ROSAT and Hipparcos Observations of Isolated Pre-Main-Sequence Stars   near HD 98800
  We present new observations of the isolated young stars HD 98800 and CD-33_7795. Pointed ROSAT observations show that their X-ray properties, including X-ray luminosity and variability, are consistent with those of pre-main-sequence (PMS) stars. These observations do not reveal any additional PMS candidates in 40' fields centered on HD 98800 and CD-33_7795. Hipparcos observations of TW Hya (Wichmann et al. 1998) and HD 98800 (Soderblom et al. 1998) show that both stars are roughly 50 pc away and are PMS with ages of ~ 10^7 yr. We searched the Hipparcos catalog (complete down to ~ 2-3 L_sun at this distance) for other PMS stars in the same area. In a 10-pc radius volume of space centered on the previously known PMS stars, we find one additional candidate PMS star (CD-36_7429) with a low space velocity, X-ray emission comparable to that of HD 98800, and Li absorption. There are eight other stars in this area that have dwarf spectral types and lie above the main sequence, but based on their weak X-ray emission, high space velocities, and lack of Li in low-resolution spectra (i.e. EW(Li) < 0.1 Angstroms), these are probably mis-classified subgiants or giants. The current positions and proper motions of TW Hya, HD 98800, and CD-36_7429 are inconsistent with them having formed as a group. 
Eric L. N. Jensen, David H. Cohen, and Ralph Neuhauser