X-RAY SOURCES IN REGIONS OF STAR-FORMATION .5. THE LOW-MASS STARS OF THE UPPER SCORPIUS ASSOCIATION

We report followup investigations of Einstein x-ray observations of th e Upper Scorpius OB association. We identify 28 low mass pre-main-sequ ence stars as counterparts of x-ray sources in the approximate to 7 sq uare degrees of the OB association observed. Based on kinematics and l ithium abundances, these stars are low mass members of the OB associat ion. We use optical spectra and optical and near-IR photometry to dete rmine the stellar luminosities, effective temperatures, masses, and ag es. We show that the bolometric corrections and effective temperatures of the G and K stars are consistent with those of subgiants. The low mass stars have isochronal ages of 1-2 Myr, depending on the choice of evolutionary models, with very small dispersion (sigma approximate to 1 Myr). This age is significantly younger than the 5-6 Myr found for the more massive B stars. The small dispersion in stellar ages, less t han 10% the sound-crossing time of the association, suggests that star formation was triggered. We present two scenarios for star formation in this association. In the two-episode scenario, formation of the low mass stars was triggered by a supernova explosion, and the low mass s tars form quickly, with high efficiency. Alternatively, high and low m ass star formation was all initiated at the same time, some 5-6 Myr ag o, and the apparent systematic age difference is an artifact of how th e isochrones are dated. The effect of the supernova is to terminate ma ss accretion and yield an apparently coeval population. We show that t he incompleteness in the x-ray sampling is about 65%, and is strongly dependent on stellar mass. After correction for incompleteness, we est imate there are about 2000 low mass members (M < 2M(.)) of this associ ation. The mass function in this association is indistinguishable from that of the field. The ratio of naked to classical T Tauri stars is m uch larger than in Tau-Aur, and may be attributable to the local envir onment. We also present observations of eight ROX sources associated w ith the rho Oph cloud, and observations of non-PMS stars in our fields .