Mass segregation in the diffuse outer-halo globular cluster Palomar 14

TL;DR

This study reveals that Palomar 14 exhibits mass segregation despite its long relaxation time, suggesting it either formed with primordial segregation or experienced past expansion.

Contribution

It provides evidence of mass segregation in Palomar 14 and discusses possible origins, challenging previous assumptions about its dynamical state.

Findings

Mass function steepens with radius, indicating mass segregation.

Cluster's current relaxation time is too long for segregation to be dynamical.

Primordial segregation or past expansion likely explains observations.

Abstract

We present an analysis of the radial dependence of the stellar mass function in the diffuse outer-halo globular cluster Palomar 14. Using archival HST/WFPC2 data of the cluster's central 39 pc (corresponding to ~0.85*r_h) we find that the mass function in the mass range of 0.55 to 0.85 solar masses is well approximated by a power-law at all radii. The mass function steepens with increasing radius, from a shallow power-law slope of 0.66+/-0.32 in the cluster's centre to a slope of 1.61+/-0.33 beyond the core radius, showing that the cluster is mass-segregated. This is seemingly in conflict with its long present-day half-mass relaxation time of ~20 Gyr, and with the recent finding by Beccari et al. (2011), who interpret the cluster's non-concentrated population of blue straggler stars as evidence that dynamical segregation has not affected the cluster yet. We discuss this apparent conflict and argue that the cluster must have either formed with primordial mass segregation, or that its relaxation time scale must have been much smaller in the past, i.e. that the cluster must have undergone a significant expansion.