Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Preibisch, A. Brown, T. Bridges, E.
Guenther, and H. Zinnecker:
Exploring the full stellar population of the
Upper Scorpius OB Association
Astronomical Journal, v.124, p.404 (2002)
Abstract.
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.
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