Mass loss on the horizontal branch: An application to NGC 6791

The presence of a substantial number of hot stars in the extremely metal-ri ch open cluster NGC 6791 has been a mystery. If these hot stars are in thei r core helium-burning phase, they are significantly bluer (hotter) than pre dicted by canonical stellar evolution theory. No obvious explanation is ava ilable yet. We consider the effects of mass loss during the evolution of ho rizontal-branch (HB) stars as their possible origin. We find that the addit ion of mass loss causes HE stars to evolve to be hotter and fainter. Mass l oss has a more pronounced effect for less massive stars and thus naturally widens the temperature (and color) distribution on the HE. If mass-loss rat es are higher for more metal-rich stars, this phenomenon would be even more pronounced in the metal-rich populations, such as NGC 6791. We find that m ass loss on the HE may be a viable method of forming subdwarf B (sdB) stars both in the field and in clusters, especially when the metallicity is high .