [Replacement]

astro-ph — Wed, 15 Feb 2012 01:36:43 +0000

In this paper a new supernova catalogue containing data for 5526 extragalactic supernovae that were discovered up to 2010 December 31 is presented. It combines several catalogues that are currently available online in a consistent and traceable way. During the comparison of the catalogues inconsistent entries were identified and resolved where possible. Remaining inconsistencies are marked transparently and can be easily identified. Thus it is possible to select a high-quality sample in a most simple way. Where available, redshift-based distance estimates to the supernovae were replaced by journal-refereed distances. Examples of statistical studies that are now possible with this new catalogue are presented in this paper.

Metallicity Gradients in Disks: Do Galaxies Form Inside-Out? [Replacement]

astro-ph — Wed, 15 Feb 2012 01:32:46 +0000

We examine radial and vertical metallicity gradients using a suite of disk galaxy simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient and reconcile differences between extant models and observations within the `inside-out’ disk growth paradigm. A sample of 25 disks is used, consisting of 19 from our RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code RAMSES. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code GASOLINE, alongside disks from Rahimi et al. (GCD+) and Kobayashi & Nakasato (GRAPE-SPH). Two chemical evolution models of inside-out disk growth were employed to contrast the temporal evolution of their radial gradients with those of the simulations. We find that systematic differences exist between the predicted evolution of radial abundance gradients in the RaDES and chemical evolution models, compared with the MUGS sample; specifically, the MUGS simulations are systematically steeper at high-redshift, and present much more rapid evolution in their gradients. We find that the majority of the models predict radial gradients today which are consistent with those observed in late-type disks, but they evolve to this self-similarity in different fashions, despite each adhering to classical `inside-out’ growth. We find that radial dependence of the efficiency with which stars form as a function of time drives the differences seen in the gradients; systematic differences in the sub-grid physics between the various codes are responsible for setting these gradients. Recent, albeit limited, data at redshift z=1.5 are consistent with the steeper gradients seen in our SPH sample, suggesting a modest revision of the classical chemical evolution models may be required.

Metallicity Gradients in Simulated Disk Galaxies [Replacement]

astro-ph — Wed, 15 Feb 2012 01:32:31 +0000

The stellar metallicity and abundance ratio gradients from the fiducial late-type galaxy simulation of Stinson et al. (2010) are presented. Over 1-3 scalelengths, gradients are shown to flatten with time, consistent with empirical evidence at high- and low-redshifts. Kinematic effects, including radial migration, though, flatten these intrinsicly steep gradients such that by redshift z=0, the measured gradients of these (now) old stars are flatter than their young counterparts, in contradiction to what is observed locally. Conversely, the stellar [O/Fe] gradient is (to first order) robust against migration, remaining fairly flat for both young and old populations today.

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.

Insight Into the Formation of the Milky Way Through Cold Halo Substructure. III. Statistical Chemical Tagging in the Smooth Halo

astro-ph — Tue, 14 Feb 2012 02:17:03 +0000

We find that the relative contribution of satellite galaxies accreted at high redshift to the stellar population of the Milky Way’s smooth halo increases with distance, becoming observable relative to the classical smooth halo about 15 kpc from the Galactic center. In particular, we determine line-of-sight-averaged [Fe/H] and [alpha/Fe] in the metal-poor main-sequence turnoff (MPMSTO) population along every Sloan Extension for Galactic Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting our sample to those lines of sight along which we do not detect elements of cold halo substructure (ECHOS), we compile the largest spectroscopic sample of stars in the smooth component of the halo ever observed in situ beyond 10 kpc. We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population in the distant half of our sample beyond about 15 kpc from the Galactic center. Inside of 15 kpc however, we find no significant spatial autocorrelation in [Fe/H]. At the same time, we perform SEGUE-like observations of N-body simulations of Milky Way analog formation. While we find that halos formed entirely by accreted satellite galaxies provide a poor match to our observations of the halo within 15 kpc of the Galactic center, we do observe spatial autocorrelation in [Fe/H] in the simulations at larger distances. This observation is an example of statistical chemical tagging and indicates that spatial autocorrelation in metallicity is a generic feature of stellar halos formed from accreted satellite galaxies.

Discovery of the Host Cluster for the Fundamental Cepheid Calibrator Zeta Gem

astro-ph — Tue, 14 Feb 2012 02:13:52 +0000

New and existing CORAVEL, UBVJHKs, HST, HIP/Tycho, ARO, KPNO, and DAO observations imply that the fundamental Cepheid calibrator Zeta Gem is a cluster member. The following parameters were inferred for Zeta Gem from cluster membership and are tied to new spectral classifications (DAO) established for 26 nearby stars (e.g., HD53588/B7.5IV, HD54692/B9.5IV): E(B-V)=0.02+-0.02, log t=7.85+-0.15, and d=355+-15 pc. The mean distance to Zeta Gem from cluster membership and six recent estimates (e.g., IRSB) is d=363+-9(se)+-26(sd) pc. The results presented here support the color-excess and HST parallax derived for the Cepheid by Benedict et al. (2007). Forthcoming precise proper motions (DASCH) and Chandra/XMM-Newton observations of the broader field may be employed to identify cluster members, bolster the cluster’s existence, and provide stronger constraints on the Cepheid’s fundamental parameters.

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.

Non-LTE effects on the lead and thorium abundance determinations for cool stars

astro-ph — Tue, 14 Feb 2012 01:56:02 +0000

Knowing accurate Pb abundances of metal-poor stars provides constraints on the Pb production mechanisms in the early Galaxy. Accurately deriving Th abundances permits a nucleo-chronometric age determination of the star. We improve the calculation of the Pb I and Th II lines in stellar atmospheres based on non-LTE line formation and evaluate the influence of departures from LTE on Pb and Th abundance determinations through a range of stellar parameters. Comprehensive model atoms for Pb I and Th II are presented. The departures from LTE lead to systematically depleted total absorption in the Pb I lines and positive abundance corrections. Non-LTE removes the discrepancy between the solar and the meteoritic Pb abundance. With the Holweger & Mueller (1974) solar model atmosphere, log eps(Pb, non-LTE) = 2.09. We revise the Pb and Eu abundances of the strongly r-process enhanced (r-II) stars CS 31082-001 and HE 1523-0901 and the Roederer et al. (2010) stellar sample. Our results provide strong evidence for universal Pb/Eu relative r-process yields during course of the Galaxy evolution. The stars with -2.3<[Fe/H]< -1.4 have, on average, 0.51 dex higher Pb/Eu ratios compared with that of the r-II stars suggesting that the s-process synthesis of Pb started as early as the time when Galactic metallicity had grown to [Fe/H] = -2.3. The average Pb/Eu ratio of the -1.4<[Fe/H]< -0.59 stars is close to the solar value, in line with the predictions of Travaglio et al. (2001) that AGB stars with [Fe/H] ~ -1 provided the largest contribution to the solar s-nuclei of Pb. Non-LTE leads to weakened Th II lines. Overall, the abundance correction does not exceed +0.2 dex when collisions with H I atoms are taken into account in non-LTE calculations.

Dark matter density profiles of the halos embedding early-type galaxies: characterizing halo contraction and dark matter annihilation strength

astro-ph — Tue, 14 Feb 2012 01:50:24 +0000

Identifying dark matter and characterizing its distribution in the inner region of halos embedding galaxies are inter-related problems of broad importance. We devise a new procedure of determining dark matter distribution in halos. We first make a self-consistent bivariate statistical match of stellar mass and velocity dispersion with halo mass as demonstrated here for the first time. Then, selecting early-type galaxy-halo systems we perform Jeans dynamical modeling with the aid of observed statistical properties of stellar mass profiles and velocity dispersion profiles. Dark matter density profiles derived specifically using Sloan Digital Sky Survey galaxies and halos from up-to-date cosmological dissipationless simulations deviate significantly from the dissipationless profle of Navarro-Frenk-White or Einasto in terms of inner density slope and/or concentration. From these dark matter profiles we find that dark matter density is enhanced in the inner region of most early-type galactic halos providing an independent dynamical evidence for halo contraction. The main characteristics of halo contraction are: (1) the mean dark matter density within the effective radius has increased by a factor from ~1 for clusters with M_vir > 10^{15} M_solar to ~4-5 for galaxies with M_vir < 10^{12} M_solar where M_vir is the halo virial mass, and (2) the enhancement is more frequently realized by steepened density slope than increased concentration compared with the fiducial NFW profile. Based on our results we predict that halos of nearby elliptical and lenticular galaxies can be promising targets for $\gamma$-ray emission from dark matter annihilation.

