Mass segregation in the hyades cluster

Abstract

ABSTRACT 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 rh of 5.75 pc. The tails are extended with rh = 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 (${\bar{M}} = 0.95 M_\odot$) and a low mass (${\bar{M}} = 0.32 M_\odot$) population. We show that: (i) the high mass population has a half-mass radius rh of 4.88 pc, whilst the low mass population has rh = 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 ($\sigma \propto {\bar{M}}^{-1/2}$). 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 0.26M⊙Myr−1 due to tidal stripping by itself implies a substantial energy flow beyond the required magnitude.