The age of the oldest Open Clusters

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

Corresponding author: M. Salaris, ms@astro.livjm.ac.uk

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 , 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  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 ( 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.