The Pleiades apex and its kinematical structure

Abstract

A study of cluster characteristics and internal kinematical structure of the middle-aged Pleiades open star cluster is presented. The individual star apexes and various cluster kinematical parameters including the velocity ellipsoid parameters are determined using both Hipparcos and Gaia data. Modern astrometric parameters were taken from the Gaia Data Release 1 (DR1) in combination with the Radial Velocity Experiment Fifth Data Release (DR5). The necessary set of parameters including parallaxes, proper motions and radial velocities are used for n=17 stars from Gaia DR1+RAVE DR5 and for n=19 stars from the Hipparcos catalog using SIMBAD data base. Single stars are used to improve accuracy by eliminating orbital movements. RAVE DR5 measurements were taken only for the stars with the radial velocity errors not exceeding $2$~km/s. For the Pleiades stars taken from Gaia, we found mean heliocentric distance as $136.8 \pm 6.4$~pc, and the apex position is calculated as: $A_{CP}=92^\circ.52\pm1^\circ.72$, $D_{CP}=-42^\circ.28\pm2^\circ.56$ by the convergent point method and $A_0=95^\circ.59\pm2^\circ.30$ and $D_0=-50^\circ.90\pm2^\circ.04$ using AD-diagram method (n=17 in both cases). The results are compared with those obtained historically before the Gaia mission era.