NLTE line formation of Fe in late-type stars II: 1D spectroscopic stellar parameters

TL;DR

This study examines how non-local thermodynamic equilibrium (NLTE) effects influence iron line formation in late-type stars, revealing systematic underestimations of FeI-based abundances in certain stellar conditions and emphasizing the importance of NLTE corrections for accurate stellar parameters.

Contribution

It provides a comprehensive analysis of NLTE effects on Fe line formation in a wide range of stellar models, improving the accuracy of spectroscopic stellar parameter determinations.

Findings

FeI abundances are underestimated in hotter, low-gravity, metal-poor stars.

LTE approximation is generally valid for FeII lines.

NLTE effects significantly impact ionisation and excitation balance in certain stellar regimes.

Abstract

We investigate departures from local thermodynamic equilibrium (NLTE) in the line formation of neutral and singly ionised iron lines and their impact on spectroscopic stellar parameters. The calculations are performed for an extensive grid of 1D MARCS models of metal-rich and metal-poor late-type dwarfs and giants. We find that iron abundances derived from FeI lines are increasingly underestimated in hotter, lower surface-gravity, and more metal-poor stars, in a simple and well-defined pattern, while LTE is usually a realistic approximation for FeII lines. For the vast majority of dwarfs and giants, the perturbed ionisation balance of FeI and FeII is the main relevant NLTE effect to consider in the determination of spectroscopic parameters, while for extremely metal-poor stars and hot giant stars significant impact is seen also on the excitation balance and on the microturbulence determination from FeI lines.