Polarization due to dichroic absorption and emission by aligned grains in dusty discs

Abstract

We present a method for calculating the infrared polarization due to absorption and emission by aligned grains distributed in a disc or a torus. The temperature distribution in the torus is calculated with the axially symmetric radiative transfer code of Efstathiou & Rowan-Robinson. The polarization patterns predicted for a torus that is believed to be typical of those found in active galactic nuclei (AGN) show a characteristic flip in position angle, as the polarization changes from dichroic absorption to dichroic emission. The wavelength at which the flip occurs is very sensitive to the optical thickness of the torus but not to the fraction of aligned grains, their elongation or any other parameter we have explored. We also find that the predicted polarization is not sensitive to whether grain alignment extends into the hot (inner) part of the disc (T>250K) or not.