The Nearest OB Association: Scorpius-Centaurus (ScoOB2)
T. Preibisch and E. Mamajek
Handbook of Star Forming Regions Vol. II
The Southern Sky ASP Monograph Publications, Vol. 5.
Edited by Bo Reipurth
pg.235 (2008)
Abstract
We summarize observational results on the stellar population and star
formation history of the Scorpius-Centaurus OB Association (Sco OB2), the nearest
region of recent massive star formation. It consists of three subgroups, Upper Scorpius
(US), Upper Centaurus-Lupus (UCL), and Lower Centaurus-Crux (LCC) which have
ages of about 5, 17, and 16 Myr. While the high- and intermediate mass association
members have been studied for several decades, the low-mass population remained
mainly unexplored until rather recently.
In Upper Scorpius, numerous studies, in particular large multi-object spectroscopic
surveys, have recently revealed hundreds of low-mass association members, including
dozens of brown dwarfs. The investigation of a large representative sample
of association members provided detailed information about the stellar population and
the star formation history. The empirical mass function could be established over the
full stellar mass range from 0.1M_sun up to 20M_sun, and was found to be consistent with
recent determinations of the field initial mass function. A narrow range of ages around
5 Myr was found for the low-mass stars, the same age as had previously (and independently)
been derived for the high-mass members. This supports earlier indications that
the star formation process in US was triggered, and agrees with previous conjectures
that the triggering eventwas a supernova- and wind-driven shock-wave originating from
the nearby UCL group.
In the older UCL and LCC regions, large numbers of low-mass members have
recently been identified among X-ray and proper-motion selected candidates. In both
subgroups, low-mass members have also been serendipitously discovered through investigations
of X-ray sources in the vicinity of better known regions (primarily the
Lupus and TW Hya associations). While both subgroups appear to have mean ages of
~16Myr, they both show signs of having substructure. Their star-formation histories
may be more complex than that of the younger, more compact US group.
Sco-Cen is an important "astrophysics laboratory" for detailed studies of recently
formed stars. For example, the ages of the sub-groups of 5 Myr and ~16 Myr are
ideal for studying how circumstellar disks evolve. While no more than a few percent of
the Sco-Cen members appear to be accreting from a circumstellar disk, recent Spitzer
results suggest that at least ~35% still have cold, dusty, debris disks.
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