Shocks in dense molecular clouds: OMC-1 revisited

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Abstract
The molecular and atomic fine structure signatures of planar shocks, bow shocks and shock absorbers propagating within dense clouds are presented. A comparison with the powerful OMC-1 bipolar outflow leads to numerous conclusions. Planar and spherical C-shock models are clearly ruled out: besides the H2 line properties, the CO, O I and OH data all lack self-consistent interpretations. A bow shock reproduces the observed line strengths, with a range of excitation states present from the leading edge to the bow tail. For fast-moving bows, line ratios are influenced only by the density, abundances and bow shape. The enigmatic 4.7-μm CO vibration-rotation band and the wide OH profiles find interpretations as emission from the molecular-atomic transition zone which divides the leading dissociated (J-shock) region from the pure H2 (C-shock) region in the bow flanks. It is demonstrated that the H2O emission line spectrum will also distinguish bows from planar shocks. Only the shock absorbers (bow shocks with a high magnetic field) yield H2, O I and CO lines as wide as those observed in OMC-1. They are also distinguished by a relatively low proportion of low-J CO emission and other cool-gas discriminants due to .the large magnetosonic Mach angle in the bow tails.

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