THE NATURE OF ISOLATED T-TAURI STARS

We present the results of a search for young stellar objects around th e two isolated T Tauri stars (TTSs) TW Hya and CoD-29 degrees 8887. Fr om the spectroscopic properties of these two objects, it is obvious th at they are T Tauri stars, although they are not associated with a sta r-forming region as it is the case for most of the known TTSs. Especia lly TW Hya is the only classical TTS that is not located in a dark clo ud with star formation activity. The same is true for the weak-line T Tauri star CoD-29 degrees 8887. We searched for pre-main sequence star s using ROSAT PSPC observations pointing at our two main targets. With a sophisticated search strategy we could identify 107 X-ray sources i n our fields. For the 37 stellar-like optical counterparts we did spec troscopic follow-up observations. These show that within the viewing f ield of ROSAT there are no other X-ray emitting young stellar objects around TW Hya and CoD-29 degrees 8887. For the isolated TTSs TW Hya an d HD 98800 Hipparcos parallax measurements are available locating them at distances of 56 and 46pc, respectively. This means that they are t he closest TTSs with circumstellar dusty disks known today. The space velocities of these two objects are 3-5km/sec. From their position in the HR diagram, we obtained an age of about 10(7) years. Hence, we con clude that these two objects travelled not far away from their origina l birthplace and their parental molecular cloud dispersed meanwhile. T his is the solution why these T Tauri stars appear to be isolated.