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Paper: The Nearest OB Association: Scorpius-Centaurus (ScoOB2)
Monograph: 5, Handbook of Star Forming Regions:
Volume II, The Southern Sky
Page: 235
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 up to 20M, 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 ∼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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