Kinematics versus X-ray luminosity segregation in the Hyades

Abstract

State of the art Tycho-2 proper motions of bona fide Hyades members are used in combination with the ROSAT X-ray survey of this open cluster in Stern et al. (1995) to study the internal velocity dispersion versus X-ray luminosity by the classical convergent point method. It is shown in this paper for the Hyades members with B-V below 0.7 mag that stars brighter than 1.3x 10²⁹ erg s^-1 (0.1-1.8 keV) have much more coherent motions than those with smaller X-ray luminosities and those not detected by ROSAT at all. In fact, the proper motions of the X-ray luminous members are in agreement with a null internal dispersion of velocities for this subpopulation, whereas the intermediate bright X-ray stars exhibit a dispersion at 320 m s^-1. The non-emitters, despite their being more massive and optically brighter, have a velocity dispersion of about 440 m s^-1. Probably related to this fact, the strong X-ray emitters are more concentrated within the tidal radius (10 pc) of the cluster centre than the non-emitters. Nearly half of strong X-ray emitters with coherent motions are visual or orbiting pairs with typical separations 5 to 50 AU. This find implies that a group of intermediate mass (1.0 to 1.5 solar masses) stars in the Hyades is significantly younger than the rest and their system is not yet dynamically relaxed.