A unified model of grain alignment: radiative alignment of interstellar grains with magnetic inclusions

TL;DR

This paper develops a unified model for grain alignment in space, showing how magnetic inclusions enhance alignment efficiency, with implications for polarized dust emission and magnetic field studies.

Contribution

It introduces the MRAT mechanism, demonstrating how iron inclusions in grains lead to high-J attractor points and improved alignment, extending previous radiative torque models.

Findings

High-J attractor points achievable with ~10% iron in grains.

Large grains can be perfectly aligned with magnetic fields.

Model supports polarization studies of dust emission.

Abstract

The radiative torque (RAT) alignment of interstellar grains with ordinary paramagnetic susceptibilities has been supported by earlier studies. The alignment of such grains depends on the so-called RAT parameter $q^{\max}$ that is determined by the grain shape. In this paper, we elaborate our model of RAT alignment for grains with enhanced magnetic susceptibility due to iron inclusions, such that RAT alignment is magnetically enhanced for which we term MRAT mechanism. Such grains can get aligned with high angular momentum at the so-called high-J attractor points, achieving a high degree of alignment. Using our analytical model 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 $\psi$. We find that if about $10\%$ of total iron abundance present in silicate grains are forming iron clusters, it is sufficient to produce high-J attractor points for all reasonable values of $q^{\max}$. To calculate the degree of grain alignment, we carry out numerical simulations of MRAT alignment by including stochastic excitations from gas collisions and magnetic fluctuations. We show that large grains can achieve perfect alignment when the high-J attractor point is present, regardless of the values of $q^{\max}$. Our obtained results pave the way for physical modeling of polarized thermal dust emission as well as magnetic dipole emission. We also find that millimeter-sized grains in accretion disks may be aligned with the magnetic field if they are incorporated with iron nanoparticles.