WHITE-DWARFS IN THE HALO OF THE HYADES CLUSTER - THE CASE OF THE MISSING WHITE-DWARFS

On subtracting the cooling age of the dimmest white dwarf in the Hyades cluster (3 X 10(8) yr) from the age of the cluster (7 X 10(8) yr), we estimate the mass of the progenitor of this white dwarf to be 3.0 M., compared with the mass 2.25 M. of a main-sequence star near the turnoff point in the cluster. Adopting a conventional mass function and normalizing to the 24 brightest main-sequence stars in the Hyades, we infer that there should be at least 21 white dwarfs dimmer than the 7 confirmed white dwarf cluster members which presumably derive from stars of initial mass in the range 2.5-3 M.. Four core helium-burning red giants derive from stars of initial mass in the range 2.25-2.5 M.. Given that the current mass function of the cluster main sequence is almost flat, so that the number of stars less massive than the mass of the typical white dwarf (0.63 M.) is an order of magnitude smaller than predicted by the conventional mass function, we speculate that evaporation of light stars has reduced the original cluster population by a factor of perhaps 10 (from 4000 to 400 stars). In anticipation that the missing white dwarfs might be in the Hyades supercluster defined by Eggen, we have explored the reality of the supercluster concept (unbound stars still retaining memory of an origin in a massive progenitor cluster and moving together with a small dispersion in current velocities). We are unable to identify in the known sample of nearby white dwarfs more than a few which move within 13-degrees of the direction of the Hyades convergent point with space velocities within +/- 2 km s-1 of the Hyades cluster space velocity. On the other hand, we find that approximately two-fifths of all nearby white dwarfs are streaming within 13-degrees of the direction of the Hyades convergent point with a velocity dispersion of sigma = 5 km s-1 about the cluster speed of 46.6 km s-1. To evaluate the meaning of these results, and to inquire further into the space and velocity distribution of stars which may have escaped from an originally much larger bound Hyades cluster, we investigate the orbits of stars leaving a hypothetical cluster in random directions with excess velocities of 0.1 km s-1 relative to the cluster center of mass at a continuous rate over the past 7 X 10(8) yr, finding that the escaped stars define a "needle" of dimensions approximately 2 X (10 pc X 14 pc X 122 pc) which is centered on the cluster and points in a direction which makes an 8-degrees angle with t