New determinations of the critical velocities of C-type shock waves in dense molecular clouds: Application to the outflow source in Orion

Abstract

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) = [nH(cm-3)]172, we established that the maximum shock speed increased from 20-30km s-1 at high pre-shock densities (nH ≥ 106cm-3) to 70-80kms-1 at low densities (nH < 104 cm-3). The critical shock speed, vcrit, also increases significantly with the transverse magnetic induction, B, at a given pre-shock gas density, nH. By way of an application of these results, we demonstrate that a two-component model, comprising shock waves with velocities vs = 60 and 40 km 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 42 515 K 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 [Si I] 68.5 μm 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 [Fe II] and [S I] observed by Rosenthal et al. may also arise in the shock-heated gas.