HIPPARCOS RESULTS FOR ROSAT-DISCOVERED YOUNG STARS

Out of similar to 500 Lithium-rich ROSAT counterparts, which were pres umed to be low-mass pre-main sequence stars, 21 stars have been observ ed by HIPPARCOS. We study their parallaxes, proper motions, and photom etric data. For 7 out of 10 Taurus and Lupus stars in our sample, prop er motions and parallaxes are not inconsistent with membership to thes e associations, while most of the stars in Chamaeleon and Scorpius app ear to be young foreground stars. Combined with ground based photometr y and spectroscopy, HIPPARCOS parallaxes allow us to place 15 stars on an H-R diagram. All these 15 stars lie above the Zero-Age-Main-Sequen ce and thus are indeed pre-main sequence stars with ages from 1 to 15 Myr. Only two of the stars are located on the Hayashi-tracks, whereas the other 13 are post-T Tauri stars located on radiative tracks. Altho ugh the sample is admittedly small, containing only 3% of the total sa mple of Lithium-rich ROSAT counterparts, it does not confirm recent pr edictions by other authors: We find no stars in the age range from 20 to 100 Myr. The foreground pre-main sequence stars may have been eject ed towards us, or they belong to the Could Belt system, a plane filled with young stars.