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+ ions, and of C from graphite. The sputtering threshold energy, particularly for the ejection of Si, is significantly higher than indicated by previous studies. The computed yields are incorporated in a C-shock code to study SiO formation in gas where the density is initially nH=104 cm−3 and the transverse magnetic induction is 100 μG. Collisional processes leading to the excitation and the dissociation of H2, and to the associated ultraviolet fluorescence radiation, are included in the model. We find that the electron temperature profile is crucial to the dynamical and chemical structure of the shock. The electron temperature can be significantly enhanced by interactions with negative ions, such as PAH− . The column densities of SiO computed for shock velocities vs≈50 km s−1 are of the order of those observed in molecular outflow regions.