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In a weakly-ionized plasma undergoing ambipolar diffusion, charged dust grains drift through the neutral particles, with a speed determined by the competition between the Lorentz and gas drag forces on the grains. We show that, under some physical conditions of astrophysical interest, gas-grain drift causes the grain angular momenta to become partially aligned with respect to the local magnetic field. Consequently, the thermal emission from warm grains in a plasma undergoing ambipolar diffusion can be partially polarized. In this paper, we present accurate theoretical estimates of the far-infrared and submillimeter polarization produced by this mechanism. We solve the Langevin equation for the angular momentum distribution of drifting grains under the assumption that the grains are oblate spheroids. Our calculations include the effects of gas-grain collisions, thermal evaporation from the grain surfaces, and paramagnetic or superparamagnetic torques. We discuss the releveance of this process to recent observations of the neutral gas ring at the Galactic center. This work was supported by NASA grant NAGW-3001.
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