Open clusters as tracers of the evolution of the abundance gradients.

Involved people at OAB: Bonifazi, Bragaglia, Di Fabrizio, Tosi.

Open clusters (OC's) are excellent tools to understand the evolution of the disk of our Galaxy from both the chemical and structural points of view. Many of the existing chemical evolution models are able to reproduce well the present-day situation, but differ significantly (Tosi 2001a) in the "history" of the chemical enrichment (hence in the involved processes). In particular, they differ in the predictions for the evolution of the abundance gradients: does the gradient slope steepen or flatten with time? From the OC's we can extract fundamental information, since they can be used to describe the run of the various elemental abundances at different ages.

In order to study in more detail the metallicity and age distribution with galactocentric distance, we are analyzing with great accuracy a large sample of open clusters (our goal is to have at least 30 OC's) at various galactic locations and covering a wide range in age and metallicity. Age, distance modulus, reddening and approximate metallicity of the clusters are derived from their Color-Magnitude Diagrams (CMDs) using the synthetic CMD technique and further constrained by the observed luminosity functions. Precise and homogeneous elemental abundances are determined from high resolution spectroscopy.

During 2001, we have completed the interpretation of Pismis 2 in terms of evolutionary parameters (Di Fabrizio et al. 2001) and started that of Be 22, Be 29, NGC 4815 and NGC 6939.

Up to now only about 25 % of the $\sim$80 old OC's have ever been studied with high resolution spectroscopy, and only a handful have abundances of elements other than iron. To widen the sample, we have obtained high-res spectra of red clump stars in a dozen of OC's, with FEROS@1.5m ESO, SARG@TNG, and UVES@VLT: analysis is completed for NGC 6819 and under way for the others (Bragaglia et al. 2001a, 2001c, 2002; Carretta et al. 2002). Preparatory work for FLAMES@VLT GTO observations has also begun in 2001.

This research is in collaboration with Carretta and Gratton (Padova Obs.), Marconi and Andreuzzi (Roma Obs.).