Formation and evolution of helium white dwarfs in the old metal-rich open clusters NGC6791

A. D. ROMERO, J. A. PANEI & L. G. ALTHAUS

La Plata, Argentina

Workshop; First La Plata International School on Astronomy and Geophysics: Compact Objects and Their Emissions; 2008

Facultad de Ciencias Astronmicas y Geofisicas - UNLP

NGC 6791 is both the oldest (~8 Gyr) and most metal-rich ([Fe&H]~+0.4) open cluster in our Galaxy. This cluster contains a large blue-strangler population, and an extremely blue horizontal branch. In addition, it has been shown that two-thirds of the NGC6791 members white dwarfs have masses below ~0.46 solar masses at which helium is ignited in the core. In particular Extreme Horizontal Branch stars, and also low mass helium white dwarf, can be formed as a result of increased mass loss in post main sequence evolutionary stages, since mass loss is much more efficient in high metallicity stages, since mass loss is much more efficient in high metallicity environments. In this work we explore the formation and  evolution of low mass helium white dwarfs in a high metallicity environment. We compute full evolutionary sequences from the beginning of the main sequence to the white dwarf cooling curve, considering the single stellar evolutionary channel with strong mass loss on the red giant branch.