EDP Sciences
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Volume 388, Number 2, June III 2002
Page(s) 492 - 503
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20020554

A&A 388, 492-503 (2002)
DOI: 10.1051/0004-6361:20020554

Homogeneous age dating of 55 Galactic globular clusters

Clues to the Galaxy formation mechanisms
M. Salaris1, 2 and A. Weiss2

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-Str. 1, 85748 Garching, Germany

(Received 21 February 2002 / Accepted 8 April 2002)

We present homogeneous age determinations for a large sample of 55 Galactic globular clusters, which constitute about 30% of the total Galactic population. A study of their age distribution reveals that all clusters from the most metal poor ones up to intermediate metallicities are coeval, whereas at higher [Fe/H] an age spread exists, together with an age-metallicity relationship. At the same time, all clusters within a certain galactocentric distance appear coeval, whereas an age spread is present further away from the Galactic centre, without any correlation with distance. The precise value of [Fe/H] and galactocentric distance for the onset of the age spread and the slope of the age-metallicity relationship are strongly affected by the as yet uncertain [Fe/H] scale. We discuss how differences in the adopted [Fe/H] scale and cluster sample size may explain discrepant results about the clusters age distribution reached by different authors. Taking advantage of the large number of objects included in our sample, we also tested the possibility that age is the global second parameter which determines the Horizontal Branch morphology, and found indications that age could explain the global behaviour of the second parameter effect.

Key words: Galaxy: formation -- Galaxy: halo -- globular clusters: general -- stars: Hertzsprung-Russell diagram -- stars: Population II

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

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