2008A&A...482..809G

We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si-bearing material in refractory grain cores, which are taken to be olivine. We calculate also the rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady-state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models, with shock speeds in the range 20<v_s<50km/s and preshock gas densities 10⁴<n_H<10⁶cm^–3, has been generated. We compare our results with those of an earlier study (Schilke et al., 1997A&A...321..293S). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si (≲10%) being released into the gas phase. Erosion of grain cores is significant (>1%) only for C-type shock speeds v_s>25km/s, given the adopted properties of olivine. More realistic assumptions concerning the initial fractional abundance of O₂ lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG ``slab'' models. The fractional abundance of SiO is deduced and found to be in the range 4x10^–8≲n(SiO)/n_H≲3x10^–7. Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam.