A Large Spectroscopic Survey for Young Low-Mass Members of the Upper Scorpius OB Association

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Abstract
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 ≤ R ≤ 16.5 in a 6 deg2 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⊙ M* 0.5 M⊙) members is at least as high as expected from the known number of high-mass (M* 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.