NLTE line formation of Fe for late-type stars. I. Standard stars with 1D and <3D> model atmospheres

TL;DR

This study examines non-LTE effects on Fe line formation in late-type stars using 1D and <3D> models, revealing improved consistency in stellar parameters and metallicities with advanced modeling techniques.

Contribution

It introduces a new Fe atom model and compares LTE and NLTE line formation in 1D and mean 3D atmospheres, highlighting the importance of NLTE effects and atmospheric structure.

Findings

NLTE ionization balance is well achieved with 1D and mean 3D models.

Strong low-excitation Fe I lines are sensitive to atmospheric structure.

Mean 3D models yield better excitation balance for Fe I lines.

Abstract

We investigate departures from LTE in the line formation of Fe for a number of well-studied late-type stars in different evolutionary stages. A new model of Fe atom was constructed from the most up-to-date theoretical and experimental atomic data available so far. Non-local thermodynamic equilibrium (NLTE) line formation calculations for Fe were performed using 1D hydrostatic MARCS and MAFAGS-OS model atmospheres, as well as the spatial and temporal average stratifications from full 3D hydrodynamical simulations of stellar convection computed using the Stagger code. It is shown that the Fe I/Fe II ionization balance can be well established with the 1D and mean 3D models under NLTE including calibrated inelastic collisions with H I calculated from the Drawin's (1969) formulae. Strong low-excitation Fe I lines are very sensitive to the atmospheric structure; classical 1D models fail to provide consistent excitation balance, particularly so for cool metal-poor stars. A better agreement between Fe I lines spanning a range of excitation potentials is obtained with the mean 3D models. Mean NLTE metallicities determined for the standard stars using the 1D and mean 3D models are fully consistent. Also, the NLTE spectroscopic effective temperatures and gravities from ionization balance agree with that determined by other methods, e.g., infrared flux method and parallaxes, if one of the stellar parameters is constrained independently.