3D non-LTE corrections for Li abundance and 6Li/7Li isotopic ratio in solar-type stars I. Application to HD 207129 and HD 95456

TL;DR

This study provides practical 3D NLTE correction formulas for lithium abundance and isotopic ratios in solar-type stars, improving the accuracy of spectroscopic analyses over traditional 1D LTE methods.

Contribution

It introduces analytical corrections for 3D NLTE effects on lithium measurements, applicable to G and F dwarf stars, enhancing the precision of isotopic ratio determinations.

Findings

3D NLTE corrections range from -0.01 to +0.11 dex for A(Li)

Corrections for 6Li/7Li ratio are between -4.9% and -0.4%

1D LTE analysis can overestimate 6Li presence by up to 4.9 percentage points

Abstract

Convective motions in solar-type stellar atmospheres induce Doppler shifts that affect the strengths and shapes of spectral absorption lines and create slightly asymmetric line profiles. 1D LTE studies of elemental abundances are not able to reproduce this phenomenon, which becomes particularly important when modeling the impact of isotopic fine structure, like the subtle depression created by the 6Li isotope on the red wing of the Li I resonance doublet line. The purpose of this work is to provide corrections for the lithium abundance, A(Li), and the 6Li/7Li ratio that can easily be applied to correct 1D LTE lithium abundances in G and F dwarf stars of approximately solar mass and metallicity for 3D and non-LTE (NLTE) effects. The corrections for A(Li) and 6Li/7Li are computed using grids of 3D NLTE and 1D LTE synthetic lithium line profiles, generated from 3D hydro-dynamical CO5BOLD and 1D hydrostatic model atmospheres, respectively. The 3D NLTE corrections are then approximated by analytical expressions as a function of the stellar parameters. These are applied to adjust the 1D LTE isotopic lithium abundances in two solar-type stars, HD 207129 and HD 95456. The derived 3D NLTE corrections range between -0.01 and +0.11 dex for A(Li), and between -4.9 and -0.4% for the 6Li/7Li ratio, depending on the adopted stellar parameters. In the case of the 6Li/7Li ratio, our corrections are always negative, showing that 1D LTE analysis can significantly overestimate (up to 4.9 percentage points) the presence of 6Li in the atmospheres of solar-like dwarf stars. These results emphasize the importance of reliable 3D model atmospheres combined with NLTE line formation for deriving precise isotopic lithium abundances. Although 3D NLTE spectral synthesis implies an extensive computational effort, the results can be made accessible with parametric tools like the ones presented in this work.