Exploring the full stellar population of the Upper Scorpius OB Association

Thomas Preibisch, Anthony Brown, Terry Bridges, Eike Guenther, & Hans Zinnecker

Astronomical Journal 124, 404-416 (2002)


We investigate the stellar population and star formation history of the Upper Scorpius OB association, the most nearby region of recent massive star formation, over the full stellar mass range from 0.1 M_sun to 20 M_sun. The first part of this paper describes an extension of our large spectroscopic survey (Preibisch et al. 2001) for low-mass pre-main sequence (PMS) stars in Upper Scorpius. Utilizing the multi-object spectrograph 2dF at the Anglo-Australian-Telescope, we obtained spectra of 469 stars with magnitudes R = 12.5-18.0 in a 6 square-degree area. Among these we find 68 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. The total area covered by our 2dF survey is now 9 square-degrees and contains 166 new PMS stars. Combining these results with our earlier investigation (Preibisch & Zinnecker 1999) yields a sample of 250 PMS stars in the mass range ~ 0.1 M_sun to ~ 2 M_sun. The location of these stars in the HR diagram suggests a mean age of 5 Myr without a significant age spread. In the second part of this paper, we also consider the population of 114 high-mass members identified in detailed Hipparcos studies. We construct a combined HR diagram for the 364 high- and low-mass members and find that the whole stellar population is very well characterized by a very narrow age distribution around 5 Myr. We estimate individual masses for all members and construct an empirical mass function covering the mass range from 0.1 M_sun up to 20 M_sun. A power-law fit to the mass function gives a slope of alpha ~ -2.6 above ~ 2 M_sun and a much flatter slope (alpha ~ -0.9) below ~ 0.6 M_sun. The initial mass function of Upper Sco is not identical, but within the errors consistent with recent determinations of the field initial mass function. There is certainly no deficit of low-mass stars in the Upper Sco OB association, but rather a small excess of low-mass stars. Our results on the stellar age distribution confirm earlier indications that the star formation process in Upper Sco was triggered, and support previous conjectures that the triggering event was a supernova shock-wave originating from the nearby Upper Centaurus Lupus association.

Retrieve the compressed Postscript file for a preprint here or follow this link to the original paper.

You might be also interested in the previous paper about the young stars in Upper Scorpius, which was the basis of this work.

Some nice images of the Upper Scorpius association can be found here .

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