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 cluster IC 1311 are presented. A comparison with previously published results on NGC 7044, based on measurements secured during the same observing run, leads to self-consistent constraints to the parameters of the cluster. The location of selected samples of giant and unevolved dwarf members in the U-B vs B-V diagram is used to simultaneously compute the 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 theoretical isochrones, based on models computed with and without the consideration 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=1.6×109 yr. The mass function for the cluster main sequence has been estimated, with a slope x= -2.58, significantly steeper than the Salpeter IMF (-1.35). The uncertainties in the adopted values for the cluster parameters, due to interstellar reddening, and to the calibrations of theoretical luminosity and effective temperature in terms of observational quantities, are also addressed. The discrepancies between the observational CM and the model isochrones, in particular the difference between observed and predicted luminosity of the RGC, are sensibly reduced for the models with convective core overshooting. The results indeed suggest the need of increasing the overshooting effectiveness beyond the values considered up to now in evolutionary computations.