EDP Sciences
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Volume 368, Number 3, March IV 2001
Page(s) 873 - 879
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20010069

A&A 368, 873-879 (2001)
DOI: 10.1051/0004-6361:20010069

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

V. V. Makarov1 and N. Robichon2

1  Copenhagen University Observatory, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
2  DASGAL, Observatoire de Paris-Meudon, bât. 11, 5 place Jansen, 92195 Meudon Cedex, France

(Received 27 November 2000 / Accepted 10 January 2001)

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 ( $\log L_{\rm X} > 29.1$, where $L_{\rm X}$ 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 ( $\log L_{\rm X} < 29.1$) 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 $1.6\sigma$, 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).

Key words: open clusters and associations: individual: Pleiades -- stars: kinematics -- astrometry

Offprint request: V. V. Makarov, makarov@astro.ku.dk

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© ESO 2001