1996A&A...307..898G

Silicon monoxide has recently been detected in `spiral arm' clouds, via rotational transitions seen in absorption against background continuum sources. We have obtained new data along the lines of sight to Sgr B2 and W49 A, and made first estimates of the abundances of SiO in the foreground clouds. The SiO:H₂ ratios are found to be ∼10^–10, comparable to values in warm star-forming cores, much greater than in dark clouds such as TMC1, but less than in shocked regions. This result provides support for the neutral-neutral mechanism of SiO production, as these reactions can reproduce the observed abundances in the absorbing clouds (although underproducing SiO in the coldest cloud). The SiO formation rate is most sensitive to temperature, and hence SiO is detected in the spiral arm clouds (20-60K), but not in dark clouds (∼10K). The clouds show no indications of shocked gas, as the lines are narrow and densities are only ∼10⁴cm^–3. Thus the data imply that shocks are not needed to produce moderate SiO abundances.