ADVANCED EVOLUTIONARY PHASES OF LARGE-METALLICITY LOW-MASS STARS AND THE PROBLEM OF THE ULTRAVIOLET EXCESS OF ELLIPTIC GALAXIES

We present a set of evolutionary tracks of large-metallicity low-mass stars, covering the advanced evolutionary phases from the zero-age horizontal branch to the final white dwarf cooling phase, at the onset of crystallization. These star models are intended to be representative of the hot component of the stellar population of elliptical galaxies and are suitable for population synthesis purposes. Physical parameters of the evolved stars have been selected to search for models leaving the asymptotic giant branch (AGB) phase before the onset of the thermal pulses (TP) and evolving to the blue side of the H-R diagram according to an evolutionary scheme earlier defined as post-early AGB (P-EAGB) evolution (Brocato et al.). Star models have been first evolved at constant mass. A selected sample of more massive models, which in absence of mass loss have been found to enter the TP-AGB, has been successively evolved adopting a moderately large rate of mass loss at the red side of the H-R diagram. As a consequence of mass loss, these models skip the thermal pulsing phase and evolve following a post-EAGB evolutionary path. We find that the maximum UV flux that post-AGB models can provide is not enough to account for the ultraviolet emission (UV) of the most UV-emitting elliptical galaxies, whereas models following the P-EAGB evolutionary behavior produce much more UV flux. Among the star models evolving off the early AGB, before the onset of thermal pulses, those maximizing the production of the UV flux give enough UV light to explain even the most UV upturned branches observed in the spectra of the most metal-rich elliptical galaxy. This is because in post-EAGB models the excursion toward the blue side of the H-R diagram is supported by the additional energy source provided by the He-burning shell. However, the fact that just the strongest UV emitters of the P-EAGB progeny appear able to produce the needed amount of flux to explain the most UV-emitting elliptical galaxies, and that a fine tuning of the stellar parameters is required to produce such an evolutionary behavior, which occurs for a restricted value of these parameters, suggests that it is likely that not only P-EAGB stars are at work in nature to produce the UV light of these galaxies.