Alignment of irregular grains by radiative torques: efficiency study

Abstract: We study the efficiency of grain alignment by radiative torques (RATs) for an ensemble of irregular grains. The grains are modeled as ensembles of oblate and prolate spheroids, deformed as Gaussian random ellipsoids, and their scattering interactions are solved using numerically exact methods. We define the fraction of the grains that both rotate fast and demonstrate perfect alignment with grain long axes perpendicular to the magnetic field. We demonstrate that for typical interstellar conditions the degree of alignment arising from the RAT mechanism is significantly larger than that arising from the Davis-Greenstein (DG) mechanism based on paramagnetic relaxation. We quantify a factor related to the efficacy of alignment and show that it is related to a $q_\mathrm{max}$-factor of analytical model (AMO) of the RAT theory. Our results indicate that the RAT alignment can potentially be sufficiently strong and to explain observations even if grains do not have magnetic inclusions.