A LITHIUM-SURVEY FOR PRE-MAIN-SEQUENCE STARS IN THE UPPER SCORPIUS OBASSOCIATION

We present the results of an intermediate resolution spectroscopic sur vey for pre-main sequence (PMS) stars in the Upper Scorpius OB associa tion. In a 160 square-degree area we were able to identify 39 new PMS stars by follow up observations of X-ray selected stars with the multi object spectrograph FLAIR at the UK Schmidt Telescope. We also invest igated the completeness of our X-ray selected sample by observing more than 100 stars that were not detected as X-ray sources, but have prop er motions indicating membership to Upper Sco. While the new X-ray sel ected PMS stars with known proper motions have kinematics consistent w ith membership, none of the X-ray quiet proper motion candidates is a PMS star. We conclude that our X-ray selected sample of PMS stars seem s to be rather complete. For stars in the magnitude interval 11.5 less than or similar to B less than or similar to 13.5 we derive a conserv ative lower limit of 75% completeness.