The Evolution of Stars of Medium Mass

Abstract

AbstractThe main properties of the evolution of low, intermediate, and high mass stars are reviewed focusing on a few issues tightly related to the interpretation of Pop I Cepheid stars. After a summary discussion of the physical mechanism responsible for the Cepheid pulsation, the classical results of stellar evolution theory for the main evolutionary phases (main sequence, core He-burning, and later) all over the HR diagram are presented, putting into evidence the various points of disagreement with current observational data. We then review the models incorporating the effect of convective overshoot, and present in some detail a study on the rich, young cluster NGC 1866 in the Large Magellanic Cloud, in which they are compared with the observational data. Arguments are given to favour the adoption of models with convective overshoot instead of the classical ones. The topics of the mass discrepancy and frequency-period distribution of Cepheid stars are then discussed in the light of models with convective overshoot, showing how they can better account for the observational data. Finally, we address the question of the enhancement in the radiative opacity of heavy elements in the middle temperature region (approximately where the ultimate ionization of the CNO group of elements occurs), which has been suggested to explain the mass anomalies and period ratios encountered in double-mode and bump Cepheids. Evolutionary models for intermediate mass stars incorporating the above modification in the radiative opacity are then presented and discussed in some detail.