The nature of isolated T Tauri stars

We present the results of a search for young stellar objects around the two isolated T Tauri stars (TTSs) TW Hya and CoD−29◦8887. From the spectroscopic properties of these two objects, it is obvious that they are T Tauri stars, although they are not associated with a star-forming region as it is the case for most of the known TTSs. Especially TW Hya is the only classical TTS that is not located in a dark cloud with star formation activity. The same is true for the weak-line T Tauri star CoD−29◦8887. We searched for pre-main sequence stars using ROSAT PSPC observations pointing at our two main targets. With a sophisticated search strategy we could identify 107 X-ray sources in our fields. For the 37 stellar-like optical counterparts we did spectroscopic follow-up observations. These show that within the viewing field of ROSAT there are no other X-ray emitting young stellar objects around TW Hya and CoD−29◦8887. For the isolated TTSs TW Hya and HD 98800 Hipparcos parallax measurements are available locating them at distances of 56 and 46pc, respectively. This means that they are the 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 107 years. Hence, we conclude that these two objects travelled not far away from their original birthplace and their parental molecular cloud dispersed meanwhile. This is the solution why these T Tauri stars appear to be isolated.