Radiative Torques on Interstellar Grains. II. Grain Alignment

Radiative torques on irregular dust grains, in addition to producingsuperthermal rotation, play a direct dynamical role in the alignment ofinterstellar dust with the local magnetic field. The equations governing theorientation of spinning, precessing grains are derived; H_2 formation torquesand paramagnetic dissipation are included in the dynamics. Stationary solutions(constant alignment angle and spin rate) are found; these solutions may bestable ("attractors") or unstable ("repellors"). The equations of motion arenumerically integrated for three exemplary irregular grain geometries, exposedto anisotropic radiation with the spectrum of interstellar starlight. Theresulting "trajectory maps" are classified as "noncyclic", "semicyclic", or"cyclic", with examples of each given. We find that radiative torques result in rapid grain alignment, even in theabsence of paramagnetic dissipation. It appears that radiative torques due tostarlight can account for the observed alignment of interstellar grains withthe Galactic magnetic field.Comment: 34 pages, 21 eps figures, uses aaspp4.sty . Submitted to Ap.