AAS 195th Meeting, January 2000
Session 32. Young Stars, Disks and Planets: What Can We Learn from the TW Hydrae Association?
Special Session Oral, Wednesday, January 12, 2000, 2:00-3:30pm, Regency VI

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[32.02] Kinematics and identifying new members of the TW Hydrae Association.

R. A. Webb, B. Zuckerman (UCLA)

Despite the fact that it is the nearest region of recent star formation, the TW Hya Association (TWA) has, until recently, gone largely unnoticed because it lacks the interstellar molecular clouds usually associated with very young stars. Until ~10 years ago, only the classical T Tauri star TW Hya was known and its origin was a mystery. Subsequent surveys of IRAS excess stars (de la Reza et al. 1989, Gregorio-Hetem et al. 1992) and, more recently, X-ray bright stars (Kastner et al. 1997, Webb et al. 1999, Sterzik et al. 1999) have been successful identifying TWA members which now comprise a bona fide association of at least 13 star systems.

There can be little doubt that the TWA stars share a common origin. Stars as young as 10 Myr are rare outside of star forming regions. We searched for lithium absorption features in over a hundred late-type, X-ray bright stars in various parts of the sky and found only 1 star which appears to be as young as the TWA (Webb, Fleming & Zuckerman 2000). In contrast, our survey of ~50 X-ray bright stars around TW Hya yielded 7 very young stars. Still, there has been some debate whether the measured motions support the kinematic relationship expected for stars which formed from the same parent cloud. Here we present HIRES radial velocity measurements which show the stars share a velocity of +11 km/s with a dispersion of only ±2 km/s, indicating that the members are indeed kinematically associated. We calculate UVW motions for each member of the group and determine a mean space motion of (-11,-18,-7) km/s. This motion is small with respect to the local standard of rest indicating the TWA stars are not high velocity runaways from a distant star forming region, but rather formed locally from a cloud which has since dissipated.

The author(s) of this abstract have provided an email address for comments about the abstract: webbr@astro.ucla.edu

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