GRAIN-GRAIN COLLISIONS AND SPUTTERING IN OBLIQUE C-TYPE SHOCKS

We use a model of the dissipation regions in C-type shocks in which tw o grain fluids are included to evaluate the rates at which grain-grain collisions and sputtering inject elemental silicon and water into the gas phase. For physical parameters typical of star forming regions, a nd on the assumption that a substantial fraction of the evaporated sil icon is efficiently converted in SiO, the two grain destruction mechan isms lead to gas phase SiO abundances larger than those in the quiesce nt gas by more than three orders of magnitude. This result is in good agreement with recent observations of SiO near stellar jets. Grain-gra in collisions will dominate the return of elemental silicon to the gas phase when the hydrogen nuclei number density of the region into whic h the shock is propagating is n(Ho) greater than or similar to 5 x 10( 5) cm(-3) and the shock speed is between about 25 km s(-1) and 35 km s (-1). Grain-grain collisions dominate over sputtering in the return of water to the gas phase when n(Ho) greater than or similar to 10(6) cm (-3) and at some shock speeds below about 15 km s(-1).