Mass Segregation in the Hyades Cluster

N. Wyn Evans; Semyeong Oh

doi:10.1093/mnras/stac707

paper

Mass Segregation in the Hyades Cluster

arXiv:2202.06973 · doi:10.1093/mnras/stac707

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 $r_{h}$ 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 $0.56 M_{⊙}$ to give a high mass ( $M ˉ = 0.95 M_{⊙}$ ) and a low mass ( $M ˉ = 0.32 M_{⊙}$ ) 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 ( $σ \propto M ˉ^{- 1/2}$ ). We identify a new mass segregation instability for clusters with escape speed $V$ . Populations with $V / σ ≲ 22$ 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 $9.5 \times 1 0^{- 4} M_{⊙} k m^{2} s^{- 2} My r^{- 1}$ to maintain its current configuration. The present mass loss of $0.26 M_{⊙} Myr^{- 1}$ due to tidal stripping by itself implies a substantial energy flow beyond the required magnitude.

arXiv commentMNRAS, submitted

Mass Segregation in the Hyades Cluster

Abstract

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