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Revisiting the helium abundance in globular clusters with multiple main sequences

Published online by Cambridge University Press:  23 April 2010

Luca Casagrande
Affiliation:
Max Planck Institue for Astrophysics, Karl Schwartzschild Straße 1, Garching, Germany email: luca@mpa-garching.mpg.de
Laura Portinari
Affiliation:
Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland email: lporti@utu.fi, cflynn@utu.fi
Chris Flynn
Affiliation:
Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland email: lporti@utu.fi, cflynn@utu.fi
Corresponding
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Abstract

For nearby K dwarfs, the broadening of the observed Main Sequence at low metallicities is much narrower than expected from isochrones with the standard helium–to–metal enrichment ratio ΔYZ~2. A much higher value, of order 10, is formally needed to reproduce the observed broadening, but it returns helium abundances in awkward contrast with Big Bang Nucleosynthesis. This steep enrichment ratio resembles, on a milder scale, the very high ΔYZ estimated from the multiple Main Sequences observed in some metal-poor Globular Clusters. We argue that a revision of low Main Sequence stellar models, suggested from nearby stars, could help to reduce the overwhelmingly high ΔYZ deduced so far for those clusters. Under the most favourable assumptions, the estimated helium content for the enriched populations may decrease from Y ≃ 0.4 to as low as Y ≃ 0.3, with intermediate values being plausible.

Type
Contributed Papers
Copyright
Copyright © International Astronomical Union 2010

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