Thomas Preibisch & Eric Mamajek
The Nearest OB Association: Scorpius-Centaurus (Sco OB2)
Invited chapter for the
Handbook of Star Forming Regions,
ed. B. Reipurth, Astronomical Society of the Pacific, (2008)
We summarize observational results
on the stellar
population and star formation history of the Scorpius-Centaurus OB
Association, the nearest region of recent massive star formation.
Scorpius-Centaurus 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 0.1 M_sun up to 20 M_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 event was 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
~16 Myr, 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.
A preprint is available