Improved colour-absolute magnitude and Hertzsprung{Russell diagrams

Abstract. Hipparcos parallaxes x distances to individual stars in the Hyades cluster with an accuracy of 6 per-cent. We use the Hipparcos proper motions, which have a larger relative precision than the trigonometric paral-laxes, to derive 3 times more precise distance estimates, by assuming that all members share the same space motion. An investigation of the available kinematic data conrms that the Hyades velocity eld does not contain signi cant structure in the form of rotation and/or shear, but is fully consistent with a common space motion plus a (one-dimensional) internal velocity dispersion of 0.30 km s−1. The improved parallaxes as a set are statistically consistent with the Hipparcos parallaxes. The maximum expected systematic error in the proper motion-based parallaxes for stars in the outer regions of the cluster (i.e., beyond 2 tidal radii 20 pc) is <0.30 mas. The new parallaxes conrm that the Hipparcos measurements are correlated on small angular scales, consistent with the limits specied in the Hipparcos Catalogue, though with signicantly smaller \amplitudes " than claimed by Narayanan & Gould. We use the Tycho{2 long time-baseline astrometric catalogue to derive a set of independent proper motion-based parallaxes for the Hipparcos members. The new parallaxes provide a uniquely sharp view of the three-dimensional structure of the Hyades. The colour-absolute magnitude diagram of the cluster based on the new parallaxes shows a well-dened main sequence with two \gaps"/\turn-os". These features provide the rst direct observational support of Böhm{Vitense’s prediction that (the onset of) surface convection in star