THE CONTRIBUTION OF ADVANCED POSTHAYASHI TRACK EVOLUTIONARY PHASES TO THE ULTRAVIOLET-LIGHT OF ELLIPTIC GALAXIES

The post red giant branch (RGB), the post early asymptotic giant branch (EAGB), and the post asymptotic giant branch (AGB) evolutionary phases are extensively discussed to derive information on their ultraviolet (UV) light contribution to the spectral energy distribution of old stellar systems. We first investigate the effect of the surface chemical composition on the well-known correlation between the luminosity and the critical hydrogen envelope mass M(cr)env, below which a star evolving along the Hayashi track is forced to turn back to the blue. We find that the surface metallicity represents an additional parameter which should be accounted for in the M(cr)env-luminosity relation. We find, however, that it is possible to define the value of the critical hydrogen envelope mass in a way in which M(cr)env becomes very mildly dependent on the metallicity, while its dependence on the luminosity becomes simple and single-valued whatever the post-Hayashi track evolutionary phase: post-RGB, post-EAGB or post-AGB phase. We also discuss the more commonly used correlation between the core mass and the hydrogen fuel to be burned above a certain temperature along the blueward excursion. We find that for low core masses (post-RGB stars) and large core masses (post-AGB stars) there exists one single correlation linking the fuel with the core mass while, for intermediate core masses (post-EAGB stars), the fuel correlates with the core in a quite different way, turning out to be much larger than in the two adjacent cases. We provide a detailed explanation for the origin of such a dichotomy, whose effect is that post-AGB and post-RGB stars turn out to be faint UV light sources while post-EAGB stars, which are also endowed with a He-burning shell in addition to the H-burning one turn out to be very efficient UV light sources.