Triggered star formation in the Sco OB2 association

We explore the star formation history of the Upper Scorpius OB association the youngest part of the large Sco OB2 association. After an initial wide field (160 square-degree) survey for solar-mass pre-main sequence (PMS) stars in Upper Scorpius, we have performed a deeper survey for low-mass (similar to0.1 - 0.7 Mcircle dot ) PMS stars, covering a 6 square-degree area. Our current PMS sample contains nearly 200 stars. in a detailed analysis of the locations of these stars in the H-R diagram, taking proper account of the uncertainties and the effects of unresolved binaries, we find a mean stellar age of about 5 Myr and no evidence for a significant age dispersion among these stars. This implies that the star formation history of the Upper Scorpius association was dominated by a short starburst, which started about 5 Myr ago and ended probably not more than one or two Myr later. This result strongly suggests that the shock wave of a supernova explosion in the nearby Upper Centaurus-Lupus association, which crossed Upper Scorpius just about 5 Myr ago, triggered the starburst in Upper Scorpius.