THE INTERMEDIATE AGE GALACTIC CLUSTER M11

We present a deep CCD investigation of stars in a selected region near the center of the galactic cluster M11. The cluster photometry has been extended to objects about 7 mag fainter than previous investigations, reaching a limiting magnitude of V approximately 22. The color-magnitude diagram, based on more than 5000 stars, reveals that the cluster main sequence (MS) appears well-populated down to, at least, V approximately 20.5 and, perhaps, down to the observational limit. Assuming for the cluster E(B-V) = 0.42 mag, the best fit of the MS to Pleiades gives for M11 an apparent distance modulus (m-M)v = 12.5 +/- 0.2 mag, which implies a distance d = 1.7 +/- 0.2 kpc. Relying on photometric criteria, one finds that the observed MS should be reasonably complete and free of contamination by field stars down to V approximately 18 mag. On this basis, evidence supports a strong dependence of the cluster luminosity function on the distance from the cluster center. We find that in the observed region the abundance of stars (cluster + field) with V greater-than-or-equal-to 15 mag appears about 2.5 times larger than previous estimates, following a luminosity function steeper than expected from a Salpeter's mass distribution. However, because of the relevant contribution of field stars, no firm conclusion can be made about the luminosity distribution of cluster stars. By best fitting the luminosity function of H-burning stars to theoretical data, one obtains a good agreement for a cluster age t = 150 million years. Comparison of the isochrone of 150 million years to our observational data or to the complete sample of luminous cluster stars supports such an agreement, disclosing that the various evolutionary phases appear to be populated according to theoretical constraints.