Zero-age horizontal branch structures: pseudo versus fully evolutionary models

Abstract

This paper addresses the problem of pseudo-evolutionary zero-age horizontal branch (ZAHB) models, as produced in the current literature, starting from suitable assumptions about the post-He flash structures. To test the accuracy of this procedure, we present evolutionary computations of 0.9-M ○. stellar models from their pre-main sequence through H burning and the He flash phases until the end of quiescent central He burning, for selected assumptions about the star metallicity and the amount of mass lost before the onset of He ignition. The effects of mass loss on the evolution of low-mass red giants are shortly revisited, discussing in some detail the evolution from the red giant tip to the onset of quiescent He burning as well as the chemical structure of the newborn HB stars. We find that the procedure adopted to produce self-consistent pseudo-evolutionary ZAHB models appears supported by detailed computations covering a large range of He flashing structures for different assumptions on both the rate of mass loss and the star metallicity. By comparing the evolution of both types of models through the whole phase of central He burning, we conclude that pseudo-evolutionary ZAHB models represent an excellent approximation of the actual behaviour of low-mass He-burning structures. As a relevant point, we find that no 'blue nose' is expected at the end of the pre-HB evolutionary path and, as a consequence, that pre-HB evolution does not play any role in the modal stability of RR Lyrae pulsators.