Grain alignment induced by radiative torques: effects of internal relaxation of energy and complex radiation fields

Abstract

Earlier studies of grain alignment dealt mostly with interstellar grains that have strong internal relaxation of energy which aligns grain axis of maximum moment of inertia with respect to grain's angular momentum. In this paper, we study the alignment by radiative torques for large irregular grains, e.g., grains in accretion disks, for which internal relaxation is subdominant. We use both numerical calculations and the analytical model of a helical grain introduced by us earlier. We demonstrate that grains in such a regime exhibit more complex dynamics. In particular, if initially the grain axis of maximum moment of inertia makes a small angle with angular momentum, then radiative torques can align the grain axis of maximum moment of inertia with angular momentum, and both axis of maximum moment of inertia and angular momentum are aligned with the magnetic field when attractors with high angular momentum (high-J attractors) are available. For the alignment without high-J attractors, beside the earlier studied attractors with low angular momentum (low-J attractors), there appears new low-J attractors. The former and later cases correspond to the alignment with long axes perpendicular and parallel to the angular momentum, respectively. In addition, we study the alignment of grains in the presence of strong internal relaxation, but induced not by a radiation beam as in earlier studies, instead, induced by a complex radiation field, that can be decomposed into dipole and quadrupole components. We find that in this situation, the parameter space $q^{max}$, for the existence of high-$J$ attractors is more extended, which entails higher degrees of polarization expected. Our obtained results are useful for modeling polarization arising from aligned grains in molecular clouds and accretion disks.