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

TL;DR

This study uses blue straggler stars' segregation levels to develop a dynamical clock for globular cluster evolution, revealing strong correlations with cluster age, core properties, and energy equipartition, based on Hubble data.

Contribution

It introduces the parameter A+ as a new indicator of dynamical evolution and applies it to a large sample of globular clusters, establishing its effectiveness as a dynamical clock.

Findings

A+ correlates with cluster age to relaxation time ratio

Strong relations between A+ and core radius and luminosity density

A+ relates to BSS velocity dispersion indicating energy equipartition

Abstract

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.