| Supersonic cloud-cloud collisions, which arise naturally in most models of the interstellar medium are likely to be involved in several problems of current astrophysical interest, including molecular cloud formation and star formation. Oort (1946) has proposed that inelastic cloud-cloud collisions, which result in the agglomeration of diffuse interstellar clouds, may lead to the formation of massive, gravitationally-unstable cloud complexes. Recently a number of authors have invoked the cloud agglomeration mechanism to explain the formation and structure of giant molecular clouds. Cloud-Cloud collisions have also been suggested as one of several possible mechanisms for triggering star formation; this hypothesis is supported by observations of NGC 1333 which seems to be a site of collision-induced star formation.; The ramifications of cloud-cloud collisions have been examined by one-dimensional, time-dependent numerical simulations which include the effects of molecule formation and destruction, line radiation, thermal conductivity, and self-gravity. Numerical simultations have been computed for interstellar clouds with widely varying initial conditions.; The results of these simulations suggest that cloud-cloud collisions fall into two classes depending upon whether the clouds are originally atomic or molecular. A collision between standard atomic clouds usually results in the formation of a diffuse molecular cloud, while a collision between molecular clouds is nearly isothermal.; The fact that a collision between atomic clouds forms a diffuse molecular cloud tends to support the hypothesis of cloud agglomeration. However, the frequency of cloud-cloud collisions is such that random cloud-cloud collisions cannot be responsible for the formation of giant molecular clouds.; The question of whether or not cloud-cloud collisions can trigger star formation can be studied in only a very rudimentary form in one dimension. Nonetheless, it can be shown that an isolated, supersonic collision between molecular clouds can trigger star formation only if the Mach number of the collision is less than five.; The time scale of the rarefaction stage of a cloud-cloud collision is longer than the average time between collisions. This implies that there is no such thing as an isolated cloud-cloud collision in the interstellar medium. It also suggests that the cloud model of the interstellar medium is not an equilibrium model. |
