Helium enrichment and Carbon-star Production in Metal-rich Populations

Abstract

We present new theoretical stellar evolutionary models of metal-rich asymptotic giant branch (AGB) stars. Stellar models are evolved with initial masses between 1Msun and 7Msun at Z=0.007, and 1Msun and 8Msun at Z=0.014 (solar) and at Z=0.03. We evolve models with a canonical helium abundance and with helium enriched compositions (Y=0.30, 0.35, 0.40) at Z=0.014 and Z=0.03. The efficiency of third dredge-up and the mass range of carbon stars decreases with an increase in metallicity. We predict carbon stars form from initial masses between 1.75-7Msun at Z=0.007 and between 2-4.5Msun at solar metallicity. At Z=0.03 the mass range for C-star production is narrowed to 3.25-4Msun. The third dredge-up is reduced when the helium content of the model increases owing to the reduced number of thermal pulses on the AGB. A small increase of Delta Y = 0.05 is enough to prevent the formation of C stars at Z=0.03, depending on the mass-loss rate, whereas at Z=0.014, an increase of Delta Y = 0.1 is required to prevent the formation of C stars. We speculate that the probability of finding C stars in a stellar population depends as much on the helium abundance as on the metallicity. To explain the paucity of C stars in the inner region of M31 we conclude that the observed stars have Y > 0.35 or that the stellar metallicity is higher than [Fe/H] ~ 0.1.