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

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.

Accepted for publication in ApJ, references updated

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• The blue straggler population of the old open cluster Berkeley 17
• Model computations of blue stragglers and W UMa-type stars in globular clusters
• The dynamical state of the globular clusters Rup 106 and IC 4499
• The fractions of post-binary-interaction stars and evolved blue straggler stars on the red giant branch of globular clusters
• Multiple Populations in Low-mass Globular Clusters: Eridanus
• On the Age Calibration of Open Clusters using Red Clump Stars