The central Blue Straggler population in four outer-halo globular clusters

TL;DR

This study compares Blue Straggler Star populations in four distant outer-halo globular clusters, revealing their similar ages, metallicities, and degree of dynamical evolution, with BSSs more centrally concentrated than red giants.

Contribution

It provides the first extended radial density profiles for these distant clusters and analyzes their BSS segregation, linking it to their dynamical states.

Findings

BSSs are more centrally segregated than red giants in all clusters.

Clusters have similar ages (10.5-11 Gyr) and metallicities.

AM 1 and Eridanus are slightly more dynamically evolved.

Abstract

Using HST/WFPC2 data, we have performed a comparative study of the Blue Straggler Star (BSS) populations in the central regions of the globular clusters AM 1, Eridanus, Palomar 3, and Palomar 4. Located at distances RGC > 50 kpc from the Galactic Centre, these are (together with Palomar 14 and NGC 2419) the most distant clusters in the Halo. We determine their colour-magnitude diagrams and centres of gravity. The four clusters turn out to have similar ages (10.5-11 Gyr), significantly smaller than those of the inner-Halo globulars, and similar metallicities. By exploiting wide field ground based data, we build the most extended radial density profiles from resolved star counts ever published for these systems. These are well reproduced by isotropic King models of relatively low concentration. BSSs appear to be significantly more centrally segregated than red giants in all globular clusters, in agreement with the estimated core and half-mass relaxation times which are smaller than the cluster ages. Assuming that this is a signature of mass segregation, we conclude that AM 1 and Eridanus are slightly dynamically more evolved than Pal 3 and Pal 4.