Mass Segregation in the Hyades Cluster
  Using the Gaia colour-magnitude diagram, we assign masses to a catalogue of
979 confirmed members of the Hyades cluster and tails. By fitting the
cumulative mass profile, stars within the tidal radius have a Plummer-like
profile with half-mass radius $r_{\rm h}$ of 5.75 pc. The tails are extended
with $r_{\rm h} = 69.35$ pc and fall off more slowly than Plummer with density
proportional to distance$^{-1.36}$. The cluster stars are separated into two
groups at BP-RP $=2$ or $0.56 M_\odot$ to give a high mass (${\bar M} = 0.95
M_\odot$) and a low mass (${\bar M} = 0.32 M_\odot$) population. We show that:
(i) the high mass population has a half-mass radius $r_{\rm h}$ of 4.88 pc,
whilst the low mass population has $r_{\rm h} = 8.10$ pc; (ii) despite the
differences in spatial extent, the kinematics and binarity properties of the
high and low mass populations are similar. They have isotropic velocity
ellipsoids with mean 1d velocity dispersions $\sigma$ of 0.427 and 0.415 km
s$^{-1}$ respectively. The dynamical state of the Hyades is far from energy
equipartition ($\sigma \propto {\bar M}^{-1/2}$). We identify a new mass
segregation instability for clusters with escape speed $V$. Populations with
$V/\sigma \lesssim 2\sqrt{2}$ can never attain thermal equilibrium and
equipartition. This regime encompasses many Galactic open and globular
clusters. For the Hyades, there must be an outward energy flux of at least $9.5
\times 10^{-4} M_\odot\,{\rm km^2\, s^{-2} Myr^{-1}}$ to maintain its current
configuration. The present mass loss of $0.26 M_\odot {\rm Myr}^{-1}$ due to
tidal stripping by itself implies a substantial energy flow beyond the required
magnitude.
