DUST GRAINS AND THE STRUCTURE OF STEADY C-TYPE MAGNETOHYDRODYNAMIC SHOCK-WAVES IN MOLECULAR CLOUDS

I examine the role of dust grains in determining the structure of stea dy, cold, oblique C-type shocks in dense molecular gas. Gas pressure, the inertia of the charged components and changes in ionization are ne glected. The grain charge and rate coefficients for electron-neutral a nd grain-neutral elastic scattering are assumed constant at values app ropriate to the shock interior. An MRN size distribution is accounted for by estimating an effective grain abundance and Hall parameter for single-size grains. A one-parameter family of intermediate shocks exis ts for each shock speed upsilon(s) between the intermediate signal spe ed upsilon(A)cos theta and root 2 upsilon(A)cot theta, where upsilon(A ) is the pre-shock Alfven speed and a is the angle between the pre-sho ck magnetic field and the normal to the shock front. In addition, ther e is a unique fast shock for each upsilon(s) > upsilon(A). If the pre- shock density n(H) greater than or similar to 10(5) cm(-3) and the pre -shock magnetic field satisfies B(mG)/n(H)(10(5) cm(-3)) less than or similar to 1, grains are partially decoupled from the magnetic field a nd the field and velocity components within fast shocks do not lie in the plane containing the preshock field and the shock normal. The resu lting shock structure is significantly thinner than in models that do not take this into account. Existing models systematically underestima te the grain-neutral drift speed and the heating rate within the shock front. At densities in excess of 10(8) cm(-3) these effects may be re duced by the nearly equal abundances of positive and negative grains.