Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...358l..63m&link_type=abstract
Astronomy and Astrophysics, v.358, p.L63-L66 (2000)
Astronomy and Astrophysics
Astrophysics
6
Galaxy: Open Clusters And Associations: Individual: Hyades, Stars: Kinematics, X-Rays: Stars
Scientific paper
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.
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