The degree to which interstellar grains align with respect to the interstellar magnetic field depends on disaligning as well as aligning mechanisms. For decades, it was assumed that disalignment was due primarily to the random angular impulses a grai
n receives when colliding with gas-phase atoms. Recently, a new disalignment mechanism has been considered, which may be very potent for a grain that has a time-varying electric dipole moment and drifts across the magnetic field. We provide quantitative estimates of the disalignment times for silicate grains with size > approximately 0.1 micron. These appear to be shorter than the time-scale for alignment by radiative torques, unless the grains contain superparamagnetic inclusions.
Radiative torques, due to the absorption and scattering of starlight, are thought to play a major role in the alignment of grains with the interstellar magnetic field. The absorption of radiation also gives rise to recoil torques, associated with the
photoelectric effect and photodesorption. The recoil torques are much more difficult to model and compute than the direct radiative torque. Here, we consider the relatively simple case of a spheroidal grain. Given our best estimates for the photoelectric yield and other relevant grain physical properties, we find that the recoil torques contribute at the 10% level or less compared with the direct radiative torque. We recommend that the recoil torques not be included in models of radiation-driven grain alignment at this time. However, additional experimental characterization of the surface properties and photoelectric yield for sub-micron grains is needed to better quantify the magnitude of these torques.
