GRAIN ALIGNMENT BY AMBIPOLAR DIFFUSION IN MOLECULAR CLOUDS

We show that the dust grains in a weakly ionized molecular cloud under going ambipolar diffusion will become partially aligned by Gold's mech anism with their angular momentum vectors oriented preferentially para llel to the local magnetic field. We present accurate numerical calcul ations on the efficiency of Gold's mechanism for oblate, spheroidal, c ore-mantle grains which include the effects of Barnett relaxation, Lar mor precession, gas-grain collisions, the evaporation of molecules fro m ice mantles, and paramagnetic or superparamagnetic relaxation. We ca lculate the polarized far-infrared emission from warm grains in a plas ma undergoing ambipolar diffusion, on the ad hoc assumption that Gold' s mechanism is the only process that aligns the grains. Our calculatio ns include an accurate treatment of the systematic gas-grain drift ind uced by electromagnetic and gas drag forces as well as the stochastic drift associated with random fluctuations in the grain charge. For rea sonable grain shapes and favorable magnetic field geometries, the line ar polarization attributable to ambipolar diffusion in a typical cloud core exceeds approximate to 1% wherever the ion-neutral drift speed e xceeds a few tenths of a kilometer per second. Large polarizations gre ater than or similar to 10% are also possible under optimal conditions where the ion-neutral drift speed is much larger than the gas thermal speed. We show that the maximum efficiency of Gold's mechanism is in excellent agreement with the largest alignment inferred from far-infra red polarimetry of molecular clouds.