  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.
