The influence of grains on the propagation and structure of C-type shock waves in interstellar molecular clouds

Abstract

Grains contribute only about 1% to the mass of material in the interstellar medium (ISM). However, in dark clouds, where the charged component of the grains is believed to be a significant fraction of the total and the degree of ionization of the gas is low, the mass density of the charged grains is expected to greatly exceed that of the gaseous component. By the term ‘grains’, we understand both bulk materials, such as graphite or silicates, and large organic molecules (LOM), which may comprise polycyclic aromatic hydrocarbons. Grains are known to be important as both sources and sinks of molecules, notably H2. In the context of dynamics of the ISM, the charged grains may also be important for the modes of shock propagation. In particular, the formation of steady-state C-type shock waves is constrained by the requirement that the magnetosonic speed in the charged fluid, given (at zero temperature) by $$ v_{ims} = \frac{B} {{\left( {4\pi \rho _c } \right)^{\frac{1} {2}} }}, $$ (1) should exceed the shock speed, v s. In (1), B is the magnetic induction in the preshock gas, perpendicular to the direction of propagation of the shock wave, and ρc is the mass density of the charged fluid. As the mass density of charged grains dominates ρc in dark clouds, the fraction of the grains which is charged is an important parameter.