2022MNRAS.512.3846E

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_h of 5.75 pc. The tails are extended with r_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 0.56M_☉ to give a high mass ({bar}M = 0.95 M_☉) and a low mass ({bar}M = 0.32 M_☉) population. We show that: (i) the high mass population has a half-mass radius r_h of 4.88 pc, whilst the low mass population has r_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 σ of 0.427 and 0.415 km s^–1, respectively. The dynamical state of the Hyades is far from energy equipartition (σ ∝{bar}M^–1/2). We identify a new mass segregation instability for clusters with escape speed V. Populations with V/σ ≲2sqrt(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 ×10^–4 M_☉ km² s^–2 Myr^–1 to maintain its current configuration. The present mass-loss of 0.26M_☉Myr^–1 due to tidal stripping by itself implies a substantial energy flow beyond the required magnitude.