Internal kinematics and binarity of X-ray stars in the Pleiades open cluster

Abstract

The classical convergent point analysis is implemented for the Pleiades stars with proper motions in the Tycho-2 Catalogue and X-ray fluxes measured by the ROSAT satellite. It is demonstrated that, with the standard astrometric errors as given in Tycho-2, strong X-ray sources in the cluster (log L_X > 29.1, where L_X is in erg s^-1) exhibit a velocity dispersion in one component of only 0.20 km s^-1, while the distributions of velocity components of moderate (log L_X < 29.1) sources and stars not detected by ROSAT at all are consistent with a velocity dispersion of 0.64 km s^-1. The difference is statistically significant at the level of 1.6sigma , or 0.95 confidence limit. This result is a clue to the kinematics/X-ray luminosity segregation, similar to that previously discovered in the Hyades open cluster. It is discussed that the segregation may be caused by a wide spread of ages of the member stars. The occurrence of high X-ray luminosities is found to correlate very well with visual binarity and multiplicity (separations > 10 AU).