Hyades dynamics from N-body simulations: Accuracy of astrometric radial velocities from Hipparcos

The internal velocity structure in the Hyades cluster as seen by Hipparcos is compared with realistic N-body simulations using the NBODY6 code, which includes binary interaction, stellar evolution and the Galactic tidal field. The model allows to estimate reliably the accuracy of astrometric radial velocities in the Hyades as derived by Lindegren et al. (2000) and Madsen et al. (2002) from Hipparcos data, by applying the same estimation procedure on the simulated data. The simulations indicate that the current cluster velocity dispersion decreases from 0.35 km s-1 at the cluster centre to a minimum of 0.20 km s-1 at 8 pc radius (2-3 core radii), from where it slightly increases outwards. A clear negative correlation between dispersion and stellar mass is seen in the central part of the cluster but is almost absent beyond a radius of 3 pc. It follows that the (internal) standard error of the astrometric radial velocities relative to the cluster centroid may be as small as 0.2 km s-1 for a suitable selection of stars, while a total (external) standard error of 0.6 km s-1 is found when the uncertainty of the bulk motion of the cluster is included. Attempts to see structure in the velocity dispersion using observational data from Hipparcos and Tycho-2 are inconclusive.