Grain Alignment by Radiative Torques in Special Conditions and Implications

TL;DR

This paper investigates radiative torque-induced grain alignment across various astrophysical environments, providing quantitative polarization predictions and exploring new alignment mechanisms with implications for cosmic microwave background studies.

Contribution

It offers comprehensive predictions of dust polarization in different environments and introduces a novel electric field alignment mechanism for dust grains.

Findings

Predicted polarization signatures match recent observations in the interstellar medium.

Identified the dependence of alignment efficiency on radiation field orientation in accretion disks.

Proposed electric field alignment as an alternative to magnetic field alignment, explaining cometary CP features.

Abstract

Grain alignment by radiative torques (RATs) has been extensively studied for various environment conditions, including interstellar medium, dense molecular clouds, and accretion disks, thanks to significant progress in observational, theoretical and numerical studies. In this paper, we explore the alignment by RATs and provide quantitative predictions of dust polarization for a set of astrophysical environments that can be tested observationally. We first consider the alignment of grains in the local interstellar medium and compare predictions for linear polarization by aligned grains with recent observational data for nearby stars. We then revisit the problem of grain alignment in accretions disks by taking into account the dependence of RAT alignment efficiency on the anisotropic direction of radiation fields relative to magnetic fields. Moreover, we study the grain alignment in interplanetary medium, including diffuse Zodiacal cloud and cometary comae, and calculate the degree of circular polarization (CP) of scattered light arising from single scattering by aligned grains. We also discuss a new type of grain alignment, namely the alignment with respect to the ambient electric field instead of the alignment with the magnetic field. We show that this type of alignment can allow us to reproduce the systematic features of CP observed across a cometary coma. Our findings suggest that polarized Zodiacal dust emission may be an important polarized foreground component, which should be treated carefully in cosmic microwave background experiments.