A unified model of grain alignment: Radiative Alignment of Interstellar Grains with magnetic inclusions

Thiem Hoang, Alex Lazarian

Published 2016-05-10Version 1

The radiative torque (RAT) alignment of interstellar grains with ordinary paramagnetic susceptibilities has been supported by a number of earlier studies. The alignment of such grains depends on the so-called RAT parameter $q^{\max}$ that is determined by the grain shape. For interstellar grains with a broad range of $q^{\max}$, a significant fraction of grains is expected to get aligned with low angular momentum at the so-called low-J attractor points, which entail degrees of alignment between 20 or 30 percent, irrespectively of the strength of RATs. The latter value may not be sufficient for explaining the observed interstellar alignment in the diffuse medium. In this paper, we elaborate our model of radiative alignment for grains with enhanced magnetic susceptibility due to magnetic inclusions, such that both Magnetic torque and RAdiative Torque (MRAT) play a role in grain alignment. Such grains can get aligned with high angular momentum at the so-called high-J attractor points, which achieve a high degree of alignment. Using our analytical model (AMO) of RATs we derive the critical value of the magnetic relaxation parameter $\delta_{m}$ to produce high-J attractor points as functions of $q^{\max}$ and the anisotropic radiation angle relative to the magnetic field $\psi$. To calculate degree of grain alignment, we carry out numerical simulations of MRAT alignment by including stochastic excitations by gas collisions and magnetic fluctuations. We numerically demonstrate that the degree of MRAT alignment varies with $\psi$ and increases with increasing grain size for different values of $\delta_{m}$. Our obtained results pave the way for physical modeling of polarization spectrum of thermal dust emission as well as magnetic dipole emission from grains of arbitrary magnetic susceptibilities.