We explore the star formation history of the Upper Scorpius OB association , the youngest part of Sco OB2 .
A wide field ( 160 square-degree ) survey for low-mass pre-main sequence ( PMS ) stars enabled us to increase the number of known low-mass members of Upper Scorpius to nearly 100 stars .
In a detailed analysis of the locations of these stars in the HR 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 star-burst , which started about 5 Myr ago and ended probably not more than one or two Myr later .
Interestingly , the structure and kinematics of the HI shells surrounding the Sco OB2 association show that the shock wave of a supernova explosion in the nearby Upper Centaurus-Lupus association , the oldest part of Sco OB2 , crossed Upper Scorpius just about 5 Myr ago .
This strongly suggests that this supernova shock wave triggered the star-burst in Upper Scorpius .