Mass Segregation in the Hyades Cluster

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 $r_{\mathrm{h}}$ of 5.75 pc. The tails are extended with $r_{\mathrm{h}}=69.35$ 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 $0.56M_{\odot }$ to give a high mass ($\bar{M}=0.95M_{\odot }$) and a low mass ($\bar{M}=0.32M_{\odot }$) population. We show that: (i) the high mass population has a half-mass radius $r_{\mathrm{h}}$ of 4.88 pc, whilst the low mass population has $r_{\mathrm{h}}=8.10$ 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 $9.5\times 10^{-4}{M}_{\odot }\mathrm{k}{\mathrm{m}}^2{\mathrm{s}}^{-2}\mathrm{My}{\mathrm{r}}^{-1}$ to maintain its current configuration. The present mass loss of $0.26M_{\odot }{\mathrm{Myr}}^{-1}$ due to tidal stripping by itself implies a substantial energy flow beyond the required magnitude.

MNRAS, submitted

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Cited by in corpus (2)

• Ongoing hierarchical massive cluster assembly: the LISCA II structure in the Perseus complex
• The Eccentricity Distribution of Wide Binaries