Mass Loss from Cool Stars: Impact on the Evolution of Stars and Stellar Populations

This review emphasizes the mass loss processes that affect the fates of single stars with initial masses between one and nine solar masses. Just one epoch of mass loss has been clearly demonstrated to be important for these stars; that is the episode that ends their evolution up the asymptotic giant branch. Quite a clear picture of this evolutionary stage is emerging from current studies. Mass loss rates increase precipitously as stars evolve toward greater luminosity and radius and decreased effective temperature. As a result, empirical relationships between mass loss rates and stellar parameters are determined mostly by selection effects and tell us which stars are losing mass rather than how stars lose mass. After detailed theoretical models are found to match observational constraints, the models may be used to extrapolate to populations not available for study nearby, such as young stars with low metallicity. The fates of stars are found to depend on both their initial masses and their initial metallicities; a larger proportion of low-metallicity stars should end up with core masses reaching the Chandrasekhar limit, giving rise to Type 1.5 supernovae, and the remnant white dwarfs of low-Z populations will be both fewer and more massive than those in Population I. There are also clear indications that some stars lose one to several tenths of a solar mass during the helium core flash, but neither models nor observations reveal any details of this process yet. The observational and theoretical bases for a variety of mass loss formulae in current use are also reviewed in this article, and the relations are compared in a series of figures.