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.

The TW Hydrae association: trigonometric parallaxes and kinematic analysis

“…4.1 and is identical to the value of σ v = 0.8 +0.3 −0.2 km s −1 derived by Mamajek (2005) in his analysis 2 . Makarov et al (2005) and de la Reza et al (2006) pioneered the idea of accurately deriving the dynamical age of the TWA. Makarov et al (2005) investigated the path of several proba-ble members of the association and showed that five objects form a rapidly expanding association with an expansion age of 4.7±0.6 My.…”

“…Makarov et al (2005) and de la Reza et al (2006) pioneered the idea of accurately deriving the dynamical age of the TWA. Makarov et al (2005) investigated the path of several proba-ble members of the association and showed that five objects form a rapidly expanding association with an expansion age of 4.7±0.6 My. De la Reza et al (2006) showed that four H stars belonging to the association were converging back in time toward a minimum volume corresponding to the dynamical age of the association.…”

“…Makarov et al (2005) derived an expansion age of 4.7 Myr using H data for TWA 1, 4, 11 and two other young nearby stars (HD 139084 and HD 220476). De la Reza et al (2006) derived an expansion age of 8.3 ± 0.8 Myr using H data for TWA 1, 4, 11, and 19.…”

“…The situation for radial velocities was equivalent and trigonometric parallaxes were available for only five H stars. An important step in the evaluation of the age of the association was pioneered by Makarov et al (2005) and another was developed by de la Reza et al (2006) who applied a traceback strategy to the five H TWA stars. They derived the epoch of minimum volume corresponding to the dynamical age of the association.…”

Lithium Depletion of Nearby Young Stellar Associations

“…However, the likely complex dynamical evolution of TWA has led to several plausible evolutionary scenarios. Makarov et al (2005) attribute this complex evolution to a chance encounter with Vega, while Lawson & Crause (2005) suggest that TWA is composed of two separate groups, based on bimodal rotation period distributions with distinctly separate ages of $10 and $17 Myr. More recently, Barrado y Navascués (2006) found a conservative age of 10 þ10 À7 Myr by comparing ages from HR diagram isochrone comparisons (from BCAH98) and lithium abundances.…”