The initial helium abundance of the Galactic globular cluster system

Free access article

Issue		A&A Volume 420, Number 3, June IV 2004


Page(s)		911 - 919
Section		Galactic structure, stellar clusters and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20035607

A&A 420, 911-919 (2004)
DOI: 10.1051/0004-6361:20035607

The initial helium abundance of the Galactic globular cluster system

M. Salaris^{1, 2}, M. Riello^{3, 4}, S. Cassisi⁵ and G. Piotto³

¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85758 Garching, Germany
³ Dipartimento di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
e-mail: piotto,riello@pd.astro.it
⁴ INAF - Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁵ INAF - Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
e-mail: cassisi@te.astro.it

(Received 31 October 2003 / Accepted 17 March 2004 )

Abstract
In this paper we estimate the initial He content in about 30% of the Galactic globular clusters (GGCs) from new star counts we have performed on the recently published HST snapshot database of Colour Magnitude Diagrams (Piotto et al. 2002). More specifically, we use the so-called R-parameter and estimate the He content from a theoretical calibration based on a recently updated set of stellar evolution models. We performed an accurate statistical analysis in order to assess whether GGCs show a statistically significant spread in their initial He abundances, and whether there is a correlation with the cluster metallicity. As in previous works on the subject, we do not find any significant dependence of the He abundance on the cluster metallicity; this provides an important constraint for models of Galaxy formation and evolution. Apart from GGCs with the bluest Horizontal Branch morphology, the observed spread in the individual helium abundances is statistically compatible with the individual errors. This means that either there is no intrinsic abundance spread among the GGCs, or that this is masked by the errors. In the latter case we have estimated a firm 1 $\sigma$ upper limit of 0.019 to the possible intrinsic spread. In case of the GGCs with the bluest Horizontal Branch morphology we detect a significant spread towards higher abundances inconsistent with the individual errors; this can be fully explained by additional effects not accounted for in our theoretical calibrations, which do not affect the abundances estimated for the clusters with redder Horizontal Branch morphology. In the hypothesis that the intrinsic dispersion on the individual He abundances is zero, taking into account the errors on the individual R-parameter estimates, as well as the uncertainties on the cluster metallicity scale and theoretical calibration, we have determined an initial He abundance mass fraction $Y_{\rm GGC}=0.250\pm0.006$ . This value is in perfect agreement with current estimates based on Cosmic Microwave Background radiation analyses and cosmological nucleosynthesis computations.

Key words: Galaxy: globular clusters: general -- Galaxy: evolution -- stars: evolution -- stars: Population II

Offprint request: M. Salaris, ms@astro.livjm.ac.uk

SIMBAD Objects

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.