Horizontal-Branch Stars as Sources of the UV Upturn in Early-Type Galaxies

摘要

It is now believed that the UV upturn sources in nearby early-type galaxies are horizontal-branch (HB) stars and their progeny. Recent studies have suggested that the HB temperature variation is mainly caused by age along with metallicity, which makes the UV upturn attractive as a potential age indicator for presumably the oldest populations in the universe—elliptical galaxies. However, the age explanation for HB temperature variation still has a number of hurdles to pass. Here, we summarize the pros and cons of the age hypothesis and report our recent progress in understanding of HB stars. Finally, we discuss whether it is valid to assume that the UV upturn can reliably date early-type galaxies.