Radiative Torques of Irregular Grains: Describing the Alignment of a Grain Ensemble

Abstract

The radiative torque (RAT) mechanism is the most promising way of explaining observed polarization arising from aligned grains. We explore the efficiency of the grain alignment by an anisotropic radiation flow for an extensive ensemble of grain shapes, grain sizes ( a ), and wavelength ( λ ). We calculate the distribution of the ratios of the amplitudes of the two major components of the RATs, which is an essential parameter that is used in the theory of RAT alignment in Lazarian & Hoang. While this distribution is different for the different classes of grain shapes that we considered, the most probable values of the parameter are centered in the range of q max  ∼ 0.5–1.5. The functional form from the calculated RATs is in good agreement with the analytical model. We find that the RAT efficiency scales as ( λ / a ) −2.6 for λ  ≳ 1.8 a , which has a slightly shallower slope than previously found in LH07. This increases the power of predictions obtained with the RAT theory. We also confirm that superparamagnetic inclusions are necessary for achieving high degrees of alignment, and constrain the parameter space describing the requirements for achieving these alignment degrees.