  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 at the cluster centre to a minimum of 0.20
km/s 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 for a
suitable selection of stars, while a total (external) standard error of 0.6
km/s 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.
