Non-LTE line formation of Fe in late-type stars – I. Standard stars with 1D and 〈3D〉 model atmospheres

Abstract

We investigate departures from local thermodynamic equilibrium (LTE) in the line formation of Fe for a number of well‐studied late‐type stars in different evolutionary stages. A new model of the Fe atom was constructed from the most up‐to‐date theoretical and experimental atomic data available so far. Non‐LTE (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 Drawin's 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. Moreover, the NLTE spectroscopic effective temperatures and gravities from ionization balance agree with that determined by other methods, e.g. the infrared flux method and parallaxes, if one of the stellar parameters is constrained independently.

A comprehensive chemical abundance study of the outer halo globular cluster M 75

“…3). We note that the differences are larger for cooler stars, which might be due to the use of plane-parallel models instead of spherical or 3D ones (Bergemann et al 2012), departure from local thermodynamic equilibrium (LTE; Heiter & Eriksson 2006;Bergemann et al 2012;Ruchti et al 2013), or unknown blends. Additionally, the use of iron lines with a broad range of excitation potentials (from 1 to 5 eV) could also cause some discrepancy, as noted by and .…”

GaiaFGK benchmark stars: Metallicity

“…All aforementioned works explain this effect as due to departures from LTE, surface granulations, incomplete opacities, chromospheric and magnetic activity, and so on. For an extensive discussion on this issue for five of our GBS (the Sun, Procyon, HD 122563, HD 140283, HD 84937, and HD 122563), see also Bergemann et al (2012).…”

“…The effect is typically larger 1 http://physics.nist.gov/PhysRefData/ASD/lines_form. html for metal-poor and giant stars (Thévenin & Idiart 1999;Bergemann et al 2012;Lind et al 2012). Only a few methods make corrections to the abundances due to these effects (e.g.…”

The Galactic chemical evolution of oxygen inferred from 3D non-LTE spectral-line-formation calculations

“…These were pre-calculated assuming LTE and the chemical composition of the atmosphere being modelled (Collet et al 2005;Bergemann et al 2012).…”