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
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
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
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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
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