Stellar Evolution and Nucleosynthesis

Abstract

Here we address the phases of stellar evolution and nucleosynthesis which are more important in connection to chemical enrichment and galactic chemical evolution. In the following one should keep in mind that the stellar evolution results are influenced by the physics adopted in the models. In particular, two physical processes are important in determining the galactic chemical enrichment, the mass loss occurring during various phases of stellar evolution and the treatment of convection. Various types of mixing are expected to occur during the life of a star: convection in central cores, in shells, in external envelopes, overshooting, rotation induced turbulent diffusion. For the sake of clarity in the next sections we will define here what is defined as “overshooting”. Usually the limit of a stellar convective core is set at the layer where the acceleration of the fluid elements is zero. However, the zero acceleration point does not coincide with the zero velocity point which should be the real limit of the convective core. Consideration of this effect in stellar evolution leads to overshooting (see Chiosi 1987). Most of the formulations for overshooting are based on the mixing-length theory and use the mean free path of convective elements expressed as α_over =1/H _p (H _p is the pressure scale height and is a free parameter). Classical stellar evolution ignores the effects of many of those physical processes.