KINEMATIC SIGNATURES OF VIOLENT FORMATION OF GALACTIC OB ASSOCIATIONSFROM HIPPARCOS MEASUREMENTS

Proper motions measured by Hipparcos confirm the large anomalous veloc ities of the OB associations located around the Cygnus Superbubble (Cy gnus OB1, OB3, OB7, and OB9), and reveal a clearly organized expanding pattern in Canis Major OB1. At the distances of these associations, t he organized velocity patterns imply LSR velocities of up to similar t o 60 km s(-1) for the associations in Cygnus, and about similar to 15 km s(-1) in Canis Major OB1. The magnitude and spatial arrangement of the expanding motions suggests that very energetic phenomena are respo nsible for the formation of the present OB associations. This is indep endently supported by observations of the associated interstellar medi um carried out in other wavelengths. The gravitational instability sce nario proposed by Comeron & Torra 1994 (ApJ 423, 652) to account for t he formation of the stars in the Cygnus Superbubble region is reviewed in the light of the new kinematic data. It is found that the energeti c requirements set by the highest velocities on the OB association pow ering the Superbubble, Cygnus OB2, are too large by orders of magnitud e. However, the scenario can still account for the formation of most o f the stars if, as can be reasonably expected, the stars with the high est measured velocities are actually runaways from Cygnus OB2 itself. As for Canis Major OB1, we consider their formation in a supernova rem nant, as suggested by Herbst & Assousa 1977 (ApJ, 217, 473). The detec tion of a new runaway star, HIC 35707 (=HD 57682), whose motion is dir ected away from the derived center of expansion, supports this scenari o and provides an independent age for the supernova remnant, assuming that the runaway star was the binary companion of the supernova. Based on a number of arguments, however, we find it unlikely that the stars are a direct consequence of instabilities in the expanding shell. We propose instead that their formation was triggered in preexisting clou ds, accelerated and compressed by the supernova explosion.