Refining the dynamical clock for star clusters

Abstract

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 {A}{rh}+, I.e., the value of the parameter determined out to the half-mass radius in each system. We find that {A}{rh}+ 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.