THE STRUCTURE OF MHD SHOCKS IN MOLECULAR OUTFLOWS - GRAIN SPUTTERING AND SIO FORMATION

We compute the yields of Si and O in the sputtering of SiO2 by He+ ion s, and of C from graphite. The sputtering threshold energy, particular ly for the ejection of Si, is significantly higher than indicated by p revious studies, The computed yields are incorporated in a C-shock cod e to study SiO formation in gas where the density is initially n(H) = 10(4) cm(-4) and the transverse magnetic induction is 100 mu G. Collis ional processes leading to the excitation and the dissociation of H-2, and to the associated ultraviolet fluorescence radiation, are include d in the model. We find that the electron temperature profile is cruci al to the dynamical and chemical structure of the shock. The electron temperature can be significantly enhanced by interactions with negativ e ions, such as PAH(-). The column densities of SiO computed for shock velocities nu(s) approximate to 50 km s(-1) are of the order of those observed in molecular outflow regions.