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 \leq m \leq 0.80 M _ { \odot } , 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 \pm 0.2 in the cluster ’ s core to a slope of 1.6 \pm 0.3 beyond 18.6 pc .
This is seemingly in conflict with the finding by Beccari et al .
( 8 ) , 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 .