Investigation of the Pleiades cluster ⋆

On the basis of the best available member list and duplicity information, we have studied the radial distribution of 270 stars and multiple systems earlier than K0 in the Pleiades. Five new long period spectroscopic binaries have been identified from the CORAVEL observations. We have found a clear mass segregation between binaries and single stars, which is explained by the greater average mass of the multiple systems. The mass function of the single stars and primaries appears to be significantly different. While the central part of the cluster is spherical, the outer part is clearly elliptical, with an el-lipticity of 0.17. The various parameters describing the Pleiades are (for a distance of 125 pc): core radius r c = 0. • 6 (1.4 pc), tidal radius r t = 7. • 4 (16 pc), half mass radius r m/2 = 0. • 88 (1.9 pc), harmonic radius r = 1. • 82 (4 pc). Low-mass stars (later than K0) probably extend further out and new proper motion and radial velocity surveys over a larger area and to fainter magnitudes would be very important to improve the description of the cluster structure and complete mass function.