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Issue A&A
Volume 414, Number 1, January IV 2004
Page(s) 163 - 174
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20031578



A&A 414, 163-174 (2004)
DOI: 10.1051/0004-6361:20031578

The age of the oldest Open Clusters

M. Salaris1, 2, A. Weiss2 and S. M. Percival1

1  Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
    e-mail: smp@astro.livjm.ac.uk
2  Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85758, Garching, Germany
    e-mail: weiss@mpa-garching.mpg.de

(Received 15 August 2003 / Accepted 2 October 2003 )

Abstract
We determine ages of 71 old Open Clusters by a two-step method: we use main-squence fitting to 10 selected clusters, in order to obtain their distances, and derive their ages from comparison with our own isochrones used before for Globular Clusters. We then calibrate the morphological age indicator $\delta(V)$, which can be obtained for all remaining clusters, in terms of age and metallicity. Particular care is taken to ensure consistency in the whole procedure. The resulting Open Cluster ages connect well to our previous Globular Cluster results. From the Open Cluster sample, as well as from the combined sample, questions regarding the formation process of Galactic components are addressed. The age of the oldest open clusters (NGC 6791 and Be 17) is of the order of 10 Gyr. We determine a delay by $2.0\pm1.5$ Gyr between the start of the halo and thin disk formation, whereas thin and thick disk started to form approximately at the same time. We do not find any significant age-metallicity relationship for the open cluster sample. The cumulative age distribution of the whole open cluster sample shows a moderately significant ( ${\sim} 2\sigma$ level) departure from the predictions for an exponentially declining dissolution rate with timescale of 2.5 Gyr. The cumulative age distribution does not show any trend with galactocentric distance, but the clusters with larger height to the Galactic plane have an excess of objects between 2-4 and 6 Gyr with respect to their counterpart closer to the plane of the Galaxy.


Key words: Galaxy: disk -- evolution -- open clusters and associations: general -- stellar content

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

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