X-RAY SURVEYS AND THE POST-T TAURI PROBLEM

Recent studies using the ROSAT All-Sky Survey (RASS) towards nearby st ar-forming regions have identified a widely dispersed population of X- ray active stars, and have suggested that these objects are older pre- main-sequence stars (post-T Tauri stars) located far from molecular cl ouds. We argue that the majority of these stars are not pre-main-seque nce stars, but young main sequence stars of ages up to similar to 10(8 ) yr. A simple model assuming continuing star formation over the past 10(8) yr quantitatively reproduces the number, surface density, X-ray emission, and optical properties of the RASS sources; Most of these st ars are old enough to have dispersed far from their birth sites in mol ecular clouds, producing a relatively homogeneous spatial distribution of X-ray sources near the galactic plane. We conclude that the RASS r esults yield little evidence for a post-T Tauri population. We emphasi ze the importance of recognizing this wide-spread spatial distribution of 10(8) yr stars in searches for possible older weak-emission T Taur i stars among X-ray selected samples in nearby star-forming regions. ( C) 1997 American Astronomical Society.