2002MNRAS.332..985L

We report calculations of the intensities of rovibrational transitions of H₂ emitted from C-type shock waves propagating in molecular gas. Attention was paid to the thermal balance of the gas and to the rates of collisional dissociation and ionization of H₂. We found that the maximum shock speeds which can be attained, prior to the collisional dissociation of H₂ (which results in a sonic point in the flow and hence a J-type shock wave), can be much higher than had previously been believed. Thus, adopting the `standard' scaling of the transverse magnetic induction with the gas density, B(µG)=[n_H(cm^–3)]^1/2, we established that the maximum shock speed increased from 20-30km.s^–1 at high pre-shock densities (n_H≥10⁶ cm^–3) to 70-80km.s^–1 at low densities (n_H≤10⁴ cm^–3). The critical shock speed, v_crit, also increases significantly with the transverse magnetic induction, B, at a given pre-shock gas density, n_H. By way of an application of these results, we demonstrate that a two-component model, comprising shock waves with velocities v_s=60 and 40km.s^–1, reproduces the column densities of H₂ observed by ISO -SWS up to the highest level (possibly) detected, v =0, J =27, which lies 42515K above the ground state. We found no necessity to invoke mechanisms other than thermal collisional excitation in the gas phase; but the v =1 vibrational band remains less completely thermalized than is indicated by the observations. Fine structure transitions of atoms and ions were also considered. The intensity of the [Sii] 68.5µm transition, observed by Gry et al. using ISO -LWS, is satisfactorily reproduced by the same model and may also originate in OMC-1, rather than Orion-KL as originally believed. The transitions of [Feii] and [Si] observed by Rosenthal et al. may also arise in the shock-heated gas.