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The parameter A+, defined as the area enclosed between the cumulative radialdistribution of blue straggler stars (BSSs) and that of a reference population,is a powerful indicator of the level of BSS central segregation. As part of theHubble Space Telescope UV Legacy Survey of Galactic globular clusters (GCs),here we present the BSS population and the determination of A+ in 27 GCsobserved out to about one half-mass radius. In combination with 21 additionalclusters discussed in a previous paper this provides us with a global sample of48 systems (corresponding to \sim 32\% of the Milky Way GC population), forwhich we find a strong correlation between A+ and the ratio of cluster age tothe current central relaxation time. Tight relations have been found also withthe core radius and the central luminosity density, which are expected tochange with the long-term cluster dynamical evolution. An interesting relationis emerging between A+ and the ratio of the BSS velocity dispersion relative tothat of main sequence turn-off stars, which measures the degree of energyequipartition experienced by BSSs in the cluster. These results provide furtherconfirmation that BSSs are invaluable probes of GC internal dynamics and A+ isa powerful dynamical clock.

astrophysical journal/the astrophysical journal

The Hubble Space Telescope UV Legacy Survey of Galactic Globular  Clusters - XV. The dynamical clock: reading cluster dynamical evolution from  the segregation level of blue straggler stars