White Dwarfs in NGC 6791: Avoiding the Helium Flash

Abstract

We propose that the anomalously bright white dwarf luminosity function observed in NGC 6791 (Bedin et al.) is the consequence of the formation of 0.5 Msolar white dwarfs with helium cores instead of carbon cores. This may happen if mass loss during the ascent of the red giant branch is strong enough to prevent a star from reaching the helium flash. Such a model can explain the slower white dwarf cooling (relative to standard models) and fits naturally with scenarios advanced to explain extreme horizontal branch stars, a population of which are also found in this cluster.