Dust grain dynamics in C-Type shock waves in molecular clouds

Abstract

We investigate the role and behaviour of dust grains in C-type MHD shock waves in weakly ionized, dense molecular clouds. New shock models are presented for steady, oblique C-type shock structures with shock speed v_s=18km/s, n_H=10^5cm^-3, and a grain population represented by either a single grain species or a MRN grain size distribution. The grain size distribution is calculated using Gauss-Legendre weights and the integrals over the continuous distribution of grain sizes are represented by a number of grain bins. The grain population can then be thought of as a series of separate grain size classes, each with uniquely specified properties. The dynamics of each grain size class is different through the shock front, with the smaller grain classes remaining coupled to the magnetic field and larger grains becoming partially decoupled from the magnetic field due to collisions with the neutrals. The importance of grain charging is also demonstrated. The effects of the orientation of the pre-shock magnetic field B_0 are also considered. It is found that there are critical orientations for B_0 in which the shock is no longer C-type and the transition becomes C$^*$ or J-type. The degree of non-coplanarity of the shock solution depends upon the grain model chosen, as well as the orientation of B_0.