STAR COUNT DENSITY PROFILES AND STRUCTURAL PARAMETERS OF 26 GALACTIC GLOBULAR CLUSTERS

Abstract

We used a proper combination of high-resolution HST observations and\nwide-field ground based data to derive the radial star density profile of 26\nGalactic globular clusters from resolved star counts (which can be all freely\ndownloaded on-line). With respect to surface brightness (SB) profiles (which\ncan be biased by the presence of sparse, bright stars), star counts are\nconsidered to be the most robust and reliable tool to derive cluster structural\nparameters. For each system a detailed comparison with both King and Wilson\nmodels has been performed and the most relevant best-fit parameters have been\nobtained. This is the largest homogeneous catalog collected so far of star\ncount profiles and structural parameters derived therefrom. The analysis of the\ndata of our catalog has shown that: (1) the presence of the central cusps\npreviously detected in the SB profiles of NGC 1851, M13 and M62 is not\nconfirmed; (2) the majority of clusters in our sample are fitted equally well\nby the King and the Wilson models; (3) we confirm the known relationship\nbetween cluster size (as measured by the effective radius) and galactocentric\ndistances; (4) the ratio between the core and the effective radii shows a\nbimodal distribution, with a peak at ~ 0.3 for about 80% of the clusters, and a\nsecondary peak at ~ 0.6 for the remaining 20%. Interestingly, the main peak\nturns out to be in agreement with what expected from simulations of cluster\ndynamical evolution and the ratio between these two radii well correlates with\nan empirical dynamical age indicator recently defined from the observed shape\nof blue straggler star radial distribution, thus suggesting that no exotic\nmechanisms of energy generation are needed in the cores of the analyzed\nclusters.\n