Mass Loss on the Horizontal Branch: An Application to NGC 6791

The presence of a substantial number of hot stars in the extremelymetal-rich} open cluster NGC6791 has been a mystery. If these hot stars are intheir core helium burning phase, they are significantly bluer (hotter) thanpredicted by canonical stellar evolution theory. No obvious explanation isavailable yet. We consider the effects of mass loss during the evolution ofhorizontal branch (HB) stars as their possible origin. We find that theaddition of mass loss causes HB stars to evolve to be hotter and fainter. Massloss has a more pronounced effect for less massive stars and thus naturallywidens the temperature (and color) distribution on the HB. If mass loss ratesare higher for more metal-rich stars, this phenomenon would be even morepronounced in the metal-rich populations, such as NGC6791. We find that massloss on the HB may be a viable method of forming subdwarf B (sdB) stars both inthe field and in clusters, especially when the metallicity is high.Comment: Accepted for publication in Astrophysical Journa