SUPERASSOCIATIONS - VIOLENT STAR-FORMATION INDUCED BY SHOCK-SHOCK COLLISIONS

Observational data on the internal structure of superassociations are analysed. Basing our assumptions on several well-studied examples we t ry to clarify the characteristic features of these large-scale regions of ongoing violent star formation. One of the most impressive example s of this kind is provided by the superassociation NGC 206 = OB 78 in M31. It reveals a binary internal structure: there are two definitely separated components of more or less equal (or at least comparable) si zes, with hundreds of OB stars in each. The age of these stars is less than 10 Myr. In one of the components, H II regions are observed in i ts external edge. In the other component, Cepheid stars with ages of a bout 40-50 Myr are seen. Between the components, a dust strip stretche s to a length comparable with the sizes of the components. The space-t ime structure of the superassociation suggests that there were at leas t two events or bursts of star formation in the evolutionary history o f this system: one about 50 Myr ago and the other less than 10 Myr ago . It is the latter star formation event that caused the system to beco me a superassociation. The very intensive formation of massive stars p roceeds over the whole area of the two-component system almost simulta neously. Binary (or triple, or composite) structures are always observ ed in superassociations and also, generally, those violent star format ion regions that can only be studied with a high angular resolution (f or instance, with the Hubble Space Telescope). The binary space-time s tructure may provide a new insight into the origin and the physical na ture of the superassociations. The approach we develop here is based o n the observational data of the internal structure of the superassocia tions, and supports violent gas dynamics processes in the interstellar medium of spiral and irregular galaxies as being the physics that mig ht be responsible for the origin of these systems. A key physical mech anism of violent star formation in the superassociation is assumed to be related to the formation and collisions of large-scale shock fronts involving gas masses of about one million solar masses or more. The n on-linear dynamic structures produced by violent shock-shock collision s are at the focus of our discussion here. To study them, we use a set of computer models developed at Ioffe Physico-Technical Institute. Th e main question with regard to the nature of the superassociations con cerns the physical cause of the very effective formation of massive st ars on a spatial scale of 1 kpc simultaneously over the whole area of the system. The gas dynamic processes we address here seem to provide a starting point in the search for an answer to this question. This ap proach suggests new observational studies of the superassociations, an d we give some theoretical predictions which can - in principle - be t ested by observations.