The Physical and Chemical Effects of C-Shocks in Molecular Outflows

The bow shocks which are observed in jets associated with low-mass star formation probably have C-shock characteristics over at least part of their extent. We show that ion-neutral streaming in C-type shocks can give rise to the release of significant amounts of silicon from the refractory cores of dust grains and hence to the formation of SiO in the gas phase. The computed profiles of SiO emission lines are found to be qualitatively similar to those observed in molecular outflow regions such as L1448. The question of the validity of the stationary-state assumption, adopted in all previous studies of C-shocks in the interstellar medium, is addressed. Time dependent calculations of the evolution of a C-type shock propagating in low-density gas suggests that steady state is unlikely to be attained under the dynamical conditions in outflows.