Evolutionary and Pulsational Constraints for Super-metal-rich Stars with Z=0.04

Abstract

We investigate the evolutionary behavior of stellar structures with metallicity Z=0.04 in order to disclose theoretical expectations for both evolutionary and pulsational behaviors of Super-Metal-Rich (SMR) objects, which are found in the solar neighborhood, in the Galactic bulge and in elliptical galaxies. We find that at the exhaustion of central helium SMR stars definitely undergo the gravonuclear instabilities previously found in some He burning structures with solar metallicity (Bono et al. 1997b). On the basis of the evolutionary scenario, we investigate the expected pulsational behavior of He burning SMR stars for suitable assumptions on the pulsators evolutionary parameters. Nonlinear, nonlocal, and time-dependent convective models are discussed, the modal stability is investigated for the first two modes, and the theoretical predictions about the period distribution inside the instability strip and the shape of both light and velocity curves are presented. Full amplitude, nonlinear envelope models show that the range of effective temperatures where SMR RR Lyrae variables present a stable limit cycle is smaller in comparison with pulsators characterized by lower metal abundances. We show that both periods and light curves of different groups of type II Cepheids with periods shorter than six days, presented by Diethelm (1983,1990), can be all reproduced by suitable variations in the effective temperature or in the luminosity level of our SMR post-HB models, supporting evidence of a substantial homogeneity of these variable.