Lithium nuclear destruction in stellar outer layers - A consistent theoretical view of the characteristic features observed in young and old stars

Lithium nuclear destruction in young and old stars has been computed within the framework of Zahn's theory of turbulent mixing induced by rotation. The lithium gap observed for F stars in the Hyades and other clusters by Boesgaard and her collaborators may be due to nuclear destruction, as already proposed by Boesgaard. The destruction law follows the increase of the rotation velocities with Teff in these F stars. Above 6700 K the existence of two separate zones of meridional circulation may prevent complete mixing between the bottom of the convection zone and the lithium nuclear burning layer so that lithium remains normal at the surface as observed. A fit of the observations with the theoretical results gives a parameter-free value for the mixing length, which must be equal to 1.9 times the pressure scale height, consistent with recent computations of solar evolutionary tracks and solar oscillations. For Population II stars, preliminary computations show that the observed lithium abundance of Li/H = 10-10 could also be due to nuclear destruction with an original value of 10-9. The "plateau" observed by Spite and Spite and confirmed by more recent observations may be accounted for in this framework owing to the rapid increase of the turbulent diffusion coefficient with radius. In this case the primordial lithium abundance would be 10-9 instead of 10-10. The two possibilities are discussed.