Issue |
A&A
Volume 420, Number 3, June IV 2004
|
|
---|---|---|
Page(s) | 911 - 919 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20035607 | |
Published online | 04 June 2004 |
The initial helium abundance of the Galactic globular cluster system *
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
Corresponding author: M. Salaris, ms@astro.livjm.ac.uk
Received:
31
October
2003
Accepted:
17
March
2004
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. [CITE]).
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 1σ 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
. 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
© ESO, 2004
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