Quantitative Theory of Grain Alignment: Probing Grain Environment and Grain Composition

TL;DR

This paper reviews a quantitative model of grain alignment driven by radiative torques, highlighting how polarization measurements can reveal dust properties and magnetic fields in various astrophysical environments.

Contribution

It presents an analytical model for radiative torques on irregular grains and discusses how grain composition and environment influence alignment mechanisms.

Findings

Radiative torque alignment can be modeled analytically for irregular grains.

Polarization measurements can inform about dust composition and magnetic fields.

Superparamagnetic inclusions and pinwheel torques significantly affect grain alignment.

Abstract

While the problem of grain alignment was posed more than 60 years ago the quantitative model of grain alignment that can account for the observed polarization arising from aligned grains has been formulated only recently. The quantitative predictions of the radiative torque mechanism, which is currently accepted as the dominant mechanism of grain alignment, open avenues to tracing magnetic fields in various astrophysical environments, including diffuse and dense interstellar gas, molecular clouds, circumstellar environments, accretion disks, comet tails, Zodiacal dust etc. At the same time, measurements of the absolute value of polarization and its variations can, in addition, provide unique information about the dust composition and dust environment. In the review I describe the analytical model describing well radiative torques acting on irregular grains and discuss how the alignment induced by radiative torques varies in the presence of superparamagnetic inclusions and pinwheel torques, e.g. arising from the molecular hydrogen formation over grain surface. I also describe observations that can establish whether grains are superparamagnetic and whether recoils from molecular hydrogen formations are powerful enough to give rise to substantial uncompensated torques. Answering to these questions should allow for reliable modeling of astrophysical polarization with numerous important applications, from accounting for dust contribution in Cosmic Microwave Background polarization studies to obtaining magnetic field strength using Chandrasekhar-Fermi technique.