OLD OPEN CLUSTERS: CONSTRAINTS ON DISK AGE AND EVOLUTION A. BRAGAGLIA and M. TOSI Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy G. MARCONI Osservatorio Astronomico di Roma, Via Osservatorio 5, I-00040 Monte Porzio, Roma, Italy 1. Why Open Clusters? Open clusters are found in different regions of the disk, cover a large interval in age (a few Myr to ∼ 10 Gyr), and in metallicity ([Fe/H] ≥−1.0); moreover, their ages and distances can be measured more accurately than for any single field star. Hence, they offer information on the age of the Galactic disk (on relative and absolute ages, on the possible time continuity between halo and disk, etc.), and on the evolution of the disk metallicity (both in space and time). Any attempt to define reliable age or metallicity rankings must be based on the study of a large sample of clusters homogeneously treated (see e.g. Janes and Phelps, 1994; Carraro and Chiosi, 1994; Friel, 1995). To this end we have started to analyze a sample of clusters at various Galactic locations, and covering a large range in ages and metallicities. We obtain metallicity, reddening, distance, and age from photometric observations (using the synthetic colour-magnitude method, see e.g. Tosi et al., 1991). We use a numerical code for CMD simulation based on stellar evolutionary tracks (taken from different groups, to study the effects of the adoption of different models), and taking into account theoretical and observational uncertainties (e.g. photometric errors, incompleteness, binary fraction). We derive the best combin- ation of metallicity, reddening, distance, and age by comparing the observed and the synthetic CMDs, on the basis of: a) morphology (e.g. shape of the MS, SGB, RGB; position of the red clump and of the TO; gaps), and b) population ratios (distribution in colour and magnitudes, e.g. the LFs). We have already analyzed several clusters with ages from about 0.1 to about 10 Gyr: NGC7790 (Romeo et al., 1989), NGC2506 (Marconi et al., 1997), Be 21 (Tosi et al., 1998), NGC6253 (Bragaglia et al., 1997), NGC2243 (Bonifazi et al., 1990), Cr261 (Gozzoli et al., 1996). Work is in progress for Be 22, To 2, Mel 71, NGC2660, NGC2849 (all intermediate-old), and NGC6603 (young). Astrophysics and Space Science is the original source of publication of this article. It is recom- mended that this article is cited as: Astrophysics and Space Science 265: 281–282, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.