HIPPARCOS results for ROSAT-discovered young stars

Out of ∼ 500 Lithium-rich ROSAT counterparts, which were presumed to be low-mass pre-main sequence stars, 21 stars have been observed by HIPPARCOS. We study their parallaxes, proper motions, and photometric data. For 7 out of 10 Taurus and Lupus stars in our sample, proper motions and parallaxes are not inconsistent with membership to these associations, while most of the stars in Chamaeleon and Scorpius appear to be young foreground stars. Combined with ground based photometry 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-Sequence 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. Although the sample is admittedly small, containing only 3 % of the total sample of Lithium-rich ROSAT counterparts, it does not confirm recent predictions 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 ejected towards us, or they belong to the Gould Belt system, a plane filled with young stars.