Grain Alignment: Role of Radiative Torques and Paramagnetic Relaxation

Abstract: Polarization arising from aligned dust grains presents a unique opportunity to study magnetic fields in the diffuse interstellar medium and molecular clouds. Polarization from circumstellar regions, accretion disks and comet atmospheres can also be related to aligned dust.To reliably trace magnetic fields quantitative theory of grain alignment is required. Formulating the theory that would correspond to observations was one of the longstanding problems in astrophysics. Lately this problem has been successfully addressed, and in this review we summarize some of the most important theoretical advances in the theory of grain alignment by radiative torques (RATs) that act on realistic irregular dust grains. We discuss an analytical model of RATs and the ways to make RAT alignment more efficient, e.g. through paramagnetic relaxation when grains have inclusions with strong magnetic response. For very small grains for which RAT alignment is inefficient, we also discuss paramagnetic relaxation and a process termed resonance relaxation. We provide an extensive analysis of the observational tests of grain alignment theory.