Magnetohydrodynamic shock waves in molecular clouds

Abstract

Calculations for the structure of shock waves in molecular clouds are presented which include the effects of ion-neutral streaming driven by the magnetic field. Results show that shock waves in molecular clouds will usually be C-type shock waves, mediated entirely by the dissipation accompanying ion-neutral streaming, and in which all of the hydrodynamic variables are continuous. The magnetohydrodynamic shock waves propagating at speeds in the range of 5-50 km/s in molecular clouds with preshock densities n(H) = 100, 10000, and 1000000/cu cm are studied in detail. The effects of chemical changes in the composition of oxygen-bearing molecules are determined, and the contributions to the cooling of the shocked gas by emission from H2, CO, OH, and H2O are evaluated. The intensities of the rotation-vibration lines of H2 and of the fine-structure lines of O I and C I are predicted. It is shown that magnetic fields may lead to a substantial increae in the limiting shock velocity above which dissociation of H2 takes place, such that for a cloud of intensity n(H) = 1000000/cu cm, the limiting shock speed is approximately 45 km/s.