Expansion of the TW Hydrae association and the encounter with Vega

Abstract

We investigate the paths of several probable members of the young association around the star TW Hydrae (TWA) with accurate distances, proper motions and radial velocities. We find that three of the previously identified members, TWA 1, TWA 4 and TWA 11, together with two other young nearby stars, HD 139084 and HD 220476, form a rapidly expanding association with an expansion age of 4.7 ± 0.6 Myr. Initial velocities of member stars with respect to the common centre of mass range from 4 to 10 km s−1. A characteristic size of the association in the initial configuration is 21 pc, which may be somewhat biased upwards due to the uncertainties in the observational data. The Lower Centaurus Crux (LCC) OB association passed near TWA, at a distance of 36 ± 6 pc, 11 Myr ago. A plausible scenario, which accounts for the difference between the isochrone age (≃10 Myr) and expansion age (5 Myr), is that star formation was stimulated in the TWA progenitor cloud by the near passage of the LCC, but that the newly formed stars were not released from the cloud until a subsequent collision with one of the other molecular clouds in the North Ophiuchus region. Vega was inside the TWA association, and close to its centre of gravity, at the time of maximum compression 4.7 Myr ago. If this alignment is a chance encounter, the powerful particular disc around Vega could have been enhanced by the passage through the TWA progenitor cloud at 8 km s−1.