Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars

TL;DR

This paper improves the accuracy of iron abundance measurements in metal-poor stars by applying advanced 3D non-LTE radiative transfer models, revealing significant differences from traditional 1D models and providing new computational resources.

Contribution

It introduces a comprehensive 3D non-LTE analysis framework for iron in metal-poor stars, including a new quantum-mechanical hydrogen collision model and publicly available correction grids.

Findings

3D non-LTE models achieve excitation and ionization balance within 0.05 dex.

Iron abundances from 3D non-LTE are about 0.1 dex higher than 1D LTE estimates.

For HD122563, the surface gravity is overestimated by 0.5 dex in current models.

Abstract

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 over-estimated 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.