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

The internal velocity structure in the Hyades cluster as seen byHipparcos is compared with realistic N-body simulations using the NBODY6code, which includes binary interaction, stellar evolution and theGalactic tidal field. The model allows to estimate reliably the accuracyof astrometric radial velocities in the Hyades as derived by Lindegrenet al. (cite{lindegren00}) and Madsen et al. (cite{madsen02}) fromHipparcos data, by applying the same estimation procedure on thesimulated data. The simulations indicate that the current clustervelocity dispersion decreases from 0.35 km s<SUP>-1</SUP> at the clustercentre to a minimum of 0.20 km s<SUP>-1</SUP> at 8 pc radius (2-3 coreradii), from where it slightly increases outwards. A clear negativecorrelation between dispersion and stellar mass is seen in the centralpart of the cluster but is almost absent beyond a radius of 3 pc. Itfollows that the (internal) standard error of the astrometric radialvelocities relative to the cluster centroid may be as small as 0.2 kms<SUP>-1</SUP> for a suitable selection of stars, while a total(external) standard error of 0.6 km s<SUP>-1</SUP> is found when theuncertainty of the bulk motion of the cluster is included. Attempts tosee structure in the velocity dispersion using observational data fromHipparcos and Tycho-2 are inconclusive.Based on observations by the ESA Hipparcos satellite, and on the N-bodycode NBODY6 by Sverre Aarseth, publicly available atftp://ftp.ast.cam.ac.uk/pub/sverre/