Astronomy and Astrophysics, volume 295, 715-724 (1995/3-3)
Influence of a stellar wind on the lithium depletion in halo stars: a new step towards the lithium primordial abundance.
VAUCLAIR S. and CHARBONNEL C.
Abstract (from CDS):
Since the first observations by Spite & Spite (1982A&A...115..357S) of lithium in halo stars, with an abundance one order of magnitude smaller than the abundance derived for young stars, the question has often been addressed of the primordial lithium abundance. Has lithium been destroyed in halo stars, or is the presently observed lithium abundance representative of the primordial value? If no macroscopic motions occur below the outer convection zone in these stars, microscopic diffusion (gravitational and thermal settling) must take place and deplete lithium in a way which is in contradiction with the observations. Rotation-induced turbulence has been invoked in previous papers to prevent microscopic diffusion. Such a turbulence leads in turn to lithium depletion due to the mixing of the layers below the convection zone down to the regions where lithium is destroyed by nuclear reactions. This effect has been invoked to suggest that the primordial lithium abundance could be that of young stars, lithium being depleted by one order of magnitude in halo stars. It is however difficult in this framework to account for the flatness and the small dispersion of the lithium abundances in the "plateau". In the present paper we introduce a new physical process which can prevent microscopic diffusion without leading to nuclear destruction: small stellar winds. We show that winds of the order of those observed or indirectly derived for PopI stars can lead in PopII stars to lithium abundance values in very good agreement with the most recent observations: the small positive slope presently observed for the lithium plateau is reproduced; the dispersion can be due to a range of mass loss rates; the stars observed with no lithium in their spectra can be those which suffer the largest rates; the light isotope 6Li is preserved. In this framework, the primordial abundance should be obtained from the upper values of the lithium abundances in the plateau, and not from their average value. This lead to a primordial abundance of 2.5±0.1.
diffusion - stars: abundances - stars: evolution - stars: interiors - stars: mass loss - cosmology: miscellaneous