TEMPORAL EVOLUTION OF MHD SHOCKS IN THE INTERSTELLAR-MEDIUM

We undertake calculations of the time-dependent structure of shock wav es propagating in dark and diffuse interstellar clouds. The results of the time-dependent model are compared with those obtained by means of an independent steady-state code and found to agree well at sufficien tly late times. Discontinuities in the flow of the neutral fluid are h andled by introducing a pseudo-viscosity. Special procedures are adopt ed to correct for the associated widening of the discontinuity, in ord er not to distort the role of inelastic collision processes. We find t hat, in dark clouds, C shocks will tend to predominate, but are unlike ly to have attained steady state in the cloud lifetime. On the other h and, in diffuse clouds, steady state may be reached but the discontinu ity in the flow of the neutral fluid remains. We find no evidence for the existence of C shocks, in which the neutral fluid undergoes a con tinuous transition from supersonic to subsonic flow tin the reference frame of the shock wave). Attention is drawn to the possible importanc e of these results for the interpretation of H-2 rovibrational line in tensities.