The vertical motions of mono-abundance sub-populations in the Milky Way disk

astro-ph — Tue, 14 Feb 2012 01:46:04 +0000

We present the vertical kinematics of stars in the Milky Way’s stellar disk inferred from SDSS/SEGUE G-dwarf data, deriving the vertical velocity dispersion, \sigma_z, as a function of vertical height |z| and Galactocentric radius R for a set of ‘mono-abundance’ sub-populations of stars with very similar elemental abundances [\alpha/Fe] and [Fe/H]. We find that all components exhibit nearly isothermal kinematics in |z|, and a slow outward decrease of the vertical velocity dispersion: $\sigma_z (z,R\,|[\alpha/Fe],[Fe/H]) ~ \sigma_z ([\alpha/Fe],[Fe/H]) x \exp (-(R-R_0)/7 kpc})$. The characteristic velocity dispersions of these components vary from ~ 15 km/s for chemically young, metal-rich stars, to >~ 50 km/s for metal poor stars. The mean \sigma_z gradient away from the mid plane is only 0.3 +/- 0.2 km/s/kpc. We find a continuum of vertical kinetic temperatures (~\sigma^2_z) as function of ([\alpha/Fe],[Fe/H]), which contribute to the stellar surface mass density as \Sigma_{R_0}(\sigma^2_z) ~ \exp(-\sigma^2_z). The existence of isothermal mono-abundance populations with intermediate dispersions reject the notion of a thin-thick disk dichotomy. This continuum of disks argues against models where the thicker disk portions arise from massive satellite infall or heating; scenarios where either the oldest disk portion was born hot, or where internal evolution plays a major role, seem the most viable. The wide range of \sigma_z ([\alpha/Fe],[Fe/H]) combined with a constant \sigma_z(z) for each abundance bin provides an independent check on the precision of the SEGUE abundances: \delta_[\alpha/Fe] ~ 0.07 dex and \delta_[Fe/H] ~ 0.15 dex. The radial decline of the vertical dispersion presumably reflects the decrease in disk surface-mass density. This measurement constitutes a first step toward a purely dynamical estimate of the mass profile the disk in our Galaxy. [abridged]

Second-order solutions of the equilibrium statistical mechanics for self-gravitating systems [Replacement]

astro-ph — Tue, 14 Feb 2012 01:43:18 +0000

In a previous study, we formulated a framework of the entropy-based equilibrium statistical mechanics for self-gravitating systems. This theory is based on the Boltzmann-Gibbs entropy and includes the generalized virial equations as additional constraints. With the truncated distribution function to the lowest order, we derived a set of second-order equations for the equilibrium states of the system. In this work, the numerical solutions of these equations are investigated. It is found that there are three types of solutions for these equations. Both the isothermal and divergent solutions are thermally unstable and have unconfined density profiles with infinite mass, energy and spatial extent. The convergent solutions, however, seem to be reasonable. Although the results cannot match the simulation data well, because of the truncations of the distribution function and its moment equations, these lowest-order convergent solutions show that the density profiles of the system are confined, the velocity dispersions are variable functions of the radius, and the velocity distributions are also anisotropic in different directions. The convergent solutions also indicate that the statistical equilibrium of self-gravitating systems is by no means the thermodynamic equilibrium. These solutions are just the lowest-order approximation, but they have already manifested the qualitative success of our theory. We expect that higher-order solutions of our statistical-mechanical theory will give much better agreement with the simulation results concerning dark matter haloes.

A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation [Replacement]

astro-ph — Tue, 14 Feb 2012 01:40:40 +0000

We present Hubble Space Telescope Wide Field Camera 3 slitless grism spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The high sensitivity of these grism observations, with 1-sigma detections of emission lines to f > 2.5×10^{-18} erg/s/cm^2, means that the galaxies in the sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than previously studied z~2 emission-line galaxies. Despite their lower mass, the galaxies have OIII/Hb ratios which are very similar to previously studied z~2 galaxies and much higher than the typical emission-line ratios of local galaxies. The WFC3 grism allows for unique studies of spatial gradients in emission lines, and we stack the two-dimensional spectra of the galaxies for this purpose. In the stacked data the OIII emission line is more spatially concentrated than the Hb emission line with 98.1 confidence. We additionally stack the X-ray data (all sources are individually undetected), and find that the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0 obscured active galaxies and star-forming galaxies. Together the compactness of the stacked OIII spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei.

Spectroscopy of new brown dwarf members of rho Ophiuchi and an updated initial mass function [Replacement]

astro-ph — Tue, 14 Feb 2012 01:36:33 +0000

To investigate the universality hypothesis of the initial mass function in the substellar regime, the population of the rho Ophiuchi molecular cloud is analysed by including a new sample of low-mass spectroscopically confirmed members. To that end, we have conducted a large spectroscopic follow-up of young substellar candidates uncovered in our previous photometric survey. The spectral types and extinction were derived for a newly found population of substellar objects, and its masses estimated by comparison to evolutionary models. A thoroughly literature search was conducted to provide an up-to-date census of the cluster, which was then used to derive the luminosity and mass functions, as well as the ratio of brown dwarfs to stars in the cluster. These results were then compared to other young clusters. It is shown that the study of the substellar population of the rho Ophiuchi molecular cloud is hampered only by the high extinction in the cluster ruling out an apparent paucity of brown dwarfs. The discovery of 16 new members of rho Ophiuchi, 13 of them in the substellar regime, reveals the low-mass end of its population and shows the success of our photometric candidate selection with the WIRCam survey. The study of the brown dwarf population of the cluster reveals a high disk fraction of 76 (+5-8)%. Taking the characteristic peak mass of the derived mass function and the ratio of brown dwarfs to stars into account, we conclude that the mass function of rho Ophiuchi is similar to other nearby young clusters.

RAMSES-CH: A New Chemodynamics Code

astro-ph — Mon, 13 Feb 2012 02:05:06 +0000

We present a new chemodynamical code based on the adaptive mesh refinement code RAMSES. The new code uses Eulerian hydrodynamics and N-body dynamics in a cosmological framework to trace the production and advection of several chemical species. It is the first such code to follow the self-consistent evolution of chemical elements in cosmological volumes while maintaining sub-kiloparsec resolution. The code will be used to simulate disk galaxies and explore the influence of chemical evolution models and star formation on galactic abundance ratios.

Why Do Stars Form In Clusters? An Analytic Model for Stellar Correlation Functions

astro-ph — Mon, 13 Feb 2012 01:59:36 +0000

Recently, we have shown that if the ISM is governed by super-sonic turbulent flows, the excursion-set formalism can be used to calculate the statistics of self-gravitating objects over a wide range of scales. On the largest self-gravitating scales (‘first crossing’), these correspond to GMCs, and on the smallest non-fragmenting self-gravitating scales (‘last crossing’), to protostellar cores. Here, we extend this formalism to rigorously calculate the auto and cross-correlation functions of cores (and by extension, young stars) as a function of spatial separation and mass, in analogy to the cosmological calculation of halo clustering. We show that this generically predicts that star formation is very strongly clustered on small scales: stars form in clusters, themselves inside GMCs. Outside the binary-star regime, the projected correlation function declines as a weak power-law, until a characteristic scale which corresponds to the characteristic mass scale of GMCs. On much larger scales the clustering declines such that star formation is not strongly biased on galactic scales, relative to the actual dense gas distribution. The precise correlation function shape depends on properties of the turbulent spectrum, but its qualitative behavior is quite general. The predictions agree well with observations of young star and core autocorrelation functions over ~4 dex in radius. Clustered star formation is a generic consequence of supersonic turbulence if most of the power in the velocity field, hence the contribution to density fluctuations, comes from large scales. The distribution of self-gravitating masses near the sonic length is then imprinted by fluctuations on larger scales. We similarly show that the fraction of stars formed in ‘isolated’ modes should be small (<~10%).

Luminous Satellites II: Spatial Distribution, Luminosity Function and Cosmic Evolution

astro-ph — Mon, 13 Feb 2012 01:58:28 +0000

We infer the normalization and the radial and angular distributions of the number density of satellites of massive galaxies ($\log_{10}[M_{h}^*/M\odot]>10.5$) between redshifts 0.1 and 0.8 as a function of host stellar mass, redshift, morphology and satellite luminosity. Exploiting the depth and resolution of the COSMOS HST images, we detect satellites up to eight magnitudes fainter than the host galaxies and as close as 0.3 (1.4) arcseconds (kpc). Describing the number density profile of satellite galaxies to be a projected power law such that $P(R)\propto R^{\rpower}$, we find $\rpower=-1.1\pm 0.3$. We find no dependency of $\rpower$ on host stellar mass, redshift, morphology or satellite luminosity. Satellites of early-type hosts have angular distributions that are more flattened than the host light profile and are aligned with its major axis. No significant average alignment is detected for satellites of late-type hosts. The number of satellites within a fixed magnitude contrast from a host galaxy is dependent on its stellar mass, with more massive galaxies hosting significantly more satellites. Furthermore, high-mass late-type hosts have significantly fewer satellites than early-type galaxies of the same stellar mass, likely a result of environmental differences. No significant evolution in the number of satellites per host is detected. The cumulative luminosity function of satellites is qualitatively in good agreement with that predicted using subhalo abundance matching techniques. However, there are significant residual discrepancies in the absolute normalization, suggesting that properties other than the host galaxy luminosity or stellar mass determine the number of satellites.

Identifying High Metallicity M Giants at Intragroup Distances with SDSS

astro-ph — Mon, 13 Feb 2012 01:53:05 +0000

Tidal stripping and three-body interactions with the central supermassive black hole may eject stars from the Milky Way. These stars would comprise a set of `intragroup’ stars that trace the past history of interactions in our galactic neighborhood. Using the Sloan Digital Sky Survey DR7, we identify candidate solar metallicity red giant intragroup stars using color cuts that are designed to exclude nearby M and L dwarfs. We present 677 intragroup candidates that are selected between 300 kpc and 2 Mpc, and are either the reddest intragroup candidates (M7-M10) or are L dwarfs at larger distances than previously detected.

