The Hyades: Distance, Structure and Dynamics

Extract

The Hipparcos parallaxes and proper motions together provide a consistent picture of the Hyades distance, structure and dynamics. They yield a cluster convergent point motion consistent with the individual trigonometric parallaxes, and together explain the large distance modulus derived from the most recent ground-based proper motion investigations as originating from differences in the space velocity and small systematic effects in the ground-based proper motions. Conversely, the smaller distance modulus generally derived from a variety of ground-based trigonometric parallax programmes originates from systematically larger values of the ground-based parallaxes. Although recent distance determinations to individual objects in the cluster, most notably the results of Torres et al. (1997a, b, c), are in excellent agreement with the Hipparcos trigonometric parallaxes, extrapolation to a mean cluster distance is again affected by systematic effects in the ground-based proper motion studies.

The combination of the Hipparcos astrometry with radial velocity measurements from ground-based programmes provides three-dimensional velocities allowing candidate membership selection to be based on 3-dimensional positional and kinematical criteria. A number of new cluster members have been found within 20 pc of the cluster centre. No evidence for systematic internal velocity structure is found; rather, the results are fully consistent with auniform cluster space motion with an internal velocity dispersion of about 0.3 km s^-1. Spatial distribution, mass segregation, and binary distributions are consistent with N-body simulations.