Revisiting ΔYZ from multiple main sequences in globular clusters: insight from nearby stars

Authors

  • Laura Portinari,

    Corresponding author
    1. Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland
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  • Luca Casagrande,

    Corresponding author
    1. Max-Planck Institute for Astrophysics, Karl Schwartzschild Straße 1, Garching, Germany
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  • Chris Flynn

    Corresponding author
    1. Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland
    2. FINCA – Finnish Centre for Astronomy with ESO, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland
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E-mail: lporti@utu.fi (LP); luca@mpa-garching.mpg.de (LC); cflynn@utu.fi (CF)

ABSTRACT

For nearby K dwarfs, the broadening of the observed main sequence (MS) at low metallicities is much narrower than expected from isochrones with the standard helium-to-metal enrichment ratio ΔYZ∼ 2. Though the latter value fits well the MS around solar metallicity, and agrees with independent measurements from H ii regions as well as with theoretical stellar yields and chemical evolution models, a much higher ΔYZ∼ 10 is necessary to reproduce the broadening observed for nearby subdwarfs. This result resembles, on a milder scale, the very high ΔYZ estimated from the multiple MSs in ω Cen and NGC 2808. Although not ‘inverted’ as in ω Cen, where the metal-rich MS is bluer than the metal-poor one, the broadening observed for nearby subdwarfs is much narrower than stellar models predict for a standard helium content. We use this empirical evidence to argue that a revision of lower MS stellar models, suggested from nearby stars, could significantly reduce the helium content inferred for the subpopulations of those globular clusters. A simple formula based on empirically calibrated homology relations is constructed for an alternative estimate of ΔYZ in multiple MSs. We find that, under the most favourable assumptions, the estimated helium content for the enriched populations could decrease from Y≃ 0.4 to as low as Y≃ 0.3.

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