Radiative torques: Analytical Model and Basic Properties

TL;DR

This paper introduces an analytical toy model for radiative torques on grains, revealing key components influencing grain alignment and demonstrating the model's consistency with complex irregular grain calculations.

Contribution

The paper presents a simple analytical model of radiative torques that captures essential features of grain alignment, facilitating understanding and analysis of RATs across different radiation spectra.

Findings

One RAT component affects precession but not alignment.

Two RAT components determine the grain alignment direction.

Self-similarity of RATs enables analysis across broad radiation spectra.

Abstract

We attempt to get a physical insight into grain alignment processes by studying basic properties of radiative torques (RATs). For this purpose we consider a simple toy model of a helical grain that reproduces well the basic features of RATs. The model grain consists of a spheroidal body with a mirror attached at an angle to it. Being very simple, the model allows analytical description of RATs that act upon it. We show a good correspondence of RATs obtained for this model and those of irregular grains calculated by DDSCAT. Our analysis of the role of different torque components for grain alignment reveals that one of the three RAT components does not affect the alignment, but induces only for grain precession. The other two components provide a generic alignment with grain long axes perpendicular to the radiation direction, if the radiation dominates the grain precession, and perpendicular to magnetic field, otherwise. We study a self-similar scaling of RATs as a function of $\lambda/a_{eff}$. We show that the self-similarity is useful for studying grain alignment by a broad spectrum of radiation, i.e. interstellar radiation field.