Spatially Resolved [FeII] 1.64 \mu m Emission in NGC 5135. Clues for Understanding the Origin of the Hard X-rays in Luminous Infrared Galaxies

astro-ph — Mon, 13 Feb 2012 01:47:26 +0000

Spatially resolved near-IR and X-ray imaging of the central region of the Luminous Infrared Galaxy NGC 5135 is presented. The kinematical signatures of strong outflows are detected in the [FeII]1.64 \mu m emission line in a compact region at 0.9 kpc from the nucleus. The derived mechanical energy release is consistent with a supernova rate of 0.05-0.1 yr$^{-1}$. The apex of the outflowing gas spatially coincides with the strongest [FeII] emission peak and with the dominant component of the extranuclear hard X-ray emission. All these features provide evidence for a plausible direct physical link between supernova-driven outflows and the hard X-ray emitting gas in a LIRG. This result is consistent with model predictions of starbursts concentrated in small volumes and with high thermalization efficiencies. A single high-mass X-ray binary (HMXB) as the major source of the hard X-ray emission although not favoured, cannot be ruled out. Outside the AGN, the hard X-ray emission in NGC 5135 appears to be dominated by the hot ISM produced by supernova explosions in a compact star-forming region, and not by the emission due to HMXB. If this scenario is common to U/LIRGs, the hard X-rays would only trace the most compact (< 100 pc) regions with high supernova and star formation densities, therefore a lower limit to their integrated star formation. The SFR derived in NGC 5135 based on its hard X-ray luminosity is a factor of two and four lower than the values obtained from the 24 \mu m and soft X-ray luminosities, respectively.

A tidally distorted dwarf galaxy near NGC 4449

astro-ph — Mon, 13 Feb 2012 01:44:18 +0000

NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8 megaparsecs. It is wreathed in an extraordinary neutral hydrogen (H I) complex, which includes rings, shells and a counter-rotating core, spanning 90 kiloparsecs. NGC 4449 is relatively isolated, although an interaction with its nearest known companion-the galaxy DDO 125, some 40 kpc to the south-has been proposed as being responsible for the complexity of its HI structure. Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of -13.4 and a half-light radius of 2.7 kpc, with a full extent of around 8 kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but has hitherto been seen only in a dissolving globular cluster. We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20 kpc from the galaxy’s centre.

Cataclysmic Variables in Globular Clusters [Replacement]

astro-ph — Mon, 13 Feb 2012 01:36:48 +0000

Every massive globular cluster (GC) is expected to harbour a significant population of cataclysmic variables (CVs). In this review, I first explain why GC CVs matter astrophysically, how many and what types are theoretically predicted to exist and what observational tools we can use to discover, confirm and study them. I then take a look at how theoretical predictions and observed samples actually stack up to date. In the process, I also reconsider the evidence for two widely held ideas about CVs in GCs: (i) that there must be many fewer dwarf novae than expected; (ii) that the incidence of magnetic CVs is much higher in GCs than in the Galactic field.

The structure of star clusters in the outer halo of M31 [Replacement]

astro-ph — Mon, 13 Feb 2012 01:32:23 +0000

We present a structural analysis of halo star clusters in M31 based on deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The clusters in our sample span a range in galactocentric projected distance from 13 to 100 kpc and thus reside in rather remote environments. Ten of the clusters are classical globulars, while four are from the Huxor et al. (2005, 2008) population of extended, old clusters. For most clusters, contamination by M31 halo stars is slight, and so the profiles can be mapped reliably to large radial distances from their centres. We find that the extended clusters are well fit by analytic King (1962) profiles with ~20 parsec core radii and ~100 parsec photometric tidal radii, or by Sersic profiles of index ~1 (i.e. approximately exponential). Most of the classical globulars also have large photometric tidal radii in the range 50-100 parsec, however the King profile is a less good fit in some cases, particularly at small radii. We find 60% of the classical globular clusters exhibit cuspy cores which are reasonably well described by Sersic profiles of index ~2-6. Our analysis also reinforces the finding that luminous classical globulars, with half-light radii <10 parsec, are present out to radii of at least 100 kpc in M31, which is in contrast to the situation in the Milky Way where such clusters (other than the unusual object NGC 2419) are absent beyond 40 kpc.

Spatially resolved Halpha maps and sizes of 57 strongly star-forming galaxies at z~1 from 3D-HST: evidence for rapid inside-out assembly of disk galaxies

astro-ph — Fri, 10 Feb 2012 01:56:50 +0000

We investigate the build-up of galaxies at z~1 using maps of Halpha and stellar continuum emission for a sample of 57 galaxies with rest-frame Halpha equivalent widths >100 Angstroms in the 3D-HST grism survey. We find that the Halpha emission broadly follows the rest-frame R-band light but that it is typically somewhat more extended and clumpy. We quantify the spatial distribution with the half-light radius. The median Halpha effective radius r_e(Halpha) is 4.2+-0.1 kpc but the sizes span a large range, from compact objects with r_e(Halpha) ~ 1.0 kpc to extended disks with r_e(Halpha) ~ 15 kpc. Comparing Halpha sizes to continuum sizes, we find =1.3+-0.1 for the full sample. That is, star formation, as traced by Halpha, typically occurs out to larger radii than the rest-frame R-band stellar continuum; galaxies are growing their radii and building up from the inside out. This effect appears to be somewhat more pronounced for the largest galaxies. Using the measured Halpha sizes, we derive star formation rate surface densities. We find that they range from ~0.05 Msun yr^{-1} kpc^{-2} for the largest galaxies to ~5 Msun yr^{-1} kpc^{-2} for the smallest galaxies, implying a large range in physical conditions in rapidly star-forming z~1 galaxies. Finally, we infer that all galaxies in the sample have very high gas mass fractions and stellar mass doubling times < 500 Myr. Although other explanations are also possible, a straightforward interpretation is that we are simultaneously witnessing the rapid formation of compact bulges and large disks at z~1.

Exploring Halo Substructure with Giant Stars: Substructure in the Local Halo as Seen in the Grid Giant Star Survey Including Extended Tidal Debris from Omega Centauri

astro-ph — Fri, 10 Feb 2012 01:55:40 +0000

We present the latitude-normalized radial velocity (vb) distribution of 3318 subsolar metallicity, V<13.5 stars from the Grid Giant Star Survey (GGSS) in Southern Hemisphere fields. The sample includes giants mostly within ~5 kpc from the Galactic disks and halo. The nearby halo is found to (1) exhibit significant kinematical substructure, and (2) be prominently represented by several velocity coherent structures, including a very retrograde "cloud" of stars at l~285 deg and extended, retrograde "streams" visible as relatively tight l-vb sequences. One sequence in the fourth Galactic quadrant lies within the l-vb space expected to contain tidal debris from the "star cluster" wCentauri. Not only does wCen lie precisely in this l-vb sequence, but the positions and vb of member stars match those of N-body simulations of tidally disrupting dwarf galaxies on orbits ending with wCen's current position and space motion. But the ultimate proof that we have very likely found extended parts of the wCen tidal stream comes from echelle spectroscopy of a subsample of the stars that reveals a very particular chemical abundance signature known to occur only in wCen. The newly discovered wCen debris accounts for almost all fourth Galactic quadrant retrograde stars in the southern GGSS, which suggests wCen is a dominant contributor of retrograde giant stars in the inner Galaxy.

Long-term monitoring of the high-energy gamma-ray emission from LS I +61{\deg} 303 and LS 5039

astro-ph — Fri, 10 Feb 2012 01:53:34 +0000

The Fermi Large Area Telescope (LAT) reported the first definitive GeV detections of the binaries LS I +61\degree 303 and LS 5039 in the first year after its launch in June, 2008. These detections were unambiguous as a consequence of the reduced positional uncertainty and the detection of modulated gamma-ray emission on the corresponding orbital periods. An analysis of new data from the LAT, comprising 30 months of observations, identifies a change in the gamma-ray behavior of LS I +61\degree 303. An increase in flux is detected in March 2009 and a steady decline in the orbital flux modulation is observed. Significant emission up to 30GeV is detected by the LAT; prior datasets led to upper limits only. Contemporaneous TeV observations no longer detected the source, or found it -in one orbit- close to periastron, far from the phases at which the source previously appeared at TeV energies. The detailed numerical simulations and models that exist within the literature do not predict or explain many of these features now observed at GeV and TeV energies. New ideas and models are needed to fully explain and understand this behavior. A detailed phase-resolved analysis of the spectral characterization of LS I +61\degree 303 in the GeV regime ascribes a power law with an exponential cutoff spectrum along each analyzed portion of the system’s orbit. The on-source exposure of LS 5039 is also substantially increased with respect to our prior publication. In this case, whereas the general gamma-ray properties remain consistent, the increased statistics of the current dataset allows for a deeper investigation of its orbital and spectral evolution.

