Radial trends in Galactic globular clusters and their possible origin

TL;DR

This paper investigates the unexpected radial segregation in Galactic globular clusters, proposing stellar collisions as a key factor influencing their internal dynamics and star distribution.

Contribution

It introduces the role of stellar collisions in explaining the observed radial segregation and the distribution of stars of different masses in globular clusters.

Findings

Stellar collisions occur frequently in collapsing nuclei of GGCs.

Observed blue stragglers are fewer than expected from lower-mass stars.

Mass range of stars at a given luminosity is broader due to collisions.

Abstract

The relaxation time at the half-mass radius of Galactic globular clusters (GGCs) is typically within a few Gyr. Hence, the majority of GGCs are expected to be well relaxed systems, given their age is around 12-13 Gyr. So any initial radial segregation between stars of the same initial mass on the main sequence (MS), in particular, the progenitors of the present day sub-giant and red-giant branch (SGB, RGB) stars should already have dissipated. However, a body of evidence contradicting to these expectations has been accumulated to date. The paradox could be solved by taking into account the effect of stellar collisions. They occur at particularly high rate in collapsing nuclei of GGCs and seem to be mainly responsible for unrelaxed central regions and the radial segregation observed. We draw attention that actually observed collisional blue stragglers should be less numerous than their lower-mass counterparts formed and accumulated at and below the present day MS turnoff. The effect of this is that MS/SGB/RGB stars of a given luminosity are not of the same mass but fall in a range of mass.