ar X iv : a st ro - p h / 01 12 36 5 v 2 1 6 Ja n 20 02 Grain Dynamics In Magnetized Interstellar Gas

The interstellar medium is turbulent and this induces relative motions of dust grains. We calculate relative velocities of charged grains in a partially ionized magnetized gas. We account for anisotropy of magnetohydrodynamic (MHD) turbulence, grain coupling with magnetic field, and the turbulence cutoff arising from the ambipolar drag. We obtain grain velocities for turbulence with parameters consistent with those in HI and dark clouds. These velocities are smaller than those in earlier papers, where MHD effects were disregarded. Finally, we consider grain velocities arising from photoelectric emission, radiation pressure and H 2 thrust. These are still lower than relative velocities induced by turbulence. We conclude that turbulence should prevent these mechanisms from segregating grains by size.