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. We calculate the distribution of the ratios of the amplitudes of the two major components of the RATs, that is an essential parameter that enters the theory of RAT alignment in Lazarian & Hoang (2007, LH07). While this distribution is different for different classes of grain shapes that we considered, the most probable values of the parameter are centered in the range of $q^{max}\sim 0.5-1.5$. The functional form from RATs calculated is in good agreement with the analytical model (AMO). We find that the RAT efficiency scales as $(\lambda/a)^{-3}$ for $\lambda\gg a$ as previously found in LH07. This increases the power of predictions obtained with the RAT theory. We also confirm that superparamagnetic inclusions are necessary in achieving high degrees of alignment, and constrain the parameter space describing the requirements for achieving these alignment degrees.