Partition functions and equilibrium constants for diatomic molecules and atoms of astrophysical interest

Partition functions and dissociation equilibrium constants are presented for 291 diatomic molecules for temperatures in the range from near absolute zero to 10000 K, thus providing data for many diatomic molecules of astrophysical interest at low temperature. The calculations are based on molecular spectroscopic data from the book of Huber and Herzberg with significant improvements from the literature, especially updated data for ground states of many of the most important molecules by Irikura. Dissociation energies are collated from compilations of experimental and theoretical values. Partition functions for 284 species of atoms for all elements from H to U are also presented based on data collected at NIST. The calculated data are expected to be useful for modelling a range of low density astrophysical environments, especially star-forming regions, protoplanetary disks, the interstellar medium, and planetary and cool stellar atmospheres. The input data, which will be made available electronically, also provides a possible foundation for future improvement by the community.Comment: 13 pages, 8 figures, 8 tables. Full tables 1, 2, 4, 5, 6, 7 and 8 to be made available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A

The Stagger-grid: A grid of 3D stellar atmosphere models - V. Fe line shapes, shifts and asymmetries

We present a theoretical study of the effects and signatures of realistic velocity field and atmospheric inhomogeneities associated with convective motions at the surface of cool late-type stars on the emergent profiles of iron spectral lines for a large range in stellar parameters. We compute 3D spectral line flux profiles under the assumption of local thermodynamic equilibrium (LTE) by employing state-of-the-art, time-dependent, 3D, radiative-hydrodynamical atmosphere models from the Stagger-grid. A set of 35 real unblended, optical FeI and FeII lines of varying excitation potential are considered. Additionally, fictitious Fe i and Fe ii lines (5000A and 0, 2, 4 eV) are used to construct general curves of growth and enable comparison of line profiles with the same line strength to illustrate systematical trends stemming from the intrinsic structural differences among 3D model atmospheres with different stellar parameters. Theoretical line shifts and bisectors are derived to analyze the shapes, shifts, and asymmetries imprinted in the full 3D line profiles emerging self-consistently from the convective simulations with velocity fields and atmospheric inhomogeneities. We find systematic variations in line strength, shift, width, and bisectors, that can be related to the respective physical conditions at the height of the line formation in the stellar atmospheric environment, in particular the amplitude of the vertical velocity field. Line shifts and asymmetries arise due to the presence of convective velocities and the granulation pattern that are ubiquitously found in observed stellar spectra of cool stars.Comment: 11 pages, 8 figures, 2 tables, submitted to A&

The Stagger-grid: A Grid of 3D Stellar Atmosphere Models - II. Horizontal and Temporal Averaging and Spectral Line Formation

We study the implications of averaging methods with different reference depth scales for 3D hydrodynamical model atmospheres computed with the Stagger-code. The temporally and spatially averaged (hereafter denoted as ) models are explored in the light of local thermodynamic equilibrium (LTE) spectral line formation by comparing spectrum calculations using full 3D atmosphere structures with those from averages. We explore methods for computing mean stratifications from the Stagger-grid time-dependent 3D radiative hydro- dynamical atmosphere models by considering four different reference depth scales (geometrical depth, column-mass density, and two optical depth scales). Furthermore, we investigate the influence of alternative averages (logarithmic or enforced hydrostatic equilibrium, flux-weighted temperatures). For the line formation we compute curves of growth for Fe i and Fe ii lines in LTE . The resulting stratifications for the four reference depth scales can be considerably different. We find typically that in the upper atmosphere and in the superadiabatic region just below the optical surface, where the temperature and density fluctuations are highest, the differences become considerable and increase for higher Teff, lower logg, and lower [Fe/H]. The differential comparison of spectral line formation shows distinctive differences depending on which model is applied. The averages over layers of constant column-mass density yield the best mean representation for LTE line formation, while the averages on layers at constant geometrical height are the least appropriate. Unexpectedly, the usually preferred averages over layers of constant optical depth are prone to the increasing interference of the reversed granulation towards higher effective temperature, in particular at low metallicity.Comment: Accepted for publication in A&A, 18 pages, 16 figure

The Stagger-grid: A grid of 3D stellar atmosphere models - IV. Limb darkening coefficients

