New parameters for star-cluster dynamics: Observational results

We recently used a large set of Monte Carlo simulations of globular clusters (GCs) to define new, fully empirical parameters (named A 51 , P 51 , and S 2.5 ) able to trace the internal dynamical evolution of dense stellar systems. These parameters are specifically designed to quantify the steepness of the cumulative radial distribution of stars in the innermost region of the host system, which tends to progressively increase with dynamical aging due to core contraction. Following the original definitions, here we measure A 51 and P 55 in a sample of 40 Galactic GCs homogeneously surveyed through HST photometric observations. In agreement with the predictions of our simulations, the largest values of A 55 and P 55 are found for the most dynamically evolved GCs - i.e., those previously classified as post-core-collapse systems based on the shape of their density profile - and those characterized by the shortest central relaxation times. Moreover, the new dynamical parameters here measured strongly correlate with A + rh , another fully empirical, independent parameter that traces the dynamical age of star clusters through the level of central segregation of blue straggler stars.