Mass segregation in the outer halo globular cluster Palomar 14
  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.