We compute the emergent stellar spectra from the UV to far infrared for different viewing angles using realistic 3D model atmospheres for a large range in stellar parameters to predict the stellar limb darkening. We have computed full 3D LTE synthetic spectra based on 3D radiative hydrodynamic atmosphere models from the Stagger-grid. From the resulting intensities at different wavelength, we derived coefficients for the standard limb darkening laws considering a number of often-used photometric filters. Furthermore, we calculated theoretical transit light curves, in order to quantify the differences between predictions by the widely used 1D model atmosphere and our 3D models. The 3D models are often found to predict steeper limb darkening compared to the 1D models, mainly due to the temperature stratifications and temperature gradients being different in the 3D models compared to those predicted with 1D models based on the mixing length theory description of convective energy transport. The resulting differences in the transit light curves are rather small; however, these can be significant for high-precision observations of extrasolar transits, and are able to lower the residuals from the fits with 1D limb darkening profiles. We advocate the use of the new limb darkening coefficients provided for the standard four-parameter non-linear power law, which can fit the limb darkening more accurately than other choices.Comment: Accepted for publication in A&A, 10 pages, 9 figures, 1 tabl

HE 1327-2326, an unevolved star with [Fe/H]<-5.0. II. New 3D-1D corrected abundances from a VLT/UVES spectrum

We present a new abundance analysis of HE 1327-2326, the currently most iron-poor star, based on observational data obtained with VLT/UVES. We correct the 1D LTE abundances for 3D effects to provide an abundance pattern that supersedes previous works, and should be used to observationally test current models of the chemical yields of the first-generation SNe. Apart from confirming the 1D LTE abundances found in previous studies before accounting for 3D effects, we make use of a novel technique to apply the 3D-1D corrections for CNO which are a function of excitation potential and line strength for the molecular lines that comprise the observable CH, NH, and OH features. We find that the fit to the NH band at 3360 A is greatly improved due to the application of the 3D-1D corrections. This may indicate that 3D effects are actually observable in this star. We also report the first detection of several weak Ni lines. The cosmologically important element Li is still not detected; the new Li upper limit is extremely low, A(Li)<0.62, and in stark contrast with results not only from WMAP but also from other metal-poor stars. We also discuss how the new corrected abundance pattern of HE 1327-2326 is being reproduced by individual and integrated yields of SNe.Comment: 43 pages, incl. 17 figures, accepted for publication in Ap

A Grid of 3D Stellar Atmosphere Models of Solar Metallicity: I. General Properties, Granulation and Atmospheric Expansion

Present grids of stellar atmosphere models are the workhorses in interpreting stellar observations, and determining their fundamental parameters. These models rely on greatly simplified models of convection, however, lending less predictive power to such models of late type stars. We present a grid of improved and more reliable stellar atmosphere models of late type stars, based on deep, 3D, convective, stellar atmosphere simulations. This grid is to be used in general for interpreting observations, and improve stellar and asteroseismic modeling. We solve the Navier Stokes equations in 3D and concurrent with the radiative transfer equation, for a range of atmospheric parameters, covering most of stellar evolution with convection at the surface. We emphasize use of the best available atomic physics for quantitative predictions and comparisons with observations. We present granulation size, convective expansion of the acoustic cavity, asymptotic adiabat, as function of atmospheric parameters. These and other results are also available in electronic form.Comment: 16 pages, 12 figures. Accepted for publication in ApJ, 201

The amplitude of solar p-mode oscillations from three-dimensional convection simulations

The amplitude of solar p-mode oscillations is governed by stochastic excitation and mode damping, both of which take place in the surface convection zone. However, the time-dependent, turbulent nature of convection makes it difficult to self-consistently study excitation and damping processes through the use of traditional one-dimensional hydrostatic models. To this end, we carried out \textit{ab initio} three-dimensional, hydrodynamical numerical simulations of the solar atmosphere to investigate how p-modes are driven and dissipated in the Sun. The description of surface convection in the simulations is free from the tuneable parameters typically adopted in traditional one-dimensional models. Mode excitation and damping rates are computed based on analytical expressions whose ingredients are evaluated directly from the three-dimensional model. With excitation and damping rates both available, we estimate the theoretical oscillation amplitude and frequency of maximum power, $\nu_{\max}$, for the Sun. We compare our numerical results with helioseismic observations, finding encouraging agreement between the two. The numerical method presented here provides a novel way to investigate the physical processes responsible for mode driving and damping, and should be valid for all solar-type oscillating stars.Comment: 11 pages, 8 figures, accepted for publication in Ap