Stellar graveyards: Clustering of compact objects in globular clusters
  NGC 3201 and NGC 6397
  We analyse Gaia EDR3 and re-calibrated HST proper motion data from the
core-collapsed and non core-collapsed globular clusters NGC 6397 and NGC 3201,
respectively, with the Bayesian mass-orbit modelling code MAMPOSSt-PM. We use
Bayesian evidence and realistic mock data sets constructed with AGAMA to select
between different mass models. In both clusters, the velocities are consistent
with isotropy within the extent of our data. We robustly detect a dark central
mass (DCM) of roughly 1000 solar masses in both clusters. Our MAMPOSSt-PM fits
strongly prefer an extended DCM in NGC 6397, while only presenting a mild
preference for it in NGC 3201, with respective sizes of a roughly one and a few
per cent of the cluster effective radius. We explore the astrophysics behind
our results with the CMC Monte Carlo N-body code, whose snapshots best matching
the phase space observations lead to similar values for the mass and size of
the DCM. The internal kinematics are thus consistent with a population of
hundreds of massive white dwarfs in NGC 6397, and roughly 100 segregated
stellar-mass black holes in NGC 3201, as previously found with CMC. Such
analyses confirm the accuracy of both mass-orbit modelling and Monte Carlo
N-body techniques, which together provide more robust predictions on the DCM of
globular clusters (core-collapsed or not). This opens possibilities to
understand a vast range of interesting astrophysical phenomena in clusters,
such as fast radio bursts, compact object mergers, and gravitational waves.
