NLTE abundances of Cr in the Sun and metal-poor stars

TL;DR

This study performs the first NLTE calculations for chromium in late-type stars, revealing significant corrections to LTE-based abundances and clarifying the Cr/Fe trend in the Galaxy.

Contribution

It introduces NLTE modeling for Cr in metal-poor stars, improving abundance accuracy and resolving discrepancies in spectroscopic analyses.

Findings

NLTE corrections to Cr I lines are +0.3 to +0.5 dex at low metallicity.

NLTE [Cr/Fe] trend is flat in halo and disk stars.

LTE assumptions lead to large abundance errors.

Abstract

We investigate statistical equilibrium of Cr in the atmospheres of late-type stars. The main goal is to ascertain the reason for a systematic abundance discrepancy between Cr I and Cr II lines, which is often encountered in spectroscopic analyses of metal-poor stars. Up to now, all these studies relied on the assumption of local thermodynamic equilibrium (LTE) in the spectrum modelling. For the first time, we perform NLTE calculations in subdwarfs and subgiants of different metallicities. We show that the LTE assumption is inadequate to describe excitation-ionization equilibrium of Cr I/Cr II in stellar atmospheres and, as a result, leads to large errors in abundances. In particular, the NLTE abundance corrections to Cr I lines range from $+0.3$ to $+0.5$ dex at low [Fe/H]. The NLTE [Cr/Fe] trend in the halo and the disk is flat and can be reproduced by most of the models of Galactic chemical evolution with standard prescriptions for Cr and Fe nucleosynthesis.