Mass Loss and the Evolution of Massive Stars

Mass loss is a dominant effect of massive star evolution and together with rotation it has a deep impact on the evolution of star clusters. Mass loss influences the age estimates, the CNO abundances at the stellar surface, the properties of Be, B[e], LBV and WR stars, the supernova progenitors, the rotation properties of pulsars, the chemical yields and the spectral evolution of galaxies.

We first examine how mass loss behaves as a function of mass in comparison with internal mixing. The asymmetries of the mass loss by stellar winds in rotating stars influence the loss of angular momentum, a question of importance regarding the occurrence of GRBs. Rotation also globally enhances the mass loss rates. We discuss the filiations between the various kinds of massive stars, the effects of mass loss on the rotation velocities, on the changes of chemical abundances and on the chemical yields. A big issue concerns the stellar winds at low metallicity Z. Amazingly, if massive stars lose little mass by stellar winds, they reach critical rotation and experience a high rotational mass loss. We show that the CNO, Mg, Al anomalies observed in the most metal deficient stars of the galactic halo, as well as the He--rich stars in the globular cluster ω Cen, remarkably correspond to enrichments by rotational mass loss, which seems to play a great role in the early galactic evolution.