Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination

TL;DR

This study investigates how convective asymmetries in stellar atmospheres affect the measurement of lithium isotopic ratios, revealing that previous estimates of 6Li may be overestimated due to line asymmetry effects.

Contribution

It introduces two methods to assess convection-related line asymmetries and demonstrates their impact on 6Li/7Li ratio determinations in halo stars.

Findings

Convective asymmetries can mimic 6Li signals in stellar spectra.

Previous 6Li/7Li measurements may be overestimated due to line asymmetries.

A systematic reappraisal of 6Li abundances in halo stars is necessary.

Abstract

Context: Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the 6Li/7Li isotopic ratio is prone to suffering from such asymmetries, as the contribution of 6Li is a slight blending reinforcement of the red wing of each component of the corresponding 7Li line, with respect to its blue wing. Aims: The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination. Method: Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li I, producing absorption lines of approximately the same equivalent width as individual components of the 7Li I resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection. Results: The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the 7Li absorption feature that mimics the presence of 6Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on 6Li/7Li has thus far been derived. The second method confirms these findings. Conclusions: From this work, it appears that a systematic reappraisal of former determinations of 6Li abundances in halo stars is warranted.