Journal Article

Revisiting Δ<i>Y</i>/Δ<i>Z</i> from multiple main sequences in globular clusters: insight from nearby stars

Laura Portinari, Luca Casagrande and Chris Flynn

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 406, issue 3, pages 1570-1582
Published in print August 2010 | ISSN: 0035-8711
Published online August 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.16811.x
Revisiting ΔY/ΔZ from multiple main sequences in globular clusters: insight from nearby stars

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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.

Keywords: stars: abundances; stars: fundamental parameters; subdwarfs; globular clusters: individual: ω Cen, NGC 2808; Hertzsprung–Russell and colour-magnitude diagrams

Journal Article.  9708 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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