Nonextensive Effects on Chandrasekhar’s Dynamical Friction

astro-ph — Fri, 10 Feb 2012 01:52:56 +0000

The motion of a point like object of mass M passing through the background potential of massive collisionless particles (m << M) suffers a steady deceleration named dynamic friction. In his classical work, Chandrasekhar assumed a Maxwellian velocity distribution in the halo and neglected the self gravity of the wake induced by the gravitational focusing of the mass M. In this paper, by relaxing the validity of the Maxwellian distribution due to the presence of long range forces, we derive an analytical formula for the dynamic friction in the context of the q-nonextensive kinetic theory. In the extensive limiting case (q = 1), the classical Gaussian Chandrasekhar result is recovered. As an application, the dynamic friction timescale for Globular Clusters spiraling to the galactic center is explicitly obtained. Our results suggest that the problem concerning the large timescale as derived by numerical N-body simulations or semi-analytical models can be understood as a departure from the standard extensive Maxwellian regime as measured by the Tsallis nonextensive q-parameter.

The sino-german 6cm polarization survey of the galactic plane: A summary

astro-ph — Fri, 10 Feb 2012 01:52:24 +0000

We have finished the 6cm polarization survey of the Galactic plane using the Urumqi 25m radio telescope. It covers 10deg

Breaking the law: the M_{bh}-M_{spheroid} mass relations for core-Sersic and Sersic galaxies

astro-ph — Fri, 10 Feb 2012 01:52:05 +0000

The popular log-linear relation between supermassive black hole mass, M_bh, and the dynamical mass of the host spheroid, M_sph, is shown to require a significant correction. Core galaxies, typically with M_bh > 2×10^8 M_Sun and thought to be formed in dry merger events, are shown to be well described by a linear relation for which the median black hole mass is 0.36% – roughly double the old value of constancy. Of greater significance is that M_bh ~ (M_sph)^2 among the (non-pseudobulge) lower-mass systems: specifically, log[M_bh/M_Sun] = (1.92+/-0.38)log[M_sph/7x10^{10}M_Sun] + (8.38+/-0.17). `Classical’ spheroids hosting a 10^6 M_Sun black hole will have M_bh/M_sph ~ 0.025%. These new relations (i) bring consistency to the relation M_bh ~ sigma^5 and the fact that L ~ sigma^x with exponent x equal to 5 and 2 for bright (M_B < -20.5 mag) and faint spheroids, respectively, (ii) mimic the non-(log-linear) behavior in the M_bh-(Sersic n) diagram, (iii) necessitate the existence of a previously over-looked M_bh ~ L^{2.5} relation for Sersic (i.e.\ not core-Sersic) galaxies, and (iv) resolve past conflicts (in mass prediction) with the M_bh-sigma relation at the low-mass end. Furthermore, the bent nature of the M_bh-M_sph relation for `classical' spheroids will have a host of important implications that relate to (i) galaxy/black hole formation theories, (ii) searches for the fundamental black hole scaling relation, (iii) black hole mass predictions in other galaxies, (iv) alleged pseudobulge detections, (v) estimates of the black hole mass function and mass density based on luminosity functions, (vi) predictions for space-based gravitational wave detections, (vii) connections with nuclear star cluster scaling relations, (viii) evolutionary studies over different cosmic epochs, (ix) comparisons and calibrations matching inactive black hole masses with low-mass AGN data, and more.

Fast Integrated Spectra Analyzer: A New Computational Tool For Age and Reddening Determination of Small Angular Diameter Open Clusters

astro-ph — Fri, 10 Feb 2012 01:47:06 +0000

We present a new algorithm called ‘Fast Integrated Spectra Analyzer” (FISA) that permits fast and reasonably accurate age and reddening determinations for small angular diameter open clusters by using their integrated spectra in the (3600-7400) \AA \ range and currently available template spectrum libraries. This algorithm and its implementation help to achieve astrophysical results in shorter times than from other methods. A brief review is given of the integrated spectroscopic technique applied to the study of open clusters as well as the basic assumptions that justify its use. We describe the numerical algorithm employed in detail, show examples of its application, and provide a link to the code. Our method has successfully been applied to integrated spectroscopy of open clusters, both in the Galaxy and in the Magellanic Clouds, to determine ages and reddenings.

The structure of star clusters in the outer halo of M31

astro-ph — Fri, 10 Feb 2012 01:41:29 +0000

We present a structural analysis of halo star clusters in M31 based on deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The clusters in our sample span a range in galactocentric projected distance from 13 to 100 kpc and thus reside in rather remote environments. Ten of the clusters are classical globulars, while four are from the Huxor et al. (2005, 2008) population of extended, old clusters. For most clusters, contamination by M31 halo stars is slight, and so the profiles can be mapped reliably to large radial distances from their centres. We find that the extended clusters are well fit by analytic King (1962) profiles with ~20 parsec core radii and ~100 parsec photometric tidal radii, or by Sersic profiles of index ~1 (i.e. approximately exponential). Most of the classical globulars also have large photometric tidal radii in the range 50-100 parsec, however the King profile is a less good fit in some cases, particularly at small radii. We find 60 of the classical globular clusters exhibit cuspy cores which are reasonably well described by Sersic profiles of index ~2-6. Our analysis also reinforces the finding that luminous classical globulars, with half-light radii <10 parsec, are present out to radii of at least 100 kpc in M31, which is in contrast to the situation in the Milky Way where such clusters (other than the unusual object NGC 2419) are absent beyond 40 kpc.

Detection of C60 in embedded young stellar objects, a Herbig Ae/Be star and an unusual post-AGB star [Replacement]

astro-ph — Fri, 10 Feb 2012 01:36:13 +0000

The first detection of the C60 (Buckminsterfullerene) molecule in massive embedded young stellar objects (YSOs) is reported. Observations with Spitzer IRS reveal the presence of C60 in YSOs ISOGAL-P J174639.6-284126 and SSTGC 372630 in the Central Molecular Zone in the Galactic centre, and in a YSO candidate, 2MASS J06314796+0419381, in the Rosette nebula. The first detection of C60 in a Herbig Ae/Be star, HD 97300, is also reported. These observations extend the range of astrophysical environments in which C60 is found to YSOs and a pre-main sequence star. C60 excitation and formation mechanisms are discussed in the context of these results, together with its presence and processes in post-AGB objects such as HR 4049.

Comparison of Convective Overshooting Models and Their Impact on Abundances from Integrated Light Spectroscopy of Young ($<$ 3 Gyr) Star Clusters

astro-ph — Thu, 09 Feb 2012 01:58:58 +0000

As part of an ongoing program to measure detailed chemical abundances in nearby galaxies, we use a sample of young to intermediate age clusters in the Large Magellanic Cloud with ages of 10 Myr to 2 Gyr to evaluate the effect of isochrone parameters, specifically core convective overshooting, on Fe abundance results from high resolution, integrated light spectroscopy. In this work we also obtain fiducial Fe abundances from high resolution spectroscopy of the cluster individual member stars. We compare the Fe abundance results for the individual stars to the results from isochrones and integrated light spectroscopy to determine whether isochrones with convective overshooting should be used in our integrated light analysis of young to intermediate age (10 Myr -3 Gyr) star clusters. We find that when using the isochrones from the Teramo group, we obtain more accurate results for young and intermediate age clusters over the entire age range when using isochrones without convective overshooting. While convective overshooting is not the only uncertain aspect of stellar evolution, it is one of the most readily parametrized ingredients in stellar evolution models, and thus important to evaluate for the specific models used in our integrated light analysis. This work demonstrates that our method for integrated light spectroscopy of star clusters can provide unique tests for future constraints on stellar evolution models of young and intermediate age clusters.

NANTEN 12CO (J=1-0) observations around the star WR 55

astro-ph — Thu, 09 Feb 2012 01:55:36 +0000

Context: A complete study of the molecular and ionized gas in the environs of the nebula RCW 78 around WR 55 is presented. Aims: The aim of this work is to investigate the spatial distribution, physical characteristics, and kinematical properties of the molecular gas linked to the galactic nebula RCW 78 to achieve a better understanding of its interaction with the star and with the ionized gas. Methods: This study was based on 12CO(1-0) fully sampled observations of a region of ~0.45{\deg} in size around the star WR 55 and the nebula RCW 78 obtained with the 4-m NANTEN telescope, radio continuum archival data at 1.4 and 4.85 GHz, obtained from SGPS and PMNRAO Southern Radio Survey, respectively, and available infrared MIPSGAL images at 24 microns. Results: A molecular gas component in the velocity range from ~ -58 to -45 km s-1, compatible with the velocity of the ionized gas, was found to be associated with the optical nebula. Adopting a distance of ~ 5 kpc, the mass of this molecular component is about 3.4 x 10^4 Msun. The analysis of the molecular data revealed the presence of a velocity gradient, in agreement with the Halpha line. New radiocontinuum flux density determinations confirm the thermal nature of RCW 78. This indicates that the ionized gas in RCW 78 arises from photoionization of the molecular gas component in the velocity range from -58 km s-1 to -45 km s-1. A molecular concentration at a velocity of -56.1 km s-1 (identified as C1) is very likely associated with the star HD 117797 and with a collection of candidate YSOs, lying at a distance of 3.9 kpc, while the rest of the molecular gas at velocities between -56 and -46 km s-1 constitute an incomplete ring-like structure which expands around WR 55 at a velocity of about ~ 5 km s-1. Mechanical energy and time requirements indicate that WR 55 is very capable of sustaining the expansion of the nebula.

