THE GALACTIC SYSTEM OF OLD OPEN CLUSTERS - AGE CALIBRATION AND AGE-METALLICITY RELATION

In this paper, we present a new homogeneous compilation of ages for th e system of intermediate age and old open clusters of the Galaxy, and the accompanying age-metallicity relation (AMR). This study stands on the analysis by Carraro et al. (1991, 1993a,b), who have obtained good estimates for the color excess, distance modulus, and age for a sampl e of ten clusters, for which modern color-magnitude diagrams (CMD) of good photometric quality and spectroscopic data on the metallicity (Fr iel & Janes 1991, 1993) were available. Firstly, we revise the results by Carraro et al. (1991, 1993a,b) to take into account recent develop ments in the libraries of stellar models (Alongi et al. 1993; Bressan et al. 1993; Fagotto et al. 1993), and secondly we present useful age calibrations based on the correlation between metallicity, age, and ma gnitude difference between the turn-off and red clump luminosities. Th e age calibration does not depend on the color excess, distance modulu s, but only weakly on the metallicity. With the aid of the new age cal ibration, we assign the age to a more numerous sample of clusters. The resulting ages span the range from 0.5 x 10(9) yr for NGC 5822 to 8.0 x 10(9) yr for NGC 6791. With such a compilation, and adopting an hom ogeneous source for the metal content, we propose a new AMR for the fa mily of Galactic open clusters. The AMR is also corrected for the effe ct of the gradient in metallicity across the Galactic Disk. Although a t any given age the spread in metallicity is high, the AMR together wi th the distribution of clusters with different age and metallicity acr oss the Galactic Plane, confirms previous suggestions that the metalli city of a cluster is more related to the position than the age.