Shock waves in outflows are generated by the impact of jets, associated with low-mass star formation, on the surrounding molecular gas. These shocks give rise to a strong H2 rovibrational emission spectrum which has been observed by the ISO satellite in several star formation regions. The dynamical time scales associated with these outflows are estimated to be a few thousand years and can be, in some regions, as short as a few hundred years. On the other hand, the time required to reach steady state for a C-shock is about 104 years. Under such circumstances, the shocks are unlikely to have attained a state of equilibrium, and a time dependent approach has to be considered. Non stationary C-shocks are found to exhibit both C-and J-type characteristics. The H2 rotational excitation diagram can provide a measure of the age of the shock; in the case of the outflow observed in Cepheus A West by the ISO satellite, the shock age is estimated to be approximately 1.5 × 103 yr.

Time scales

Steady state shocks

Shocks propagating in the interstellar medium are expected to modify profoundly the local physical and chemical conditions. Even in the simplest case of planar shocks, the structure of the shock can take a number of different forms, from ‘jump’ or J-type structure, in which changes in density, velocity and temperature occur quasi-discontinuously, to ‘continuous’ or C-type, where the variations take place smoothly over a much larger distance scale.