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
  The parameter A+, defined as the area enclosed between the cumulative radial
distribution 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 the
Hubble Space Telescope UV Legacy Survey of Galactic globular clusters (GCs),
here we present the BSS population and the determination of A+ in 27 GCs
observed out to about one half-mass radius. In combination with 21 additional
clusters discussed in a previous paper this provides us with a global sample of
48 systems (corresponding to \sim 32\% of the Milky Way GC population), for
which we find a strong correlation between A+ and the ratio of cluster age to
the current central relaxation time. Tight relations have been found also with
the core radius and the central luminosity density, which are expected to
change with the long-term cluster dynamical evolution. An interesting relation
is emerging between A+ and the ratio of the BSS velocity dispersion relative to
that of main sequence turn-off stars, which measures the degree of energy
equipartition experienced by BSSs in the cluster. These results provide further
confirmation that BSSs are invaluable probes of GC internal dynamics and A+ is
a powerful dynamical clock.
