FAR-INFRARED WATER EMISSION FROM MAGNETOHYDRODYNAMIC SHOCK-WAVES

Nondissociative, magnetohydrodynamic, C-type shock waves are expected to be a prodigious source of far-infrared water emissions in dense int erstellar regions. We have constructed a model to calculate the far-in frared H2O line spectra that emerge from such shocks. Using the best e stimates currently available for the radiative cooling rate and the de gree of ion-neutral coupling within the shocked gas, we modeled the te mperature structure of MHD shocks using standard methods in which the charged and neutral particles are treated separately as two weakly cou pled, interpenetrating fluids. Then we solved the equations of statist ical equilibrium to find the populations of the lowest 179 and 170 rot ational states of ortho- and para-H2O. We have completed an extensive parameter study to determine the emergent H2O line luminosities as a f unction of preshock density in the range n(H-2) = 10(4)-10(6.5) cm(-3) and shock velocity in the range upsilon(s) = 5-40 km s(-1). We find t hat numerous rotational transitions of water are potentially observabl e using the Infrared Space Observatory and the Submillimeter Wave Astr onomy Satellite and may be used as diagnostics of the shocked gas. We have also computed the rotational and re-vibrational emissions expecte d from H-2, CO, and OH, and we discuss how complementary observations of such emissions may be used to further constrain the shock condition s. In common with previous studies, we come close to matching the obse rved H-2 and high-J CO emissions from the Orion-KL star-forming region on the basis of a single shock model. We present our predictions for the strengths of H2O line emission from the Orion shock, and we show h ow our results may be scaled to other regions where molecular shocks a re likely to be present.