Grain Alignment by Radiation in Dark Clouds and Cores

Abstract

We study alignment of grains by radiative torques. We found a steep rise in radiative torque efficiency as grain size increases. This allows the larger grains that are known to exist within molecular clouds to be aligned by the attenuated and reddened interstellar radiation field. In particular, we found that, even deep inside giant molecular clouds, e.g., at optical depths corresponding to AV ≲ 10, large grains can still be aligned by radiative torques. This means that, contrary to earlier claims, far-infrared/submillimeter polarimetry provides a reliable tool to study magnetic fields of prestellar cores. Our results show that the grain size distribution is important for determining the relation between the degree of polarization and intensity.