Infrared Variability of Evolved Protoplanetary Disks: Evidence for Scale Height Variations in the Inner Disk

astro-ph — Thu, 09 Feb 2012 01:54:43 +0000

We present the results of a multi-wavelength multi-epoch survey of five evolved protoplanetary disks in the IC 348 cluster that show significant infrared variability. Using 3-8micron and 24micron photometry along with 5-40micron spectroscopy from the Spitzer Space Telescope, as well as ground-based 0.8-5micron spectroscopy, optical spectroscopy and near-infrared photometry, covering timescales of days to years, we examine the variability in the disk, stellar and accretion flux. We find substantial variations (10-60%) at all infrared wavelengths on timescales of weeks to months for all of these young stellar objects. This behavior is not unique when compared to other cluster members and is consistent with changes in the structure of the inner disk, most likely scale height fluctuations on a dynamical timescale. Previous observations, along with our near-infrared photometry, indicate that the stellar fluxes are relatively constant; stellar variability does not appear to drive the large changes in the infrared fluxes. Based on our near-infrared spectroscopy of the Pa-beta and Br-gamma lines we find that the accretion rates are variable in most of the evolved disks but the overall rates are probably too small to cause the infrared variability. We discuss other possible physical causes for the variability, including the influence of a companion, magnetic fields threading the disk, and X-ray flares.

The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae

astro-ph — Thu, 09 Feb 2012 01:53:43 +0000

Sulfur abundances derived from optical emission line measurements and ionization correction factors in planetary nebulae are systematically lower than expected for the objects’ metallicities. We have carefully considered a large range of explanations for this “sulfur anomaly”, including: (1) correlations between the size of the sulfur deficit and numerous nebular and central star properties; (2) ionization correction factors which under-correct for unobserved ions; (3) effects of dielectronic recombination on the sulfur ionization balance; (4) sequestering of S into dust and/or molecules; and (5) excessive destruction of S or production of O by AGB stars. It appears that all but the second scenario can be ruled out. However, we find evidence that the sulfur deficit is generally reduced but not eliminated when S^+3 abundances determined directly from IR measurements are used in place of the customary sulfur ionization correction factor. We tentatively conclude that the sulfur anomaly is caused by the inability of commonly used ICFs to properly correct for populations of ionization stages higher than S^+2.

Evolution of dust in the Orion Bar with Herschel: I. Radiative transfer modelling

astro-ph — Thu, 09 Feb 2012 01:50:19 +0000

Interstellar dust is a key element in our understanding of the interstellar medium and star formation. The manner in which dust populations evolve with the excitation and the physical conditions is a first step in the comprehension of the evolution of inter- stellar dust. Within the framework of the Evolution of interstellar dust Herschel key program, we have acquired PACS and SPIRE spec- trophotometric observations of various photodissociation regions, to characterise this evolution. The aim of this paper is to trace the evolution of dust grains in the Orion Bar photodissociation region. We use Herschel/PACS (70 and 160 mic) and SPIRE (250, 350 and 500 mic) together with Spitzer/IRAC observations to map the spatial distribution of the dust populations across the Bar. Brightness profiles are modelled using the DustEM model coupled with a radiative transfer code. Thanks to Herschel, we are able to probe finely the dust emission of the densest parts of the Orion Bar with a resolution from 5.6″ to 35.1″. These new observations allow us to infer the temperature of the biggest grains at different positions in the Bar, which reveals a gradient from \sim 80 K to 40 K coupled with an increase of the spectral emissivity index from the ionization front to the densest regions. Combining Spitzer/IRAC observations, which are sensitive to the dust emission from the surface, with Herschel maps, we have been able to measure the Orion Bar emission from 3.6 to 500 mic. We find a stratification in the different dust components which can be re- produced quantitatively by a simple radiative transfer model without dust evolution. However including dust evolution is needed to explain the brightness in each band. PAH abundance variations, or a combination of PAH abundance variations with an emissivity enhancement of the biggest grains due to coagulation give good results.

Galactic cold cores III. General cloud properties

astro-ph — Thu, 09 Feb 2012 01:47:22 +0000

We examine the cloud structure around the Planck detections in 71 fields observed with the Herschel SPIRE instrument. We wish to determine the general physical characteristics of the fields and to examine the morphology of the clouds where the cold high column density clumps are found. We derive colour temperature and column density maps of the fields. We examine the infrared spectral energy distributions of the main clumps. The clouds are categorised according to their large scale morphology. With the help of recently released WISE satellite data, we look for signs of enhanced mid-infrared scattering (coreshine), an indication of growth of the dust grains, and examine the star formation activity associated with the cold clumps. The mapped clouds have distances ranging from ~100pc to several kiloparsecs and cover a range of sizes and masses from cores of less than 10 solar masses to clouds with masses in excess of 10000 solar mass. Most fields contain some filamentary structures and in about half of the cases a filament or a few filaments dominate the morphology. In one case out of ten, the clouds show a cometary shape or have sharp boundaries indicative of compression by an external force. The width of the filaments is typically ~0.2-0.3pc. However, there is significant variation from 0.1pc to 1pc and the estimates are sensitive to the methods used and the very definition of a filament. Enhanced mid-infrared scattering, coreshine, was detected in four clouds with six additional tentative detections. The cloud LDN183 is included in our sample and remains the best example of this phenomenon. About half of the fields are associated with active star formation as indicated by the presence of mid-infrared point sources. The mid-infrared sources often coincide with structures whose sub-millimetre spectra are still dominated by the cold dust.

Kinematical and chemical vertical structure of the Galactic thick disk I. Thick disk kinematics

astro-ph — Thu, 09 Feb 2012 01:39:53 +0000

The variation of the kinematical properties of the Galactic thick disk with Galactic height Z are studied by means of 412 red giants observed in the direction of the south Galactic pole up to 4.5 kpc from the plane. We confirm the non-null mean radial motion toward the Galactic anticenter found by other authors, but we find that it changes sign at |Z|=3 kpc, and the proposed inward motion of the LSR alone cannot explain these observations. The rotational velocity decreases with |Z| by -30 km/s/kpc, but the data are better represented by a power-law with index 1.25, similar to that proposed from the analysis of SDSS data. All the velocity dispersions increase with |Z|, but the vertical gradients are small. The dispersions grow proportionally, with no significant variation of the anisotropy. The ratio sigma_U/sigma_W=2 suggests that the thick disk could have formed from a low-latitude merging event. The vertex deviation increases with Galactic height, reaching ~20 degrees at |Z|=3.5 kpc. The tilt angle also increases, and the orientation of the ellipsoid in the radial-vertical plane is constantly intermediate between the alignment with the cylindrical and the spherical coordinate systems. The tilt angle at |Z|=2 kpc coincides with the expectations of MOND, but an extension of the calculations to higher |Z| is required to perform a conclusive test. Finally, between 2.5 and 3.5 kpc we detect deviations from the linear trend of many kinematical quantities, suggesting that some kinematical substructure could be present.

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

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

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

The molecular gas in Luminous Infrared Galaxies II: extreme physical conditions, and their effects on the X_{co} factor

astro-ph — Thu, 09 Feb 2012 01:39:07 +0000

In this work we conclude the analysis of our CO line survey of Luminous Infrared Galaxies (LIRGs: L_{IR}>=10^{11}L_{sol}) in the local Universe (Paper\,I), by focussing on the influence of their average ISM properties on the total molecular gas mass estimates via the so-called X_{co}=M(H_2)/L_{co,1-0} factor. One-phase radiative transfer models of the global CO Spectral Line Energy Distributions (SLEDs) yield an X_{co} distribution with: \sim(0.6+/-0.2) M_{sol}(K km s^{-1} pc^2)^{-1} over a significant range of average gas densities, temperatures and dynamical states. The latter emerges as the most important parameter in determining X_{co}, with unbound states yielding low values and self-gravitating states the highest ones. Nevertheless in many (U)LIRGs where available higher-J CO lines (J=3–2, 4–3, and/or J=6–5) or HCN line data from the literature allow a separate assesment of the gas mass at high densities (>=10^{4}cm^{-3}) rather than a simple one-phase analysis a near-Galactic, X_{co}\sim(3-6) M_{sol} (K\,km s^{-1} pc^2)^{-1} become possible….. …and may have thus resulted to systematic underestimates of molecular gas mass in ULIRGs.

Keplerian Ensemble Approximation. The issue of motions of compact objects in Galactic halo [Replacement]

astro-ph — Thu, 09 Feb 2012 01:35:51 +0000

The motion of compact objects in Galactic halo is investigated in the approximation of point mass under the assumption that the external halo is not gravitationally dominating and consists of test bodies. Then, the phase space distribution function is that of a spherically symmetric collection of confocal elliptical orbits of various energies and eccentricities, called Keplerian ensemble, and can be determined in a systematic way from the radial velocity dispersion profile of the halo objects. In particular, the anisotropy of the velocity dispersion tensor becomes a function of Galactocentric distance. We find that the observed motion of halo objects can be made consistent with Galaxy mass of $2.1\times10^{11}\msun$

Heat Transfer and Reconnection Diffusion in Turbulent Magnetized Plasmas [Replacement]

astro-ph — Thu, 09 Feb 2012 01:35:14 +0000

It is well known that magnetic fields constrain motions of charged particles, impeding the diffusion of charged particles perpendicular to magnetic field direction. This modification of transport processes is of vital importance for a wide variety of astrophysical processes including cosmic ray transport, transfer of heavy elements in the interstellar medium, star formation etc. Dealing with these processes one should keep in mind that in realistic astrophysical conditions magnetized fluids are turbulent. In this review we single out a single transport process, namely, heat transfer and consider how it occurs in the presence of the magnetized turbulence. We show that the ability of magnetic field lines to constantly change topology and connectivity is at the heart of the correct description of the 3D magnetic field stochasticity in turbulent fluids. This ability is ensured by fast magnetic reconnection in turbulent fluids and puts forward the concept of reconnection diffusion at the core of the physical picture of heat transfer in astrophysical plasmas. Appealing to reconnection diffusion we describe the ability of plasma to diffuse between different magnetized eddies explaining the advection of the heat by turbulence. Adopting the structure of magnetic field that follows from the modern understanding of MHD turbulence, we also discuss thermal conductivity that arises as electrons stream along stochastic magnetic field lines. We compare the effective heat transport that arise from the two processes and conclude that in many astrophysically-motivated cased eddy advection of heat dominates. Finally, we discuss the concepts of sub and superdiffusion and show that the subdiffusion requires rather restrictive settings. At the same time, accelerated diffusion or superdiffusion of heat is possible on the scales less than the injection scale of the turbulence.

