Dependence of the old star clusters' dynamical clock on the host galaxy gravitational field

TL;DR

This study analyzes how the dynamical evolution of old star clusters, measured by the $A^+$ parameter, depends on both internal relaxation processes and the gravitational field of the host galaxy, revealing a new way to disentangle these effects.

Contribution

It demonstrates that the dynamical clock parameter $A^+$ is influenced by the host galaxy's gravitational potential, highlighting the combined impact of internal and external dynamical effects.

Findings

$A^+$ correlates with $N_{relax}$ differently depending on galaxy location.

Host galaxy gravitational field affects the dynamical evolution of star clusters.

Disentangling internal relaxation and tidal effects is possible through $A^+$ analysis.

Abstract

I report outcomes of the analysis of the $A^+$ parameter, which measures the level of radial segregation of blue straggler stars in old star clusters, commonly known as the dynamical clock for the long-term internal dynamical evolution. I used $A^+$ values available in the literature for 48 Milky Way globular clusters. I found that the relationship of $A^+$ and the number of central relaxation times which have elapsed ($N_{relax}$) shows a non negligible dependence on the strength of the host galaxy gravitational potential, in addition to depending on the two-body relaxation mechanism. Indeed, a measured $A^+$ value corresponds to relatively smaller or larger $N_{relax}$ values for star clusters located farther or closer to the galaxy center. From an observational point of view, this finding reveals the possibility of disentangling for the first time the dynamical evolutionary stage due to two-body relaxation and tidal effect, that affect the whole star clusters' body concurrently.