Environment assisted electron capture

Electron capture by {\it isolated} atoms and ions proceeds by photorecombination. In this process a species captures a free electron by emitting a photon which carries away the excess energy. It is shown here that in the presence of an {\it environment} a competing non-radiative electron capture process can take place due to long range electron correlation. In this interatomic (intermolecular) process the excess energy is transferred to neighboring species. The asymptotic expression for the cross section of this process is derived. We demonstrate by explicit examples that under realizable conditions the cross section of this interatomic process can clearly dominate that of photorecombination

Be Star Disk Models in Consistent Vertical Hydrostatic Equilibrium

A popular model for the circumstellar disks of Be stars is that of a geometrically thin disk with a density in the equatorial plane that drops as a power law of distance from the star. It is usually assumed that the vertical structure of such a disk (in the direction parallel to the stellar rotation axis) is governed by the hydrostatic equilibrium set by the vertical component of the star's gravitational acceleration. Previous radiative equilibrium models for such disks have usually been computed assuming a fixed density structure. This introduces an inconsistency as the gas density is not allowed to respond to temperature changes and the resultant disk model is not in vertical, hydrostatic equilibrium. In this work, we modify the {\sc bedisk} code of \citet{sig07} so that it enforces a hydrostatic equilibrium consistent with the temperature solution. We compare the disk densities, temperatures, H$\alpha$ line profiles, and near-IR excesses predicted by such models with those computed from models with a fixed density structure. We find that the fixed models can differ substantially from the consistent hydrostatic models when the disk density is high enough that the circumstellar disk develops a cool ($T\lesssim10,000$K) equatorial region close to the parent star. Based on these new hydrostatic disks, we also predict an approximate relation between the (global) density-averaged disk temperature and the $T_{\rm eff}$ of the central star, covering the full range of central Be star spectral types.Comment: 25 pages; 11 figure

HyRec: A fast and highly accurate primordial hydrogen and helium recombination code

We present a state-of-the-art primordial recombination code, HyRec, including all the physical effects that have been shown to significantly affect recombination. The computation of helium recombination includes simple analytic treatments of hydrogen continuum opacity in the He I 2 1P - 1 1S line, the He I] 2 3P - 1 1S line, and treats feedback between these lines within the on-the-spot approximation. Hydrogen recombination is computed using the effective multilevel atom method, virtually accounting for an infinite number of excited states. We account for two-photon transitions from 2s and higher levels as well as frequency diffusion in Lyman-alpha with a full radiative transfer calculation. We present a new method to evolve the radiation field simultaneously with the level populations and the free electron fraction. These computations are sped up by taking advantage of the particular sparseness pattern of the equations describing the radiative transfer. The computation time for a full recombination history is ~2 seconds. This makes our code well suited for inclusion in Monte Carlo Markov chains for cosmological parameter estimation from upcoming high-precision cosmic microwave background anisotropy measurements.Comment: Version accepted by PRD. Numerical integration switches adapted to be well behaved for a wide range of cosmologies (Sec. V E). HyRec is available at http://www.tapir.caltech.edu/~yacine/hyrec/hyrec.htm

PPAK Integral Field Spectroscopy survey of the Orion Nebula: Data Release

We present a low-resolution spectroscopic survey of the Orion nebula which data we release for public use. In this article, we intend to show the possible applications of this dataset analyzing some of the main properties of the nebula. We perform an integral field spectroscopy mosaic of an area of ~5' X 6' centered on the Trapezium region of the nebula, including the ionization front to the south-east. The analysis of the line fluxes and line ratios of both the individual and integrated spectra allowed us to determine the main characteristics of the ionization throughtout the nebula.The final dataset comprises 8182 individual spectra, which sample each one a circular area of \~2.7" diameter. The data can be downloaded as a single row-stacked spectra fits file plus a position table or as an interpolated datacube with a final sampling of 1.5"/pixel. The integrated spectrum across the field-of-view was used to obtain the main integrated properties of the nebula, including the electron density and temperature, the dust extinction, the Halpha integrated flux (after correcting for dust reddening), and the main diagnostic line ratios. The individual spectra were used to obtain line intensity maps of the different detected lines. These maps were used to study the distribution of the ionized hydrogen, the dust extinction, the electron density and temperature, and the helium and oxygen abundance...Comment: 13 pages, 8 figures, accepted for publishing in Astronomy & Astrophysic

Comparing simulations of ionisation triggered star formation and observations in RCW 120

Massive clumps within the swept-up shells of bubbles, like that surrounding the galactic HII region RCW 120, have been interpreted in terms of the Collect and Collapse (C&C) mechanism for triggered star formation. The cold, dusty clumps surrounding RCW 120 are arranged in an almost spherical shell and harbour many young stellar objects. By performing high-resolution, three-dimensional SPH simulations of HII regions expanding into fractal molecular clouds, we investigate whether the formation of massive clumps in dense, swept-up shells necessarily requires the C&C mechanism. In a second step, we use RADMC-3D to compute the synthetic dust continuum emission from our simulations, in order to compare them with observations of RCW 120 made with APEX-LABOCA at 870 micron. We show that a distribution of clumps similar to the one seen in RCW 120 can readily be explained by a non-uniform initial molecular cloud structure. Hence, a shell-like configuration of massive clumps does not imply that the C&C mechanism is at work. Rather, we find a hybrid form of triggering, which combines elements of C&C and Radiatively Driven Implosion (RDI). In addition, we investigate the reliability of deriving clump masses from their 870 micron emission. We find that for clumps with more than 100 M_sun the observational estimates are accurate to within a factor of two and that, even at these long wavelengths, it is important to account for the radiative heating from triggered, embedded protostars.Comment: 10 pages, 7 figures, submitted to MNRA

