Partition functions and dissociation equilibrium constants are presented for 291 diatomic molecules for temperatures in the range from near absolute zero to 10 000 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 & Herzberg (1979, Constants of Diatomic Molecules) with significant improvements from the literature, especially updated data for ground states of many of the most important molecules by Irikura (2007, J. Phys. Chem. Ref. Data, 36, 389). 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.
As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using ...archetypal metal-poor stars. We perform detailed 3D non-LTE radiative transfer calculations based on 3D hydrodynamic stagger model atmospheres, and employ a new model atom that includes new quantum-mechanical neutral hydrogen collisional rate coefficients. With the exception of the red giant HD122563, we find that the 3D non-LTE models achieve Fe i/Fe ii excitation and ionization balance as well as not having any trends with equivalent width to within modelling uncertainties of 0.05 dex, all without having to invoke any microturbulent broadening; for HD122563 we predict that the current best parallax-based surface gravity is overestimated by 0.5 dex. Using a 3D non-LTE analysis, we infer iron abundances from the 3D model atmospheres that are roughly 0.1 dex higher than corresponding abundances from 1D MARCS model atmospheres; these differences go in the same direction as the non-LTE effects themselves. We make available grids of departure coefficients, equivalent widths and abundance corrections, calculated on 1D MARCS model atmospheres and horizontally and temporally averaged 3D stagger model atmospheres.
Data for inelastic processes due to hydrogen atom collisions with iron are needed for accurate modelling of the iron spectrum in late-type stars. Excitation and charge transfer in low-energy Fe+H ...collisions is studied theoretically using a previously presented method based on an asymptotic two-electron linear combination of atomic orbitals model of ionic-covalent interactions in the neutral atom-hydrogen-atom system, together with the multi-channel Landau–Zener model. An extensive calculation including 166 covalent states and 25 ionic states is presented and rate coefficients are calculated for temperatures in the range 1000–20 000 K. The largest rates are found for charge transfer processes to and from two clusters of states around 6.3 and 6.6 eV excitation, corresponding in both cases to active 4d and 5p electrons undergoing transfer. Excitation and de-excitation processes among these two sets of states are also significant.
Neutral sodium is a minority species in the atmospheres of late-type stars, and line formation in local thermodynamic equilibrium (LTE) is often a poor assumption, in particular for strong lines. We ...present an extensive grid of non-LTE calculations for several Na I lines in cool stellar atmospheres, including metal-rich and metal-poor dwarfs and giants. For the first time, we constructed a Na model atom that incorporates accurate quantum mechanical calculations for collisional excitation and ionisation by electrons as well as collisional excitation and charge exchange reactions with neutral hydrogen. Similar to Li I, the new rates for hydrogen impact excitation do not affect the statistical equilibrium calculations, while charge exchange reactions have a small but non-negligible influence. The presented LTE and non-LTE curves-of-growth can be interpolated to obtain non-LTE abundances and abundance corrections for arbitrary stellar parameter combinations and line strengths. The typical corrections for weak lines are −0.1... −0.2 dex, whereas saturated lines may overestimate the abundance in LTE by more than 0.5 dex. The non-LTE Na abundances appear very robust with respect to uncertainties in the input collisional data.
Excitation and charge transfer in low-energy O+H collisions is studied; it is a problem of importance for modelling stellar spectra and obtaining accurate oxygen abundances in late-type stars ...including the Sun. The collisions have been studied theoretically using a previously presented method based on an asymptotic two-electron linear combination of atomic orbitals (LCAO) model of ionic-covalent interactions in the neutral atom-hydrogen-atom system, together with the multichannel Landau-Zener model. The method has been extended to include configurations involving excited states of hydrogen using an estimate for the two-electron transition coupling, but this extension was found to not lead to any remarkably high rates. Rate coefficients are calculated for temperatures in the range 1000–20 000 K, and charge transfer and (de)excitation processes involving the first excited S-states, 4s.5So and 4s.3So, are found to have the highest rates.
Context. Models of star formation in the early universe require a detailed understanding of accretion, fragmentation and radiative feedback in metal-free molecular clouds. Different simulations ...predict different initial mass functions of the first stars, ranging from predominantly low-mass (0.1−10 M⊙), to massive (10−100 M⊙), or even supermassive (100−1000 M⊙). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova. Aims. The photospheres of metal-poor stars are relatively transparent in the UV, which may lead to large three-dimensional (3D) effects as well as departures from local thermodynamical equilibrium (LTE), even for weak spectral lines. If 3D effects and departures from LTE (NLTE) are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708by means of the most realistic methods available, and compare the results to predicted supernova yields. Methods. A 3D hydrodynamical Staggermodel atmosphere and 3D NLTE radiative transfer were applied to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters. Results. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for Fe/H is now a factor ten larger, at Fe/H < −6.53 (3σ), than previous estimates based on ⟨ 3D ⟩NLTE (i.e., using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D−⟨ 3D ⟩NLTE differences, all of which lead to a higher abundance determination. Conclusions. We find that supernova yields for models in a wide range of progenitor masses reproduce the revised chemical composition. In addition to massive progenitors of 20−60 M⊙ exploding with low energies (1−2 B, where 1 B = 1051 erg), we also find good fits for progenitors of 10 M⊙, with very low explosion energies (<1 B). We cannot reconcile the new abundances with supernovae or hypernovae with explosion energies above 2.5 B, nor with pair-instability supernovae.
