# Influence of inelastic collisions with hydrogen atoms on the non-LTE line formation for Fe I and Fe II in the 1D model atmospheres of late-type stars

## TL;DR

This study investigates how inelastic collisions with hydrogen atoms affect non-LTE line formation of Fe I and Fe II in late-type star atmospheres, using quantum-mechanical data to improve abundance accuracy.

## Contribution

It provides a detailed analysis of hydrogen collision effects on Fe I/Fe II ionization equilibrium using new quantum-mechanical collision data.

## Key findings

- Collisions with H I are efficient thermalizers for Fe II.
- NLTE abundance corrections for Fe II are small at [Fe/H] > -3.
- Consistent Fe I and Fe II abundances are achieved with quantum data.

## Abstract

Iron plays a crucial role in studies of late-type stars. In their atmospheres, Fe I is the minority species and its lines are subject to the departures from LTE. In contrast, one believes that LTE is a realistic approximation for Fe II 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. We investigate the effect of Fe I + H I and Fe II + H I collisions and their different treatment on the Fe I/Fe II ionisation equilibrium and Fe abundance determinations for Galactic halo benchmark stars HD84937, HD122563, and HD140283 and a sample of 38 very metal-poor giants in the dwarf galaxies with well known distances. We perform the NLTE calculations for Fe I-II with using quantum-mechanical (QM) rate coefficients for collisions with H I from Barklem (2018, B18), Yakovleva, Belyaev, and Kraemer (2018, YBK18), and Yakovleva, Belyaev, and Kraemer (2019). We find that collisions with H I serve as efficient thermalisation processes for Fe II and the NLTE abundance corrections for Fe II lines do not exceed 0.02 dex at [Fe/H] > -3 and reach +0.06~dex at [Fe/H] ~ -4. For given star, the B18 and YBK18 treatments of Fe I + H I collisions lead to similar average NLTE abundances from Fe I lines, although there exist discrepancies in the NLTE corrections for individual lines. With using QM collisional data and the Gaia based surface gravity, we obtain consistent abundances from Fe I and Fe II for a red giant HD122563. For HD84937 and HD140283, we study the Fe lines in the visible and the UV (1968-2990 A) range. For both Fe I and Fe II, abundances from the visible and UV lines are consistent. The abundances from Fe I and Fe II agree within 0.10 and 0.13 dex in the YBK18 and B18 cases. The Fe I/Fe II ionisation equilibrium is achieved for each [Fe/H] > -3.5 star of our dwarf galaxy sample.