2019A&A...631A..43M

Context.Iron plays a crucial role in studies of late-type stars.In their atmospheres, neutral iron is the minority species, and lines of FeI are subject to the departures from local thermodynamic equilibrium (LTE).In contrast, one believes that LTE is a realistic approximation for FeII lines. The main source of the uncertainties in the non-LTE (NLTE) calculations for cool atmospheres is a treatment of inelastic collisions with hydrogen atoms.
Aims. Our aim is to investigate the effect of FeI+HI and FeII+HI collisions and their different treatments on the FeI/FeII ionisation equilibrium and iron abundance determinations for three Galactic halo benchmark stars (HD 84937, HD 122563, and HD 140283) and a sample of 38 very metal-poor giants in the dwarf galaxies with well known distances.
Methods. We performed the NLTE calculations for FeI-FeII by applying quantum-mechanical rate coefficients for collisions with HI from recent papers.
Results. We find that collisions with HI serve as efficient thermalisation processes for FeII, to an extent that the NLTE abundance corrections for FeII lines do not exceed 0.02dex, in absolute value, for [Fe/H]≥-3, and reach +0.06dex at [Fe/H]~-4. For a given star, different treatments of FeI+HI collisions lead to similar average NLTE abundances from the FeI lines, although discrepancies in the NLTE abundance corrections exist for individual lines. By using quantum-mechanical collisional data and the Gaia-based surface gravity, we obtain consistent abundances from the two ionisation stages, FeI and FeII, for red giant HD 122563. For turn-off star HD 84937, and subgiant HD 140283, we analyse the iron lines in the visible and the ultra-violet (UV, 1968-2990Å) ranges. For either FeI or FeII, abundances from the visible and UV lines are found to be consistent in each star. The NLTE abundances from the two ionisation stages agree within 0.10dex and 0.13dex for two different treatments of FeI + HI collisions. The FeI/FeII ionisation equilibrium is achieved for each star of our stellar sample in the dwarf galaxies, with the exception of stars at [Fe/H]≤-3.7.