Dynamical age differences among coeval star clusters as revealed by blue stragglers

TL;DR

This paper demonstrates that the distribution of blue straggler stars in globular clusters can be used to determine their relative dynamical ages, revealing different evolutionary stages among clusters of the same chronological age.

Contribution

It introduces a method to assess the dynamical age of globular clusters based on blue straggler star distributions, providing a new observational tool for studying cluster evolution.

Findings

Clusters can be grouped by blue straggler distribution patterns.

These groups correlate with their dynamical evolutionary stages.

The method allows direct measurement of a cluster's dynamical age.

Abstract

Globular star clusters that formed at the same cosmic time may have evolved rather differently from a dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different dynamical ages). Blue straggler stars have masses greater than those at the turn-off point on the main sequence and therefore must be the result of either a collision or a mass-transfer event. Because they are among the most massive and luminous objects in old clusters, they can be used as test particles with which to probe dynamical evolution. Here we report that globular clusters can be grouped into a few distinct families on the basis of the radial distribution of blue stragglers. This grouping corresponds well to an effective ranking of the dynamical stage reached by stellar systems, thereby permitting a direct measure of the cluster dynamical age purely from observed properties.