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

We report calculations of the intensities of rovibrational transitions of H-2 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 H-2 . We found that the maximum shock speeds which can be attained, prior to the collisional dissociation of H-2 (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 (muG)=[n (H) (cm(-3) )](1/2), we established that the maximum shock speed increased from 20-30 km s(-1) at high pre-shock densities (n (H) greater than or equal to10(6) cm(-3) ) to 70-80 km s(-1) at low densities (n (H) less than or equal to10(4) 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 40 km s(-1) , reproduces the column densities of H-2 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 mum 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.