Observing Lense-Thirring Precession in Tidal Disruption Flares [Replacement]

astro-ph — Thu, 09 Feb 2012 01:34:25 +0000

When a star is tidally disrupted by a supermassive black hole (SMBH), the streams of liberated gas form an accretion disk after their return to pericenter. We demonstrate that Lense-Thirring precession in the spacetime around a rotating SMBH can produce significant time evolution of the disk angular momentum vector, due to both the periodic precession of the disk and the nonperiodic, differential precession of the bound debris streams. Jet precession and periodic modulation of disk luminosity are possible consequences. The persistence of the jetted X-ray emission in the Swift J164449.3+573451 flare suggests that the jet axis was aligned with the spin axis of the SMBH during this event.

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

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

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

Implications on the X-ray emission of evolved pulsar wind nebulae based on VHE gamma-ray observations [Replacement]

astro-ph — Thu, 09 Feb 2012 01:31:05 +0000

Energetic pulsars power winds of relativistic leptons which produce photon nebulae (so-called pulsar wind nebulae, PWNe) detectable across the electromagnetic spectrum up to energies of several TeV. The spectral energy distribution has a double-humped structure: the first hump lies in the X-ray regime, the second in the gamma-ray range. The X-ray emission is generally understood as synchrotron radiation by highly energetic electrons, the gamma-ray emission as Inverse Compton scattering of energetic electrons with ambient photon fields. The evolution of the spectral energy distribution is influenced by the time-dependent spin-down of the pulsar and the decrease of the magnetic field strength with time. Thus, the present spectral appearance of a PWN depends on the age of the pulsar: while young PWNe are bright in X-rays and gamma-rays, the X-ray emission of evolved PWNe is suppressed. Hence, evolved PWNe may offer an explanation of the nature of some of the unidentified VHE gamma-ray sources not yet associated with a counterpart in other high-energy ranges. The purpose of this work is to develop a model which allows to calculate the expected X-ray fluxes of unidentified VHE gamma-ray sources considered to be PWN candidates. Such an estimate helps to evaluate the prospects of detecting the X-ray signal in deep observations with current X-ray observatories in future studies. We present a time-dependent leptonic model which predicts the broad-band emission of a PWN according to the characteristics of its pulsar. The values of the free parameters of the model are determined by a fit to observational VHE gamma-ray data. For a sample of representative PWNe, the resulting model predictions in the X-ray and gamma-ray range are compared to observations. The comparison shows that the high-energy emission of identified PWNe from different states of evolution is predicted correctly by the model.

The X-ray reflector in NGC 4945: a time and space resolved portrait

astro-ph — Wed, 08 Feb 2012 02:04:13 +0000

We present a time, spectral and imaging analysis of the X-ray reflector in NGC 4945, which reveals its geometrical and physical structure with unprecedented detail. NGC 4945 hosts one of the brightest AGN in the sky above 10 keV, but it is only visible through its reflected/scattered emission below 10 keV, due to absorption by a column density of ~4\times10^24 cm-2. A new Suzaku campaign of 5 observations spanning ~6 months, together with past XMM-Newton and Chandra observations, show a remarkable constancy (within 30-50 pc, well within the imaging capabilities of Chandra at the distance of NGC 4945 (1″~18 pc). Accordingly, the Chandra imaging reveals a resolved, flattened, ~150 pc-long clumpy structure, whose spectrum is fully due to cold reflection of the primary AGN emission. The clumpiness may explain the small covering factor derived from the spectral and variability properties.

A new probe of the small-scale primordial power spectrum: astrometric microlensing by ultracompact minihalos

astro-ph — Wed, 08 Feb 2012 01:59:57 +0000

The dark matter enclosed in a density perturbation with a large initial amplitude (delta-rho/rho > 1e-3) collapses shortly after recombination and forms an ultracompact minihalo (UCMH). Their high central densities make UCMHs especially suitable for detection via astrometric microlensing: as the UCMH moves, it changes the apparent position of background stars. A UCMH with a mass larger than a few solar masses can produce a distinctive astrometric microlensing signal that is detectable by the space astrometry mission Gaia. If Gaia does not detect gravitational lensing by any UCMHs, then it establishes an upper limit on their abundance and constrains the amplitude of the primordial power spectrum for k~3500 Mpc^{-1}. These constraints complement the upper bound on the amplitude of the primordial power spectrum derived from limits on gamma-ray emission from UCMHs because the astrometric microlensing signal produced by an UCMH is maximized if the dark-matter annihilation rate is too low to affect the UCMH’s density profile. If dark matter annihilation within UCMHs is not detectable, a search for UCMHs by Gaia could constrain the amplitude of the primordial power spectrum to be less than 1e-5; this bound is three orders of magnitude stronger than the bound derived from the absence of primordial black holes.

Homogeneous Metallicities and Radial Velocities for Galactic Globular Clusters

astro-ph — Wed, 08 Feb 2012 01:56:44 +0000

Dust growth in the interstellar medium: How do accretion and coagulation interplay?

astro-ph — Wed, 08 Feb 2012 01:54:15 +0000

Dust grains grow in interstellar clouds by accretion and coagulation. In this paper, we focus on these two grain growth processes and numerically investigate how they interplay to increase the grain radii. We show that accretion efficiently depletes grains with radii $a\la 0.001 \micron$ on a time-scale of $\la 10$ Myr in solar-metallicity molecular clouds. Coagulation also occurs on a similar time-scale, but accretion is more efficient in producing a large bump in the grain size distribution. Coagulation further pushes the grains to larger sizes after a major part of the gas phase metals are used up. Similar grain sizes are achieved by coagulation regardless of whether accretion takes place or not; in this sense, accretion and coagulation modify the grain size distribution independently. The increase of the total dust mass in a cloud is also investigated. We show that coagulation slightly ’suppresses’ dust mass growth by accretion but that this effect is slight enough to be neglected in considering the grain mass budget in galaxies. Finally we examine how accretion and coagulation affect the extinction curve: The ultraviolet slope and the carbon bump are \textit{enhanced} by accretion, while they are flattened by coagulation.

Evidence of a bisymmetric spiral in the Milky Way

astro-ph — Wed, 08 Feb 2012 01:50:50 +0000

Context: Because of our viewing point within the Galactic disc, it is extremely difficult to observe the spiral structure of the Milky Way. Aims: To clarify the structure of the Galaxy by re-examination of gas distributions and data from 2MASS; to determine stream memberships among local stars and to show the relationship between streaming motions and spiral structure. Methods: We extend the spiral pattern found from neutral gas towards the Galactic centre using data from 2MASS. We select a population of 23 075 local disc stars for which complete kinematic data is available. We plot eccentricity against the true anomaly for stellar orbits and identify streams as dense regions of the plot. We reconstruct the spiral pattern by replacing each star at a random position of the inward part of its orbit. Results: We find evidence in 2MASS of a bar of length 4.2 \pm 0.1 kpc at angle 30 \pm 10\degree. We extend spiral structure by more than a full turn toward the Galactic centre, and confirm that the Milky Way is a two-armed grand-design bisymmetric spiral with pitch angle 5.56 \pm 0.06\degree. Memberships of kinematic groups are assigned to 98% of local disc stars and it is seen that the large majority of local stars have orbits aligned with this spiral structure.

Neutron stars from young nearby associations the origin of RXJ1605.3+3249

astro-ph — Wed, 08 Feb 2012 01:49:14 +0000

Many neutron stars (NSs) and runaway stars apparently come from the same regions on the sky. This suggests that they share the same birth places, namely associations and clusters of young massive stars. To identify NS birth places, we attempt to and NS-runaway pairs that could be former companions that were disrupted in a supernova (SN). The remains of recent (

Radial Migration in Galactic Thick Discs

astro-ph — Wed, 08 Feb 2012 01:46:50 +0000

We present a study of the extent to which the Sellwood & Binney radial migration of stars is affected by their vertical motion about the midplane. We use both controlled simulations in which only a single spiral mode is excited, as well as slightly more realistic cases with multiple spiral patterns and a bar. We find that rms angular momentum changes are reduced by vertical motion, but rather gradually, and the maximum changes are almost as large for thick disc stars as for those in a thin disc. We find that particles in simulations in which a bar forms suffer slightly larger angular momentum changes than in comparable cases with no bar, but the cumulative effect of multiple spiral events still dominates. We have determined that vertical action, and not vertical energy, is conserved on average during radial migration.

