1994A&A...287..761C

In this paper, we present a new homogeneous compilation of ages for the 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 sample of ten clusters, for which modern color-magnitude diagrams (CMD) of good photometric quality and spectroscopic data on the metallicity (Friel & Janes 1991, 1993) were available. Firstly, we revise the results by Carraro et al. (1991, 1993a,b) to take into account recent developments 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 magnitude difference between the turn-off and red clump luminosities. The age calibration does not depend on the color excess, distance modulus, but only weakly on the metallicity. With the aid of the new age calibration, we assign the age to a more numerous sample of clusters. The resulting ages span the range from 0.5x10⁹yr for NGC 5822 to 8.0x10⁹yr for NGC 6791. With such a compilation, and adopting an homogeneous source for the metal content, we propose a new AMR for the family of Galactic open clusters. The AMR is also corrected for the effect of the gradient in metallicity across the Galactic Disk. Although at any given age the spread in metallicity is high, the AMR together with the distribution of clusters with different age and metallicity across the Galactic Plane, confirms previous suggestions that the metallicity of a cluster is more related to the position than the age.