Internal kinematics of the TW Hya association of young stars

Abstract

Thirty one probable kinematic members of the nearby TW Hya association of young stars are selected from the RASSBSC/Tycho-2 sample of stars luminous in X-rays, detected by ROSAT. Eight of them have been listed already as members of the association, and 23 are new candidates. The association occupies a volume of some 10⁶ pc³, the nearest member being at a distance of only 17.5 pc from the Sun. The lower bound internal velocity dispersion is estimated 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 estimated at 31 solar masses. The centre of mass lies at 73 pc from us in the direction close to the position of the prototype star TW Hya. A realistic kinematical 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, in good agreement with previous age estimations for T Tauri members. Likewise the nearby Carina-Vela moving group of X-ray stars, the TW Hya association appears to be an outpost of the Gould Belt structure rather than an isolated open cluster.