GRAIN ALIGNMENT BY AMBIPOLAR DIFFUSION IN MOLECULAR CLOUDS

We show that the dust grains in a weakly ionized molecular cloud undergoing ambipolar diffusion will become partially aligned by Gold's mechanism with their angular momentum vectors oriented preferentially parallel to the local magnetic field. We present accurate numerical calculations on the efficiency of Gold's mechanism for oblate, spheroidal, core-mantle grains which include the effects of Barnett relaxation, Larmor precession, gas-grain collisions, the evaporation of molecules from ice mantles, and paramagnetic or superparamagnetic relaxation. We calculate the polarized far-infrared emission from warm grains in a plasma undergoing ambipolar diffusion, on the ad hoc assumption that Gold's mechanism is the only process that aligns the grains. Our calculations include an accurate treatment of the systematic gas-grain drift induced by electromagnetic and gas drag forces as well as the stochastic drift associated with random fluctuations in the grain charge. For reasonable grain shapes and favorable magnetic field geometries, the linear polarization attributable to ambipolar diffusion in a typical cloud core exceeds approximate to 1% wherever the ion-neutral drift speed exceeds a few tenths of a kilometer per second. Large polarizations greater 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-infrared polarimetry of molecular clouds.