Mass Loss on the Horizontal Branch: an Application to Ngc 6791 Hwanjong Yong and Pierre Demarque

The presence of a substantial number of hot stars in the extremely metal-rich open cluster NGC 6791 has been a mystery. If these hot stars are in their core helium burning phase, they are significantly bluer (hotter) than predicted by canonical stellar evolution theory. No obvious explanation is available yet. We consider the effects of mass loss during the evolution of horizontal branch (HB) stars as their possible origin. We find that the addition of mass loss causes HB stars to evolve to be hotter and fainter. Mass loss has a more pronounced effect for less massive stars and thus naturally widens the temperature (and color) distribution on the HB. If mass loss rates 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 mass loss on the HB may be a viable method of forming subdwarf B (sdB) stars both in the field and in clusters, especially when the metallicity is high. Subject headings: stars: evolution stars: horizontal-branch stars: mass loss