REFINING the DYNAMICAL CLOCK for STAR CLUSTERS

We used a sample of 25 Galactic globular clusters to empirically measure the parameter A + recently introduced by Alessandrini et al., and defined as the area enclosed between the cumulative radial distribution of blue straggler stars (BSSs) and that of a reference population. Based on N-body simulations, this parameter is expected to efficiently measure the level of BSS central segregation. Observationally, for a proper cluster-to-cluster comparison we use , i.e., the value of the parameter determined out to the half-mass radius in each system. We find that nicely correlates with the position of the minimum of the BSS normalized radial distribution and with the cluster central relaxation time. This demonstrates that it is a sensitive indicator of the cluster dynamical age as traced by the spatial segregation of the BSS population. In the context of the "stellar system dynamical clock," this parameter provides a new clock-hand, which is easier to determine observationally and allows a more robust measure of the cluster dynamical age.