Comparison of Millimeter-wave and X-Ray Emission in Seyfert Galaxies

We compare the emission at multiple wavelengths of an extended Seyfert galaxy sample, including both types of Seyfert nuclei. We use the Caltech Submillimeter Observatory to observe the CO J = 2-1 transition line in a sample of 45 Seyfert galaxies and detect 35 of them. The galaxies are selected by their joint soft X-ray (0.1-2.4 keV) and far-infrared ({\lambda} = 60-100 {\mu}m) emission from the ROSAT/IRAS sample. Since the CO line widths (W CO) reflect the orbital motion in the gravitational potential of the host galaxy, we study how the kinematics are affected by the central massive black hole (BH), using the X-ray luminosity. A significant correlation is found between the CO line width and hard (0.3-8 keV from Chandra and XMM-Newton) X-ray luminosity for both types of Seyfert nuclei. Assuming an Eddington accretion to estimate the BH mass (M BH) from the X-ray luminosity, the W CO-L X relation establishes a direct connection between the kinematics of the molecular gas of the host galaxy and the nuclear activity, and corroborates the previous studies that show that the CO is a good surrogate for the bulge mass. We also find a tight correlation between the (soft and hard) X-ray and the CO luminosities for both Seyfert types. These results indicate a direct relation between the molecular gas (i.e., star formation activity) of the host galaxy and the nuclear activity. To establish a clear causal connection between molecular gas and the fueling of nuclear activity, high-resolution maps (<100 pc) of the CO emission of our sample will be required and provided in a forthcoming Atacama Large Millimeter Array observation

Post common envelope binaries from SDSS. II : identification of 9 close binaries with VLT/FORS2

Context. Post common envelope binaries (PCEBs) consisting of a white dwarf and a main sequence star are ideal systems to use to calibrate current theories of angular momentum loss in close compact binary stars. The potential held by PCEBs for further development of close binary evolution could so far not be exploited due to the small number of known systems and the inhomogeneity of the sample. The Sloan Digital Sky Survey is changing this scene dramatically, as it is very efficient in identifying white dwarf/main sequence (WDMS) binaries, including both wide systems whose stellar components evolve like single stars and − more interesting in the context of close binary evolution − PCEBs. Aims. We pursue a large-scale follow-up survey to identify and characterise the PCEBs among the WDMS binaries that have been found with SDSS. We use a two-step strategy with the identification of PCEBs among WDMS binaries in the first phase and orbital period determinations in the second phase. Here we present first results of our ESO-VLT/FORS2 pilot study that targets the identification of the PCEBs among the fainter (g >∼18.5) SDSSWDMS binaries. Methods. From published SDSS catalogues we selected 26 WDMS binaries to be observed with ESO-VLT/FORS2 in service mode. The design of the observations was to get two spectra per object separated by at least one night.We used the Na I λλ 8183.27, 8194.81 doublet to measure radial velocity variations of our targets and a spectral decomposition/fitting technique to determine the white dwarf effective temperatures and surface gravities, masses, and secondary star spectral types for all WDMS binaries in our sample. Results. Among the 26 observed WDMS binaries, we find 9 strong PCEB candidates showing clear (≥3σ) radial velocity variations, and we estimate the fraction of PCEBs among SDSS WDMS binaries to be ∼35 ± 12%. We find indications of a dependence of the relative number of PCEBs among SDSSWDMS binaries on the spectral type of the secondary star. These results are subject to small number statistics and need to be confirmed by additional observations. Using Magellan-Clay/LDSS3, we measured the orbital periods of two PCEB candidates, SDSS J1047+0523 and SDSS J1414–0132, to be 9.17 h and 17.48 h, respectively. Conclusions. This pilot study demonstrates that our survey is highly efficient in identifying PCEBs among the SDSSWDMS binaries, and it will indeed provide the observational parameters that are needed to constrain the theoretical models of close binary evolution

The relationship between [OIII]5007A equivalent width and obscuration in AGN

In this paper we study the relationship between the equivalent width (EW) of the [OIII]5007A narrow emission line in AGN and the level of obscuration. To this end, we combine the results of a systematic spectral analysis, both in the optical and in the X-rays, on a statistically complete sample of ~170 X-ray selected AGN from the XMM-Newton Bright Serendipitous Source sample (XBS). We find that the observed large range of [OIII]5007A equivalent widths observed in the sample (from a few A up to 500A) is well explained as a combination of an intrinsic spread, probably due to the large range of covering factors of the Narrow Line Region, and the effect of absorption. The intrinsic spread is dominant for EW below 40-50A while absorption brings the values of EW up to ~100-150A, for moderate levels of absorption (AV~0.5-2 mag) or up to ~500A for AV>2 mag. In this picture, the absorption has a significant impact on the observed EW also in type~1 AGN. Using numerical simulations we find that this model is able to reproduce the [OIII]5007A EW distribution observed in the XBS sample and correctly predicts the shape of the EW distribution observed in the optically selected sample of QSO taken from the SDSS survey.Comment: 7 pages, 5 figures. Accepted for publication in MNRA