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 in electronic form. Methods. In general, experimental transition probabilities were preferred but theoretical values were also used. Astrophysical g fvalues 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 a detailed 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 nm to 685 nm and from 850 nm to 895 nm, we identified about 200 lines of 24 species which have accurate g f-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.

[1] High-resolution, ultraviolet SO 2 photoabsorption cross section data are required for modeling density and temperature profiles of SO 2 in solar system atmospheres. We report measurements of such cross sections on SO 2 at 295 K for theB 1 B 1 -X 1 A 1 and C 1 B 2 -X 1 A 1 bands over the wavelength range 220 to 325 nm. Resolving powers of up to 550,000 were employed in order to study the congested spectrum of sharp SO 2 features in parts of this region. Our data are compared to earlier values obtained with lower resolving power.

The analysis of stellar abundances for odd‐Z Fe‐peak elements requires accurate non‐local thermodynamic equilibrium (NLTE) modelling of spectral lines fully taking into account the hyperfine structure (HFS) splitting of lines. Here, we investigate the statistical equilibrium of Co in the atmospheres of cool stars and the influence of NLTE and HFS on the formation of Co lines and abundances. Significant departures from LTE level populations are found for Co i; number densities of excited states in Co ii also differ from LTE at low metallicity. The NLTE level populations are used to determine the abundance of Co in solar photosphere, log ɛ= 4.95 ± 0.04 dex, which is in agreement with that in C i meteorites within the combined uncertainties. The spectral lines of Co i were calculated using the results of recent measurements of hyperfine interaction constants by UV Fourier transform spectrometry. For Co ii, the first laboratory measurements of HFS A and B factors were performed. These highly accurate A factor measurements (errors of the order of 3–7 per cent) allow, for the first time, reliable modelling of Co ii lines in the solar and stellar spectra and, thus, a test of the Co i/Co ii ionization equilibrium in stellar atmospheres. A differential abundance analysis of Co is carried out for 18 stars in the metallicity range −3.12 < [Fe/H] < 0. The abundances are derived by the method of spectrum synthesis. At low [Fe/H], NLTE abundance corrections for Co i lines are as large as +0.6, … , + 0.8 dex. Thus, LTE abundances of Co in metal‐poor stars are severely underestimated. The stellar NLTE abundances determined from the single UV line of Co ii are lower by ∼0.5–0.6 dex. The discrepancy might be attributed to possible blends that have not been accounted for in the solar Co ii line and its erroneous oscillator strength. The increasing [Co/Fe] trend in metal‐poor stars, as calculated from the Co i lines under NLTE, can be explained if Co is overproduced relative to Fe in massive stars. The models of Galactic chemical evolution are wholly inadequate to describe this trend suggesting that the problem is in supernova yields.

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