INFLUENCE OF A STELLAR WIND ON THE LITHIUM DEPLETION IN HALO STARS - A NEW STEP TOWARDS THE LITHIUM PRIMORDIAL ABUNDANCE

Since the first observations by Spite & Spite (1982) of lithium in hal o stars, with an abundance one order of magnitude smaller than the abu ndance derived for young stars, the question has often been addressed of the primordial lithium abundance. Has lithium been destroyed in hal o stars, or is the presently observed lithium abundance representative of the primordial value? If no macroscopic motions occur below the ou ter convection zone in these stars, microscopic diffusion (gravitation al and thermal settling) must take place and deplete lithium in a way which is in contradiction with the observations. Rotation-induced turb ulence has been invoked in previous papers to prevent microscopic diff usion. Such a turbulence leads in turn to lithium depletion due to the mixing of the layers below the convection zone down to the regions wh ere lithium is destroyed by nuclear reactions. This effect has been in voked to suggest that the primordial lithium abundance could be that o f young stars, lithium being depleted by one order of magnitude in hal o stars. It is however difficult in this framework to account for the flatness and the small dispersion of the lithium abundances in the ''p lateau''. In the present paper we introduce a new physical process whi ch can prevent microscopic diffusion without leading to nuclear destru ction: small stellar winds. We show that winds of the order of those o bserved or indirectly derived for PopI stars can lead in PopII stars t o lithium abundance values in very good agreement with the most recent observations: the small positive slope presently observed for the lit hium plateau is reproduced; the dispersion can be due to a range of ma ss loss rates; the stars observed with no lithium in their spectra can be those which suffer the largest rates; the light isotope Li-6 is pr eserved. In this framework, the primordial abundance should be obtaine d 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.