Kinematic modelling of clusters with Gaia: the Death Throes of the Hyades

TL;DR

This paper introduces a new forward-modelling method to analyze the internal kinematics of star clusters using Gaia data, revealing detailed velocity structures and tidal tail dynamics of the Hyades.

Contribution

It develops and applies a novel kinematic modelling technique that accounts for anisotropic dispersion, rotation, shear, and contamination, specifically applied to Gaia DR2 data of the Hyades.

Findings

The Hyades cluster has nearly isotropic velocity dispersion.

Tidal tails are elongated towards the Galactic centre.

The tails exhibit shear and vorticity distinct from local Oort constants.

Abstract

The precision of the Gaia data offers a unique opportunity to study the internal velocity field of star clusters. We develop and validate a forward-modelling method for the internal motions of stars in a cluster. The model allows an anisotropic velocity dispersion matrix and linear velocity gradient describing rotation and shear, combines radial velocities available for a subset of stars, and accounts for contamination from background sources via a mixture model. We apply the method to Gaia DR2 data of the Hyades cluster and its tidal tails, dividing and comparing the kinematics of stars within and beyond 10 pc, which is roughly the tidal radius of the cluster. While the velocity dispersion for the cluster is nearly isotropic, the velocity ellipsoid for the tails is clearly elongated with the major axis pointing towards the Galactic centre. We find positive and negative expansion at $\approx 2\sigma$ significance in Galactic azimuthal and vertical direction for the cluster but no rotation. The tidal tails are stretching in a direction tilted from the Galactic centre while equally contracting as the cluster in Galactic vertical direction. The tails have a shear (A) of $16.90\pm 0.92$ m/s/pc and a vorticity (B) of $-6.48\pm1.15$ m/s/pc, values distinct from the local Oort constants.