WATER MASER EMISSION FROM MAGNETOHYDRODYNAMIC SHOCK-WAVES

Slow, nondissociative, magnetohydrodynamic shock waves that propagate in dense molecular gas are a probable source of water maser emission i n regions of active star formation. We have constructed a model to com pute the water maser emission expected from such shocks. We have integ rated a set of one-dimensional hydrodynamics equations in which the ne utral species and charged particles are treated separately as two inte rpenetrating but weakly coupled fluids, and then solved the equations of statistical equilibrium to obtain the level populations of the lowe st 179 and 170 rotational states of ortho- and para-H2O. Our model inc ludes radiative cooling due to rovibrational transitions of H2O, CO an d H-2, and cooling due to dissociation of H-2 and due to gas-grain col lisions. The fractional ionization is extremely low in the dense shock s considered here and resides primarily on charged dust grains. We fin d that luminous H2O maser emission is expected from dense nondissociat ive MHD shocks: in particular, the warm molecular gas behind such shoc ks is ideal for pumping numerous low- and high-lying submillimeter mas er transitions. Here we present results for shocks with initial H-2 de nsities of 10(7)-10(9.5) cm(-3) and velocities of propagation up to si milar to 45 km s(-1). Over this entire parameter space, we have determ ined the efficiency with which shock energy is converted into maser lu minosity for each of the water maser transitions that have so far been observed in interstellar gas, under conditions where the maser action is saturated, and we have considered the geometrical effects which de termine whether or not a given maser transition will be saturated. For the range of preshock densities that we considered, nondissociative s hocks give rise to individual masing regions with sizes of similar to 10(12) to a few times 10(14) cm, and, given suitable geometries, can r eproduce the high brightness temperatures characteristic of observed m aser sources. Nondissociative shock models are also successful in acco unting for the magnetic field strengths that have been inferred from o bservations of Zeeman splitting. Maser line ratios are presented for u se as potential probes of the conditions in the masing gas. These are compared with observational data, some of which cannot be explained on the basis of fast dissociative shock models.