Bow shocks in molecular clouds: H2 line strengths

We demonstrate that molecular-hydrogen emission-line ratios can be utilized to distinguish between models for shocks in molecular clouds. We compare planar and curved shocks of both C- and J-type. Specifically, we determine here line strengths from a model for bow-shaped C-shocks under high-density molecular cloud conditions. The line ratios are independent of the bow speed once it exceeds the breakdown value (typically 30–50 km s^–1). This is because the H₂ emitting section is simply shifted towards the bow tail at higher speeds. The line ratios are sensitive only to the shape of the bow, the (shock-stimulated) H₂O abundance and the cloud density when the breakdown limit is determined by dissociation. This greatly restricts the predicted line ratios. The presently available data are interpretable as paraboloidal bows of high velocity and moderate H₂O abundance $$\rm (i.e. \varepsilon \sim1-5\times{10}^{-4})$$. J-shocks (with ε = 0) produce similar line ratios but with the shock pressure as the critical parameter.