3D non-LTE corrections for Li abundance and ⁶Li/⁷Li isotopic ratio in solar-type stars^⋆

I. Application to HD 207129 and HD 95456

¹ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: gharutyunyan@aip.de
² GEPI, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, Place Jules Janssen, 92190 Meudon, France
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain

Received: 19 February 2018
Accepted: 29 June 2019

Abstract

Context. 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. One-dimensional (1D) local thermodynamic equilibrium (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.

Aims. The purpose of this work is to provide corrections for the lithium abundance, A(Li), and the ⁶Li/⁷Li isotopic 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 three-dimensional (3D) and non-LTE (NLTE) effects.

Methods. 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. For comparative purposes, all calculations are performed for three different line lists representing the Li I λ670.8 nm spectral region. The 3D NLTE corrections are then approximated by analytical expressions as a function of the stellar parameters (T_eff, log ℊ, [Fe/H], ν sin i, A(Li), ⁶Li/⁷Li). These are applied to adjust the 1D LTE isotopic lithium abundances in two solar-type stars, HD 207129 and HD 95456, for which high-quality HARPS observations are available.

Results. The derived 3D NLTE corrections range between −0.01 and +0.11 dex for A(Li), and between −4.9 and −0.4% for ⁶Li/⁷Li, depending on the adopted stellar parameters. We confirm that the inferred ⁶Li abundance depends critically on the strength of the Si I 670.8025 nm line. Our findings show a general consistency with recent works on lithium abundance corrections. After the application of such corrections, we do not find a significant amount of ⁶Li in any of the two target stars.

Conclusions. In the case of ⁶Li/⁷Li, our corrections are always negative, showing that 1D LTE analysis can significantly overestimate the presence of ⁶Li (up to 4.9% points) 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.