PHYSICAL PARAMETERS OF THE INTERMEDIATE AGE OPEN CLUSTER IC-1311 - CLUES FOR THE THEORY OF STELLAR EVOLUTION

The results of a UBVR photometric study in the field of the open clust er IC 1311 are presented. A comparison with previously published resul ts on NGC 7044, based on measurements secured during the same observin g run, leads to self-consistent constraints to the parameters of the c luster. The location of selected samples of giant and unevolved dwarf members in the U-B vs B-V diagram is used to simultaneously compute th e color excess E(B-V) and the metallicity [Fe/H]. Distance modulus and age are estimated by the ZAMS fitting method, and by the use of theor etical isochrones, based on models computed with and without the consi deration of convective overshooting during the phases of core nuclear burning. The adopted set of values is: E(B-V)=0.28, [Fe/H]=0.0, DM=13. 9, Age 1.6x10(9) yr. The mass function for the cluster main sequence h as been estimated, with a slope x=-2.58, significantly steeper than th e Salpeter IMF (-1.35). The uncertainties in the adopted values for th e cluster parameters, due to interstellar reddening, and to the calibr ations of theoretical luminosity and effective temperature in terms of observational quantities, are also addressed. The discrepancies betwe en the observational CM and the model isochrones, in particular the di fference between observed and predicted luminosity of the RGC, are sen sibly reduced for the models with convective core overshooting. The re sults indeed suggest the need of increasing the overshooting effective ness beyond the values considered up to now in evolutionary computatio ns.