6Li in metal-poor halo stars: real or spurious?

TL;DR

This paper investigates how convective motions in metal-poor halo stars affect the measurement of lithium isotopic ratios, revealing that ignoring line asymmetries can lead to overestimating 6Li presence and questioning previous detections.

Contribution

It quantifies the impact of convective line asymmetries on 6Li/7Li ratio measurements, suggesting previous estimates may be significantly overestimated.

Findings

6Li/7Li ratios should be reduced by ~0.015 when accounting for asymmetries

Number of confirmed 6Li detections decreases from 9 to 4 stars

Questions the existence of a 6Li plateau in halo stars

Abstract

The presence of convective motions in the atmospheres of metal-poor halo stars leads to systematic asymmetries of the emergent spectral line profiles. Since such line asymmetries are very small, they can be safely ignored for standard spectroscopic abundance analysis. However, when it comes to the determination of the 6Li/7Li isotopic ratio, q(Li)=n(6Li)/n(7Li), the intrinsic asymmetry of the 7Li line must be taken into account, because its signature is essentially indistinguishable from the presence of a weak 6Li blend in the red wing of the 7Li line. In this contribution we quantity the error of the inferred 6Li/7Li isotopic ratio that arises if the convective line asymmetry is ignored in the fitting of the lithium blend at 6707 A. Our conclusion is that 6Li/7Li ratios derived by Asplund et al. (2006), using symmetric line profiles, must be reduced by typically dq(Li) ~ 0.015. This diminishes the number of certain 6Li detections from 9 to 4 stars or less, casting some doubt on the existence of a 6Li plateau.