We investigate the role and behaviour of dust grains in C-type MHD shock waves in weakly ionized , dense molecular clouds .
The structure of C-type shocks is largely determined by the coupling of the charged species and the magnetic field .
In weakly ionized clouds , charged dust grains enhance the energy and momentum transfer between the magnetic field and the neutral fluid , and dominate the neutral collisional heating rate .
New shock models are developed here for steady oblique C-type shock structures with shock speed v _ { s } = 18 \mathrm { km / s } , pre-shock number density n _ { H } = 10 ^ { 5 } { \mathrm { cm } ^ { -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 converted to a series of separate grain bins or ‘ size classes ’ .
The dynamics of each grain size class various through the shock front ; smaller grains remain coupled to the magnetic field and larger grains are partially decoupled from the magnetic field due to collisions with the neutrals .
The charges on the grains are allowed to vary , via the sticking and re-releasing of electrons , increasing with increasing electron temperature .
The increase in grain charge increases the coupling of the grains to the magnetic field , and magnetic field rotation out of the shock plane is suppressed .
MRN ( mantles ) and MRN ( PAHs ) distributions are also compared with the standard MRN model .
Increasing the grain sizes in the MRN ( mantles ) model leads to an increase in the collisional heating of the neutrals leading to hotter , thinner shock structures than those using a standard MRN distribution .
With the addition of PAHs , the electron abundance is reduced and the grain charge is held constant , resulting in less grain coupling to the magnetic field , and substantial rotation of the magnetic field out of the shock plane .
The effects of varying the orientation of the pre-shock magnetic field { \bf B _ { 0 } } with the shock normal , specified by the angle \theta , are also considered .
It is found that there are critical values of \theta below 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 angle \theta .