Helium enrichment and carbon-star production in metal-rich populations

Amanda I. Karakas*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    78 Citations (Scopus)

    Abstract

    We present new theoretical stellar evolutionary models of metal-rich asymptotic giant branch (AGB) stars. Stellar models are evolved with initial masses between 1 and 7M at Z = 0.007, and 1 and 8M 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, and 0.40) at Z = 0.014 and 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 and 7M at Z = 0.007 and between 2 and 4.5M at solar metallicity. At Z = 0.03, the mass range for C-star production is narrowed to 3.25-4M. 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 ΔY = 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 ΔY ≳ 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.

    Original languageEnglish
    Pages (from-to)347-358
    Number of pages12
    JournalMonthly Notices of the Royal Astronomical Society
    Volume445
    Issue number1
    DOIs
    Publication statusPublished - 8 Oct 2014

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