New Members of the TW Hydrae Association, β Pictoris Moving Group, and Tucana/Horologium Association

Abstract

We have identified five new members of the TW Hydrae association (TWA), 11 new members of the β Pic moving group, and 11 new Tucana/Horologium association members. These are the three youngest (30 Myr) known kinematic stellar groups near the Earth. Newly identified β Pic group members are located mostly in the northern hemisphere, and they have a slightly different U-component of Galactic velocity compared to that of previously known members. Tracing the motion of β Pic members backward in time for 12 Myr indicates that they might have formed in a small region with an initial velocity dispersion of ~8 km s^-1. A couple of mid-M spectral type β Pic members show emission features [He I λ5876+λ6678) and Na D λ5890+λ5896)] seen among earlier spectral type stars in the TWA and β Pic groups. To derive the distances of the non-Hipparcos members of these groups, we have constructed a V-K versus M_K color-magnitude diagram that is very useful in separating young K/M stars from older main-sequence counterparts and constraining theoretical pre-main-sequence evolutionary tracks. All newly identified K- and M-type members of the three groups show saturated X-ray activity (L_X/L_bol ~ 10^-3). One newly identified TWA member, SSS 101727-5354, is estimated to be only 22 pc away from Earth. Its extreme youth, late spectral type (~M5), and proximity to Earth make SSS 101727-5354 perhaps the best target for direct imaging detection of cooling planets.