Internal kinematics of the TW Hya association of young stars

Thirty one probable kinematic members of the nearby TW Hya association of y oung stars are selected from the RASSBSC/Trycho-2 sample of stars luminous in X-rays, detected by ROSAT. Eight of them have been listed already as mem bers of the association, and 23 are new candidates. The association occupie s a volume of some 10(6) pc(3), the nearest member being at a distance of o nly 17.5 pc from the Sun. The lower bound internal velocity dispersion is e stimated at 0.8 km s(-1), which is considerably larger than is expected in a gravitationally bound open cluster. The total mass of the group is estima ted at 31 solar masses. The centre of mass lies at 73 pc from us in the dir ection close to the position of the prototype star TW Hya. A realistic kine matical model fitting both the observed trigonometric parallaxes and radial velocities involves a linear expansion of the group with the rate 0.12 km s(-1) pc(-1), which defines a dynamical age of 8.3 Myr, ill good agreement with previous age estimations for T Tauri members. Likewise the nearby Cari na-Vela moving group of X-ray stars, the TW Hya association appears to be a n outpost of the Gould Belt structure rather than an isolated open cluster.