Rotating Disks and Non-Kinematic Double Peaks

astro-ph — Wed, 08 Feb 2012 01:44:57 +0000

Double-peaked line profiles are commonly considered a hallmark of rotating disks, with the distance between the peaks a measure of the rotation velocity. However, double-peaks can arise also from radiative transfer effects in optically thick non-rotating sources. Utilizing exact solutions of the line transfer problem we present a detailed study of line emission from geometrically thin Keplerian disks. We derive the conditions for emergence of kinematic double peaks in optically thin and thick disks, and find that it is generally impossible to disentangle the effects of kinematics and line opacity in observed double-peaked profiles. Unless supplemented by additional information, a double-peaked profile alone is not a reliable indicator of a rotating disk. In certain circumstances, triple and quadruple profiles might be better indicators of rotation in optically thick disks.

The CHESS survey of the L1157-B1 shock: the dissociative jet shock as revealed by Herschel–PACS

astro-ph — Wed, 08 Feb 2012 01:44:44 +0000

Outflows generated by protostars heavily affect the kinematics and chemistry of the hosting molecular cloud through strong shocks that enhance the abundance of some molecules. L1157 is the prototype of chemically active outflows, and a strong shock, called B1, is taking place in its blue lobe between the precessing jet and the hosting cloud. We present the Herschel-PACS 55–210 micron spectra of the L1157-B1 shock, showing emission lines from CO, H2O, OH, and [OI]. The spatial resolution of the PACS spectrometer allows us to map the warm gas traced by far-infrared (FIR) lines with unprecedented detail. The rotational diagram of the high-Jup CO lines indicates high-excitation conditions (Tex ~ 210 +/- 10 K). We used a radiative transfer code to model the hot CO gas emission observed with PACS and in the CO (13-12) and (10-9) lines measured by Herschel-HIFI. We derive 200

Implications on the X-ray emission of evolved pulsar wind nebulae based on VHE gamma-ray observations

astro-ph — Wed, 08 Feb 2012 01:44:12 +0000

Feedback from Central Black Holes in Elliptical Galaxies: Two-dimensional Models Compared to One-dimensional Models [Replacement]

astro-ph — Wed, 08 Feb 2012 01:38:20 +0000

We extend the black hole (BH) feedback models of Ciotti, Ostriker, and Proga to two dimensions. In this paper, we focus on identifying the differences between the one-dimensional and two-dimensional hydrodynamical simulations. We examine a normal, isolated $L_*$ galaxy subject to the cooling flow instability of gas in the inner regions. Allowance is made for subsequent star formation, Type Ia and Type II supernovae, radiation pressure, and inflow to the central BH from mildly rotating galactic gas which is being replenished as a normal consequence of stellar evolution. The central BH accretes some of the infalling gas and expels a conical wind with mass, momentum, and energy flux derived from both observational and theoretical studies. The galaxy is assumed to have low specific angular momentum in analogy with the existing one-dimensional case in order to isolate the effect of dimensionality. The code then tracks the interaction of the outflowing radiation and winds with the galactic gas and their effects on regulating the accretion. After matching physical modeling to the extent possible between the one-dimensional and two-dimensional treatments, we find essentially similar results in terms of BH growth and duty cycle (fraction of the time above a given fraction of the Eddington luminosity). In the two-dimensional calculations, the cool shells forming at 0.1–1 kpc from the center are Rayleigh–Taylor unstable to fragmentation, leading to a somewhat higher accretion rate, less effective feedback, and a more irregular pattern of bursting compared to the one-dimensional case.

Three Dimensional MHD Simulation of Circumbinary Accretion Disks: Disk Structures and Angular Momentum Transport [Replacement]

astro-ph — Wed, 08 Feb 2012 01:35:52 +0000

We present the first three-dimensional magnetohydrodynamic (MHD) simulations of a circumbinary disk surrounding an equal mass binary. The binary maintains a fixed circular orbit of separation $a$. As in previous hydrodynamical simulations, strong torques by the binary can maintain a gap of radius $\simeq 2a$. Streams curve inward from $r \simeq 2a$ toward the binary; some of their mass passes through the inner boundary, while the remainder swings back out to the disk. However, we also find that near its inner edge the disk develops both a strong $m=1$ asymmetry and growing orbital eccentricity. Because the MHD stresses introduce more matter into the gap, the total torque per unit disk mass is $\simeq 14$ times larger than found previously. The inner boundary accretion rate per unit disk mass is $\simeq 40$ times greater than found from previous hydrodynamical calculations. The implied binary shrinkage rate is determined by a balance between the rate at which the binary gains angular momentum by accretion and loses it by gravitational torque. The large accretion rate brings these two rates nearly into balance, but in net, we find that $\dot a/a < 0$, and its magnitude is about 2.7 times larger than predicted by the earlier hydrodynamic simulations. If the binary comprises two massive black holes, the accretion rate may be great enough for one or both to be AGN, with consequences for the physical state of the gas both in the disk body and in its inner gap.

Slicing and dicing the Milky Way disk in SDSS [Replacement]

astro-ph — Wed, 08 Feb 2012 01:34:19 +0000

We use the Stripe 82 proper motion catalogue of Bramich et al. (2008) to study the kinematics of Galactic disk stars in the solar neighborhood. We select samples of dwarf stars with reliable spectra and proper motions. They have cylindrical polar radius between 7 < R < 9 kpc, heights from the Galactic plane satisfying z < 2 kpc and span a range of metallicities -1.5 < [Fe/H] < 0. We develop a method for calculating and correcting for the halo contamination in our sample using the distribution of rotational velocities. Two Gaussians representing disk and halo populations are used to fit the radial and vertical velocity distributions via maximum likelihood methods. For the azimuthal velocities the same technique is used, except that a skewed non-Gaussian functional form now represents the disk velocity distribution. This enables us to compute the dispersions and cross-terms (the tilt and the vertex deviation) of the velocity ellipsoid as a function of height and metallicity. We also investigate the rotation lag of the disk, finding that the more metal-poor stars rotate significantly slower than the metal-rich stars. These samples provide important constraints on heating mechanisms in the Galactic disk and can be used for a variety of applications. We present one such application, employing the Jeans equations to provide a simple model of the potential close to the disk. Our model is in excellent agreement with others in the literature and provides an indication the disk, rather than the halo, dominates the circular speed at the solar neighborhood. We obtain a surface mass density within 1.1 kpc of around 66 M/pc^2 and estimate a local halo density of 0.015 M/pc^3 = 0.57 GeV/cm^3.

Sub-arcsecond SMA observations of the prototype Class 0 object VLA1623 at 1.3 mm: A single protostar with a structured outflow cavity ? [Replacement]

astro-ph — Wed, 08 Feb 2012 01:32:18 +0000

We present 1.3-mm subarcsecond SMA observations of the prototypical Class 0 protostar VLA1623. We report the detection of 1.3-mm continuum emission both from the central protostellar component VLA1623 and two additional sources, Knot-A and Knot-B, which have been already detected at longer wavelengths. Knot-A and Knot-B are both located along the western cavity wall opened by the protostellar outflow from VLA1623. Our SMA observations moreover show that these two continuum sources are associated with bright, high-velocity 12CO(2-1) emission, slightly shifted downstream of the outflow propagation direction with respect to the 1.3-mm continuum emission peaks. The alignment of Knot-A and Knot-B along the protostellar outflow cavity, the compactness of their 1.3-mm continuum emission and the properties of the associated CO emission suggest that these two sources trace outflow features due to shocks along the cavity wall, rather than protostellar objects. While it was considered as one of the best examples of a close protobinary system so far, the present analysis suggests that the prototypical Class 0, VLA1623, is single on the scales a>100 AU probed by our SMA observations. Moreover, we present here the second robust case of compact millimeter continuum emission produced by interactions between the protostellar jet and the envelope of a Class 0 protostar, which suggests a high occurrence of these outflow features during the embedded phase.

Vertical structure of a supernova-driven turbulent magnetized ISM [Replacement]

astro-ph — Wed, 08 Feb 2012 01:31:53 +0000

Stellar feedback drives the circulation of matter from the disk to the halo of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a vertical column of the interstellar medium with initial conditions typical of the solar circle in which supernovae drive turbulence and determine the vertical stratification of the medium. The simulations were run using a stable, positivity-preserving scheme for ideal MHD implemented in the FLASH code. We find that the majority (\approx 90 %) of the mass is contained in thermally-stable temperature regimes of cold molecular and atomic gas at T < 200 K or warm atomic and ionized gas at 5000 K < T < 10^{4.2} K, with strong peaks in probability distribution functions of temperature in both the cold and warm regimes. The 200 – 10^{4.2} K gas fills 50-60 % of the volume near the plane, with hotter gas associated with supernova remnants (30-40 %) and cold clouds ( 3 kpc. The magnetic field in our models has no significant impact on the scale heights of gas in each temperature regime; the magnetic tension force is approximately equal to and opposite the magnetic pressure, so the addition of the field does not significantly affect the vertical support of the gas. The addition of a magnetic field does reduce the fraction of gas in the cold (< 200 K) regime with a corresponding increase in the fraction of warm (~ 10^4 K) gas. However, our models lack rotational shear and thus have no large-scale dynamo, which reduces the role of the field in the models compared to reality. The supernovae drive oscillations in the vertical distribution of halo gas, with the period of the oscillations ranging from ~ 30 Myr in the T < 200 K gas to ~ 100 Myr in the 10^6 K gas, in line with predictions by Walters & Cox.

