Kinematic analysis and membership status of TWA22 AB^*,**

¹ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226 Cidade Universitária, 05508-900 São Paulo - SP, Brazil e-mail: teixeira@astro.iag.usp.br
² Observatoire Aquitaine des Sciences de l'Univers, CNRS-UMR 5804, BP 89, 33270 Floirac, France
³ Laboratoire d'Astrophysique, Observatoire de Grenoble, 414 rue de la piscine, 38400 Saint-Martin d'Hères, France
⁴ Department of Physics & Astronomy, the University of Georgia, Athens, GA 30605, USA

Received: 24 March 2009
Accepted: 29 April 2009

Abstract

Context. TWA22 was initially regarded as a member of the TW Hydrae association (TWA). In addition to being one of the youngest (≈8 Myr) and nearest (≈20 pc) stars to Earth, TWA22 has proven to be very interesting after being resolved as a tight, very low-mass binary. This binary can serve as a very useful dynamical calibrator for pre-main sequence evolutionary models. However, its membership in the TWA has been recently questioned despite due to the lack of accurate kinematic measurements.

Aims. Based on proper motion, radial velocity, and trigonometric parallax measurements, we aim here to re-analyze the membership of TWA22 to young, nearby associations.

Methods. Using the ESO NTT/SUSI2 telescope, we observed TWA22 AB during 5 different observing runs over 1.2 years to measure its trigonometric parallax and proper motion. This is a part of a larger project measuring trigonometric parallaxes and proper motions of most known TWA members at a sub-milliarcsec level. HARPS at the ESO 3.6 m telescope was also used to measure the system's radial velocity over 2 years.

Results. We report an absolute trigonometric parallax of TWA22 AB, π = 57.0±0.7 mas, corresponding to a distance 17.5±0.2 pc from Earth. Measured proper motions of TWA 22AB are = -175.8±0.8 mas/yr and = -21.3±0.8 mas/yr. Finally, from HARPS measurements, we obtain a radial velocity V_rad = 14.8±2.1 km s^-1.

Conclusions. A kinematic analysis of TWA22 AB space motion and position implies that a membership of TWA22 AB to known young, nearby associations can be excluded except for the β Pictoris and TW Hydrae associations. Membership probabilities based on the system's Galactic space motion and/or the trace-back technique support a higher chance of being a member to the β Pictoris association. Membership of TWA22 in the TWA cannot be fully excluded because of large uncertainties in parallax measurements and radial velocities and to the uncertain internal velocity dispersion of its members.