6.2. Magnetic Fields and Star Formation Onsets
As reviewed in Elmegreen (1998), spontaneous processes probably dominate the onset of star formation on a galactic scale, but triggered star formation processes sustain, amplify and disperse what large-scale instabilities begin. Triggers include (i) direct compression of clouds (a 3-dimensional squeezing), (ii) collection and compression of clouds at the edges of HII regions or supernovae or giant rings and shells in galaxy disks (a 2-dimensional push), and (iii) cloudlet-cloudlet collisions and cloud-cloud collisions (2-dimensional effect with possible lateral squirting).
Theoretical models of cloudlet collisions with magnetic fields are in their infancy (e.g., Elmegreen 1998). Small cloudlets in inelastic collisions may coalesce to form larger cloudlets. A sufficiently massive cloudlet (= clump by now) may then undergo a gravitational collapse. Globally, the large gas compression following many inelastic collisions might enhance the star formation rate. Cloudlet-cloudlet collisions have been discussed theoretically as strong collisions (such as with magnetic tangling) or as weak collisions (such as with magnetic oscillation) (e.g., Elmegreen 1988). The morphology of the cloudlets after collisions is strongly dependent on both the presence of a magnetic field and the elasticity defined as the ratio of the final to the initial kinetic energy of the cloudlets (e.g., Ricotti et al. 1997). Stone (1970a, 1970b) has identified three main evolutionary stages (compression, expansion and lateral outflow, collapse). The presence of a magnetic field parallel to the shock front between two clouds will lower the compression of the postshock gas, decrease the dissipation energy, increase the elasticity of the collision, and prevent the shattering of the clouds (Ricotti et al. 1997). In the magnetized collision case, Elmegreen (1985) has shown that energy dissipation by Alfvén wave radiation can be important at least for some clouds.
Observationally, cloudlet-cloudlet collisions have been categorized with a 12-point observational test for their easy recognition (see Table 3 in Vallée, 1995b). The application of the 12-point test, made for the observations of the cloud collision giving rise to the infrared source IRAS 2306+1451, supports the collision model of Stone (1970a, 1970b).