Magnetic Properties of Dust Grains, Effect of Precession and Radiative Torque Alignment

TL;DR

This paper explores how dust grain alignment mechanisms, influenced by magnetic fields and radiation, can be used to probe magnetic properties and environments in space, introducing new methods and constraints for astrophysical observations.

Contribution

It revisits grain alignment by Radiative Torques, introduces a new magnetic relaxation process, and proposes observational tests to constrain dust magnetic susceptibility.

Findings

Identification of a new magnetic relaxation process in dust grains.

Demonstration of how grain alignment changes can probe magnetic susceptibility.

Constraints on dust magnetic properties in various astrophysical environments.

Abstract

Alignment of dust grains in astrophysical environments results in the polarization of starlight as well as the polarization of radiation emitted by dust. We demonstrate the advances in grain alignment theory allow the use of linear and circular polarization to probe not only the magnetic field, but also dust composition, the dust environment, etc. We revisit the process of grain alignment by Radiative Torques (RATs) and focus on constraining magnetic susceptibility of grains via observations. We discuss the possibility of observational testing of the magnetic properties of grains as the alignment changes from being in respect to the magnetic field to being in respect to the radiation direction. This opens both a possibility of constraining the uncertain parameters of the RATs theory and provides a new way of measuring magnetic fields in interstellar medium and circumstellar regions. We provide a detailed discussion of the precession induced both by the magnetic field and the anisotropic radiation and revisit a number of key processes related to magnetic response of the grains. We consider various effects that increase the rate of magnetic relaxation both in silicate and carbonaceous grains. In particular, we find a new relaxation process related to the change of the amplitude of internal magnetization within a wobbling triaxial grain and identify a range of grain sizes in which this effect can dominate the internal alignment of angular momentum within grain axes. We show that these relaxation processes significantly change the dynamics of grains in the presence of RATs. We apply our analysis for observed grain alignment in special environments to put constraints on the enhanced magnetic properties of dust grains in the cloud near supernovae, in cometary coma, and protoplanetary disks.