Lithium evolution in metal-poor stars: From pre-main sequence to the Spite plateau

Abstract

Lithium abundance derived in metal-poor main-sequence (MS) stars is about three times lower than the value of primordial Li predicted by the standard big bang nucleosynthesis when the baryon density is taken from the cosmic microwave background or the deuterium measurements. This disagreement is generally referred as the lithium problem.We here reconsider the stellar Li evolution from the pre-main sequence (PMS) to the end of theMS phase by introducing the effects of convective overshooting (OV) and residual mass accretion.We show that 7Li could be significantly depleted by convective OV in the PMS phase and then partially restored in the stellar atmosphere by a tail of matter accretion which follows the Li-depletion phase and that could be regulated by EUV photoevaporation. By considering the conventional nuclear burning and microscopic diffusion along the MS, we can reproduce the Spite plateau for stars with initial mass m0 = 0.62-0.80 M⊙, and the Li declining branch for lower mass dwarfs, e.g. m0 = 0.57-0.60 M⊙, for a wide range of metallicities (Z = 0.00001 to Z = 0.0005), starting from an initial Li abundance A(Li) = 2.72. This environmental Li evolution model also offers the possibility to interpret the decrease of Li abundance in extremely metal-poor stars, the Li disparities in spectroscopic binaries and the low Li abundance in planet hosting stars.