The TW Hydrae Association : trigonometric parallaxes and kinematic   analysis
  The nearby TW Hydrae Association (TWA) is currently a benchmark for the study of formation and evolution of young low-mass stars, circumstellar disks and the imaging detection of planetary companions. For such studies, it is crucial to evaluate the distance to group members in order to access their physical properties. Membership of several stars is strongly debated and age estimates vary from one author to another with doubts about coevality. We revisit the kinematic properties of the TWA in light of new trigonometric parallaxes and proper motions to derive the dynamical age of the association and physical parameters of kinematic members. Using observations performed with the NTT/ESO telescope we measured trigonometric parallaxes and proper motions for 13 stars in TWA. With the convergent point method we identify a co-moving group with 31 TWA stars. We deduce kinematic distances for 7 members of the moving group that lack trigonometric parallaxes. A traceback strategy is applied to the stellar space motions of a selection of 16 of the co-moving objects with accurate and reliable data yielding a dynamical age for the association of t = 7.5 +/- 0.7 Myr. Using our new parallaxes and photometry available in the literature we derive stellar ages and masses from theoretical evolutionary models. With new parallax and proper motion measurements from this work and current astrometric catalogs we provide an improved and accurate database for TWA stars to be used in kinematical analysis. We conclude that the dynamical age obtained via traceback strategy is consistent with previous age estimates for the TWA, and is also compatible with the average ages derived in the present paper from evolutionary models for pre-main sequence stars. 
C. Ducourant, R. Teixeira, P. A. B. Galli, J. F. Le Campion, A.   Krone-Martins, B. Zuckerman, G. Chauvin and I. Song
astro-ph.SR