1998A&A...330..975C

Proper motions measured by Hipparcos confirm the large anomalous velocities of the OB associations located around the Cygnus Superbubble (Cygnus OB1, OB3, OB7, and OB9), and reveal a clearly organized expanding pattern in Canis Major OB1. At the distances of these associations, the organized velocity patterns imply LSR velocities of up to ∼60km/s for the associations in Cygnus, and about ∼15km/s in Canis Major OB1. The magnitude and spatial arrangement of the expanding motions suggests that very energetic phenomena are responsible for the formation of the present OB associations. This is independently supported by observations of the associated interstellar medium carried out in other wavelengths. The gravitational instability scenario proposed by Comeron & Torra (1994ApJ...423..652C) to account for the formation of the stars in the Cygnus Superbubble region is reviewed in the light of the new kinematic data. It is found that the energetic requirements set by the highest velocities on the OB association powering the Superbubble, Cygnus OB2, are too large by orders of magnitude. However, the scenario can still account for the formation of most of the stars if, as can be reasonably expected, the stars with the highest measured velocities are actually runaways from Cygnus OB2 itself. As for Canis Major OB1, we consider their formation in a supernova remnant, as suggested by Herbst & Assousa (1977ApJ...217..473H). The detection of a new runaway star, HIC 35707 (=HD 57682), whose motion is directed away from the derived center of expansion, supports this scenario 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 clouds, accelerated and compressed by the supernova explosion.