Li isotopes in metal-poor halo dwarfs, a more and more complicated story

TL;DR

This paper reviews lithium isotope abundances in metal-poor stars, highlighting discrepancies with Big Bang predictions and discussing potential astrophysical processes affecting lithium synthesis and depletion.

Contribution

It provides a comprehensive analysis of lithium isotope observations in metal-poor stars and explores possible explanations for the lithium abundance discrepancies with cosmological models.

Findings

Lithium abundances in metal-poor stars are lower than Big Bang predictions.

A high 6Li abundance has been observed in some stars, challenging existing theories.

Star-to-star scatter in lithium increases at very low metallicities.

Abstract

The nuclei of the lithium isotopes are fragile, easily destroyed, so that, at variance with most of the other elements, they cannot be formed in stars through steady hydrostatic nucleosynthesis. The 7Li isotope is synthesized during primordial nucleosynthesis in the first minutes after the Big Bang and later by cosmic rays, by novae and in pulsations of AGB stars (possibly also by the "nu" process). 6Li is mainly formed by cosmic rays. The oldest (most metal-deficient) warm galactic stars should retain the signature of these processes if, (as it had been often expected) lithium is not depleted in these stars. The existence of a "plateau" of the abundance of 7Li (and of its slope) in the warm metal-poor stars is discussed. At very low metallicity ([Fe/H]<-2.7 dex the star to star scatter increases significantly towards low Li abundances. The highest value of the lithium abundance in the early stellar matter of the Galaxy (A(7Li) = 2.2 dex) is much lower than the the value A(7Li) = 2.72 predicted by the standard Big Bang nucleosynthesis, according to the specifications found by the satellite WMAP. After gathering a homogeneous stellar sample, and analysing its behaviour, possible explanations of the disagreement between Big Bang and stellar abundances are discussed (including early astration and diffusion). On the other hand, possibilities of lower productions of 7Li in the standard and/or non-standard Big Bang nucleosyntheses are briefly evoked. A surprisingly high value (A(6Li)=0.8 dex) of the abundance of the 6Li isotope has been found in a few warm metal-poor stars. Such a high abundance of 6Li independent of the mean metallicity in the early Galaxy cannot be easily explained. But are we really observing 6Li ?