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

TL;DR

This study models the evolution of metal-rich AGB stars with varying helium content to understand carbon star formation, revealing that increased helium reduces carbon star production and may explain their scarcity in certain regions.

Contribution

It provides new theoretical models showing how helium enrichment affects carbon star formation in metal-rich populations, a novel insight into stellar evolution.

Findings

Higher metallicity narrows the mass range for carbon star formation.

Helium enrichment reduces the efficiency of third dredge-up in AGB stars.

Helium-rich populations are less likely to produce carbon stars.

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.