Carbon and oxygen are key tracers of the Galactic chemical evolution; in particular, a reported upturn in C/O towards decreasing O/H in metal-poor halo stars could be a signature of nucleosynthesis ...by massive Population III stars. We reanalyse carbon, oxygen, and iron abundances in 39 metal-poor turn-off stars. For the first time, we take into account 3D hydrodynamic effects together with departures from local thermodynamic equilibrium (LTE) when determining both the stellar parameters and the elemental abundances, by deriving effective temperatures from 3D non-LTE Hβ profiles, surface gravities from Gaia parallaxes, iron abundances from 3D LTE Fe II equivalent widths, and carbon and oxygen abundances from 3D non-LTE C I and O I equivalent widths. We find that C/Fe stays flat with Fe/H, whereas O/Fe increases linearly up to 0.75 dex with decreasing Fe/H down to −3.0 dex. Therefore C/O monotonically decreases towards decreasing C/H, in contrast to previous findings, mainly because the non-LTE effects for O I at low Fe/H are weaker with our improved calculations.
Context.
We describe the atomic and molecular data that were used for the abundance analyses of FGK-type stars carried out within the
Gaia
-ESO Public Spectroscopic Survey in the years 2012 to 2019. ...The
Gaia
-ESO Survey is one among several current and future stellar spectroscopic surveys producing abundances for Milky-Way stars on an industrial scale.
Aims.
We present an unprecedented effort to create a homogeneous common line list, which was used by several abundance analysis groups using different radiative transfer codes to calculate synthetic spectra and equivalent widths. The atomic data are accompanied by quality indicators and detailed references to the sources. The atomic and molecular data are made publicly available at the CDS.
Methods.
In general, experimental transition probabilities were preferred but theoretical values were also used. Astrophysical
gf
-values were avoided due to the model-dependence of such a procedure. For elements whose lines are significantly affected by a hyperfine structure or isotopic splitting, a concerted effort has been made to collate the necessary data for the individual line components. Synthetic stellar spectra calculated for the Sun and Arcturus were used to assess the blending properties of the lines. We also performed adetailed investigation of available data for line broadening due to collisions with neutral hydrogen atoms.
Results.
Among a subset of over 1300 lines of 35 elements in the wavelength ranges from 475 to 685 nm and from 850 to 895 nm, we identified about 200 lines of 24 species which have accurate
gf
-values and are free of blends in the spectra of the Sun and Arcturus. For the broadening due to collisions with neutral hydrogen, we recommend data based on Anstee-Barklem-O’Mara theory, where possible. We recommend avoiding lines of neutral species for which these are not available. Theoretical broadening data by R.L. Kurucz should be used for Sc
II
, Ti
II
, and Y
II
lines; additionally, for ionised rare-earth species, the Unsöld approximation with an enhancement factor of 1.5 for the line width can be used.
Conclusions.
The line list has proven to be a useful tool for abundance determinations based on the spectra obtained within the
Gaia
-ESO Survey, as well as other spectroscopic projects. Accuracies below 0.2 dex are regularly achieved, where part of the uncertainties are due to differences in the employed analysis methods. Desirable improvements in atomic data were identified for a number of species, most importantly Al
I
, S
I
, and Cr
II
, but also Na
I
, Si
I
, Ca
II
, and Ni
I
.
Mg is the a element of choice for Galactic population and chemical evolution studies because it is easily detectable in all late-type stars. Such studies require precise elemental abundances, and ...thus departures from local thermodynamic equilibrium (LTE) need to be accounted for. Our goal is to provide reliable departure coefficients and equivalent widths in non-LTE, and for reference in LTE, for diagnostic lines of Mg studied in late-type stars. Using the model atom built and tested in the preceding paper in this series, we performed non-LTE radiative transfer calculations in a grid of 3945 stellar 1D atmospheric models. We used a sub-grid of 86 models to explore the propagation of errors in the recent atomic collision calculations to the radiative transfer results. Giants tend to have negative abundance corrections while dwarfs have positive, though small, corrections. As the errors are less than or on the same order as typical corrections, we expect that we can use these results to extract Mg abundances from high-quality spectra more reliably than from classical LTE analysis.
Hydrogen Balmer lines are commonly used as spectroscopic effective temperature diagnostics of late-type stars. However, reliable inferences require accurate model spectra, and the absolute accuracy ...of classical methods that are based on one-dimensional (1D) hydrostatic model atmospheres and local thermodynamic equilibrium (LTE) is still unclear. To investigate this, we carry out 3D non-LTE calculations for the Balmer lines, performed, for the first time, over an extensive grid of 3D hydrodynamic STAGGER model atmospheres. For Hα, Hβ, and Hγ we find significant 1D non-LTE versus 3D non-LTE differences (3D effects): the outer wings tend to be stronger in 3D models, particularly for Hγ, while the inner wings can be weaker in 3D models, particularly for Hα. For Hα, we also find significant 3D LTE versus 3D non-LTE differences (non-LTE effects): in warmer stars (Teff ≈ 6500 K) the inner wings tend to be weaker in non-LTE models, while at lower effective temperatures (Teff ≈ 4500 K) the inner wings can be stronger in non-LTE models; the non-LTE effects are more severe at lower metallicities. We test our 3D non-LTE models against observations of well-studied benchmark stars. For the Sun, we infer concordant effective temperatures from Hα, Hβ, and Hγ; however the value is too low by around 50 K which could signal residual modelling shortcomings. For other benchmark stars, our 3D non-LTE models generally reproduce the effective temperatures to within 1σ uncertainties. For Hα, the absolute 3D effects and non-LTE effects can separately reach around 100 K, in terms of inferred effective temperatures. For metal-poor turn-off stars, 1D LTE models of Hα can underestimate effective temperatures by around 150 K. Our 3D non-LTE model spectra are publicly available, and can be used for more reliable spectroscopic effective temperature determinations.
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