A large spectroscopic survey for young low-mass members of the Upper Scorpius OB association

We present the results of a large intermediate-resolution spectroscopic survey for pre-main-sequence (PMS) stars in the Upper Scorpius OB association, the nearest region of recent massive star formation. Utilizing the 2dF multiobject spectrograph at the Anglo-Australian Telescope, we obtained spectra of 576 stars with magnitudes 12.5 less than or equal to R less than or equal to 16.5 in a 6 deg(2) area in Upper Scorpius. Among these objects we were able to identify 98 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. We place the new PMS stars into the H-R diagram and find that their ages agree well with the mean age of 5 Myr determined in our earlier investigation based on a smaller sample of stars. The number of low-mass (0.1 M. less than or similar to M-* less than or similar to 0.5 M.) members is at least as high as expected from the known number of high-mass (M-* greater than or similar to 3 M.) association members and from recent determinations of the field star initial mass function. Thus, there is no deficit of low-mass stars in Upper Scorpius.