Measurement of the Mass and Stellar Population Distribution in M82 with the LBT

astro-ph — Tue, 07 Feb 2012 02:11:12 +0000

We present a K-band spectroscopic study of the stellar and gas kinematics, mass distribution, and stellar populations of the archetypical starburst M82. We used the CO stellar absorption bandhead at 2.29 {\mu}m (CO_2.29) to measure the rotation curve out to nearly 4 kpc radius on both the eastern and western sides of the galaxy. In contrast with the nearly Keplerian gas dynamics suggested by previous measurements of HI and CO emission from the ISM, which imply a truncation in M82’s dark matter halo, our data show that the rotation curve is in fact flat on 1 – 4 kpc scales. The kinematics of the Br{\gamma}, H2, and HeI emission lines are consistent with, although characterized by slightly higher velocities than, the stellar kinematics. We derived M82’s mass distribution from our stellar kinematic measurements and estimate its total dynamical mass is ~10^10 Msun. We measured the equivalent width of CO_2.29 (W_2.29) as a function of distance from the center of the galaxy to investigate the spatial extent of the red supergiant (RSG) population. The variation in W_2.29 with radius clearly shows that RSGs dominate the light inside 500 pc radius. M82’s famous superwind is likely launched from this region, where we estimate the enclosed mass is <= 2\times10^9 Msun.

The JCMT 12CO(3-2) Survey of the Cygnus X Region: I. A Pathfinder

astro-ph — Tue, 07 Feb 2012 02:10:38 +0000

Cygnus X is one of the most complex areas in the sky. This complicates interpretation, but also creates the opportunity to investigate accretion into molecular clouds and many subsequent stages of star formation, all within one small field of view. Understanding large complexes like Cygnus X is the key to understanding the dominant role that massive star complexes play in galaxies across the Universe. The main goal of this study is to establish feasibility of a high-resolution CO survey of the entire Cygnus X region by observing part of it as a Pathfinder, and to evaluate the survey as a tool for investigating the star-formation process. A 2×4 degree area of the Cygnus X region has been mapped in the 12CO(3-2) line at an angular resolution of 15″ and a velocity resolution of ~0.4km/s using HARP-B and ACSIS on the James Clerk Maxwell Telescope. The star formation process is heavily connected to the life-cycle of the molecular material in the interstellar medium. The high critical density of the 12CO(3-2) transition reveals clouds in key stages of molecule formation, and shows processes that turn a molecular cloud into a star. We observed ~15% of Cygnus X, and demonstrated that a full survey would be feasible and rewarding. We detected three distinct layers of 12CO(3-2) emission, related to the Cygnus Rift (500-800 pc), to W75N (1-1.8 kpc), and to DR21 (1.5-2.5 kpc). Within the Cygnus Rift, HI self-absorption features are tightly correlated with faint diffuse CO emission, while HISA features in the DR21 layer are mostly unrelated to any CO emission. 47 molecular outflows were detected in the Pathfinder, 27 of them previously unknown. Sequentially triggered star formation is a widespread phenomenon.

Low-frequency gravitational-wave science with eLISA/NGO [Cross-Listing]

astro-ph — Tue, 07 Feb 2012 02:10:24 +0000

We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultracompact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISA’s high signal-to-noise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

Spectrum and Morphology of the Two Brightest Milagro Sources in the Cygnus Region: MGRO J2019+37 and MGRO J2031+41

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

The Cygnus region is a very bright and complex portion of the TeV sky, host to unidentified sources and a diffuse excess with respect to conventional cosmic-ray propagation models. Two of the brightest TeV sources, MGRO J2019+37 and MGRO J2031+41, are analyzed using Milagro data with a new technique, and their emission is tested under two different spectral assumptions: a power law and a power law with an exponential cutoff. The new analysis technique is based on an energy estimator that uses the fraction of photomultiplier tubes in the observatory that detect the extensive air shower. The photon spectrum is measured in the range 1 to 200 TeV using the last 3 years of Milagro data (2005-2008), with the detector in its final configuration. MGRO J2019+37 is detected with a significance of 12.3 standard deviations ($\sigma$), and is better fit by a power law with an exponential cutoff than by a simple power law, with a probability $>98$% (F-test). The best-fitting parameters for the power law with exponential cutoff model are a normalization at 10 TeV of $7^{+5}_{-2}\times10^{-10}$ $\mathrm{s^{-1}\: m^{-2}\: TeV^{-1}}$, a spectral index of $2.0^{+0.5}_{-1.0}$ and a cutoff energy of $29^{+50}_{-16}$ TeV. MGRO J2031+41 is detected with a significance of 7.3$\sigma$, with no evidence of a cutoff. The best-fitting parameters for a power law are a normalization of $2.4^{+0.6}_{-0.5}\times10^{-10}$ $\mathrm{s^{-1}\: m^{-2}\: TeV^{-1}}$ and a spectral index of $3.08^{+0.19}_{-0.17}$. The overall flux is subject to an $\sim$30% systematic uncertainty. The systematic uncertainty on the power law indices is $\sim$0.1. A comparison with previous results from TeV J2032+4130, MGRO J2031+41 and MGRO J2019+37 is also presented.

Size and velocity-dispersion evolution of early-type galaxies in a Lambda cold dark matter universe

astro-ph — Tue, 07 Feb 2012 02:06:28 +0000

Early-type galaxies (ETGs) are observed to be more compact at z>2 than in the local universe. Remarkably, much of this size evolution appears to take place in a short \sim1.8 Gyr time span between z\sim2.2 and z\sim1.3, which poses a serious challenge to hierarchical galaxy formation models where mergers occurring on a similar timescale are the main mechanism for galaxy growth. We compute the merger-driven redshift evolution of stellar mass Mstar\propto(1+z)^aM, half-mass radius Re\propto(1+z)^aR and velocity-dispersion sigma0\propto(1+z)^asigma predicted by concordance Lambda Cold Dark Matter for a typical massive ETG in the redshift range z=1.3-2.2. Neglecting dissipative processes, and thus maximizing evolution in surface density, we find -1.5

Testing Galactic Magnetic Field Models using Near-Infrared Polarimetry

astro-ph — Tue, 07 Feb 2012 02:05:21 +0000

This work combines new observations of NIR starlight linear polarimetry with previously simulated observations in order to constrain dynamo models of the Galactic magnetic field. Polarimetric observations were obtained with the Mimir instrument on the Perkins Telescope in Flagstaff, AZ, along a line of constant Galactic longitude (\ell = 150\circ) with 17 pointings of the 10′ \times 10′ field of view between -75\circ < b < 10\circ, with more frequent pointings towards the Galactic midplane. A total of 10,962 stars were photometrically measured and 1,116 had usable polarizations. The observed distribution of polarization position angles with Galactic latitude and the cumulative distribution function of the measured polarizations are compared to predicted values. While the predictions lack the effects of turbulence and are therefore idealized, this comparison allows significant rejection of A0-type magnetic field models. S0 and disk-even halo-odd magnetic field geometries are also rejected by the observations, but at lower significance. New predictions of spiral-type, axisymmetric magnetic fields, when combined with these new NIR observations, constrain the Galactic magnetic field spiral pitch angle to -6\circ \pm 2\circ.

Characterization of Turbulence from Submillimeter Dust Emission

astro-ph — Tue, 07 Feb 2012 02:05:08 +0000

In this paper we use our recent technique for estimating the turbulent component of the magnetic field to derive the structure functions of the unpolarized emission as well as that of the Stokes Q and U parameters of the polarized emission. The solutions for the structure functions to 350-um SHARP polarization data of OMC-1 allow the determination of the corresponding turbulent correlation length scales. The estimated values for these length scales are 9.4″ +/- 0.1″, 7.3″ +/- 0.1″, 12.6″ +/- 0.2″ (or 20.5 +/- 0.2, 16.0 +/- 0.2, and 27.5 +/- 0.4 mpc at 450 pc, the adopted distance for OMC-1) for the Stokes Q and U parameters, and for the unpolarized emission N, respectively. Our current results for Q and U are consistent with previous results obtained through other methods, and may indicate presence of anisotropy in magnetized turbulence. We infer a weak coupling between the dust component responsible for the unpolarized emission N and the magnetic field B from the significant difference between their turbulent correlation length scales.

Spectral components analysis of diffuse emission processes

astro-ph — Tue, 07 Feb 2012 02:02:33 +0000

We develop a novel method to separate the components of a diffuse emission process based on an association with the energy spectra. Most of the existing methods use some information about the spatial distribution of components, e.g., closeness to an external template, independence of components etc., in order to separate them. In this paper we propose a method where one puts conditions on the spectra only. The advantages of our method are: 1) it is internal: the maps of the components are constructed as combinations of data in different energy bins, 2) the components may be correlated among each other, 3) the method is semi-blind: in many cases, it is sufficient to assume a functional form of the spectra and determine the parameters from a maximization of a likelihood function. As an example, we derive the CMB map and the foreground maps for seven yeas of WMAP data. In an Appendix, we present a generalization of the method, where one can also add a number of external templates.