Diffusion, Meridional Circulation and Mass Loss in Main Sequence and Horizontal Branch Stars

Abstract

Photospheric abundances are used to determine the importance of atomic diffusion, meridional circulation, mass loss and turbulence in main sequence and horizontal branch stars. Atomic diffusion leads to the appearance of the Li gap and the AmFm and HgMn phenomena at approximately the Teff at which they are observed. It leads to a 12 to 25% reduction in the age of halo stars. The Li abundance in Halo stars is probably 50% of the original abundance. Atomic diffusion competes with meridional circulation. The V sin i at which the HgMn and AmFm phenomena disappear give an argument in favour of the meridional circulation model of Tassoul and Tassoul. Mass loss is probably present in AmFm stars and cooler F stars but only at the rate of 10-15 M⊙ yr-1. In many objects, the turbulent particle diffusion coefficient is at most 10 times larger than the atomic diffusion coefficient.