ON THE ORIGIN OF THE ULTRAVIOLET UPTURN IN ELLIPTIC GALAXIES - II - TEST OF THE HORIZONTAL-BRANCH HYPOTHESIS

Whether or not metal-rich HE stars are the dominant UV source in giant elliptical galaxies (GEs) is an important question in current astrono mical research. We follow up our previous evolutionary population synt hesis study with quantitative tests to answer this question affirmativ ely under the following three conditions: (1) Reimers's empirical mass -loss formula is proper, (2) the mass-loss efficiency parameter eta in metal-rich stars is somewhat larger than the value estimated from the metal-poor star studies, and (3) the true value of the helium-enrichm ent parameter (Delta Y/Delta Z) is positive. All three important empir ical characteristics of the UV upturn (i.e., the fact that strong UV u pturns are restricted to GEs, the positive UV upturn-metallicity corre lation, and the narrow range of the T-eff of the UV sources) are close ly reproduced for reasonable ranges of input parameters. We discuss th e major sources of uncertainties in the models, such as the production and role of hot horizontal-branch stars in GEs, and the importance of galactic nucleosynthesis.