Mass segregation in the outer halo globular cluster Palomar 14

TL;DR

This study finds evidence of mass segregation in Palomar 14, a globular cluster with an unexpectedly long relaxation time, suggesting primordial segregation or past compactness, with implications for its formation and orbital history.

Contribution

It provides the first detailed analysis of radial stellar mass function variation in Palomar 14, revealing mass segregation despite its long relaxation time, and discusses possible origins.

Findings

Mass function slope increases with radius, indicating segregation.

Palomar 14 may have been more compact or primordial segregation occurred.

Tidal shocks could explain the cluster's expansion and current state.

Abstract

We present evidence for mass segregation in the outer-halo globular cluster Palomar 14, which is intuitively unexpected since its present-day two-body relaxation time significantly exceeds the Hubble time. Based on archival Hubble Space Telescope imaging, we analyze the radial dependence of the stellar mass function in the cluster's inner 39.2 pc in the mass range of 0.53-0.80 M_sun, ranging from the main-sequence turn-off down to a V-band magnitude of 27.1 mag. The mass function at different radii is well approximated by a power law and rises from a shallow slope of 0.6+/-0.2 in the cluster's core to a slope of 1.6+/-0.3 beyond 18.6 pc. This is seemingly in conflict with the finding by Beccari et al. (2011), who interpret the cluster's non-segregated population of (more massive) blue straggler stars, compared to (less massive) red giants and horizontal branch stars, as evidence that the cluster has not experienced dynamical segregation yet. We discuss how both results can be reconciled. Our findings indicate that the cluster was either primordially mass-segregated and/or used to be significantly more compact in the past. For the latter case, we propose tidal shocks as the mechanism driving the cluster's expansion, which would imply that Palomar 14 is on a highly eccentric orbit. Conversely, if the cluster formed already extended and with primordial mass segregation, this could support an accretion origin of the cluster.