The bologna open cluster chemical evolution project: Midterm results from the photometric sample

We describe a long- term project aimed at deriving information on the chemical evolution of the Galactic disk from a large sample of open clusters. The main property of this project is that all clusters are analyzed in a homogeneous way to guarantee the robustness of the ranking in age, distance, and metallicity. Special emphasis is devoted to the evolution of the earliest phases of the Galactic disk evolution, for which clusters have superior reliability with respect to other types of evolution indicators. The project is twofold: on one hand we derive the age, distance, and reddening ( and indicative metallicity) by interpreting deep and accurate photometric data with stellar evolution models, and on the other hand, we derive the chemical abundances from high- resolution spectroscopy. Here we describe our overall goals and approaches and report on the midterm project status of the photometric part, with 16 clusters already studied, covering an age interval from 0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. The importance of quantifying the theoretical uncertainties by deriving the cluster parameters with various sets of stellar models is emphasized. Stellar evolution models assuming overshooting from convective regions appear to better reproduce the photometric properties of the cluster stars. The examined clusters show a clear metallicity dependence on the galactocentric distance and no dependence on age. The tight relation between cluster age and magnitude difference between the main- sequence turnoff and the red clump is confirmed.