The alignment of interstellar dust grains: thermal flipping and the Davis-Greenstein mechanism

TL;DR

This paper investigates the impact of thermal flipping on interstellar dust grain alignment, specifically examining the Davis-Greenstein mechanism, and finds it unlikely to efficiently align large grains in the cold neutral medium.

Contribution

The study provides detailed numerical simulations showing thermal trapping is not significant, and assesses the viability of Davis-Greenstein alignment considering thermal flipping effects.

Findings

Thermal trapping is not prevalent during crossovers.

Davis-Greenstein alignment is too inefficient for large grains in the cold neutral medium.

Small grains may contribute to ultraviolet polarization.

Abstract

Interstellar dust grains are non-spherical and, in some environments, partially aligned along the direction of the interstellar magnetic field. Numerous alignment theories have been proposed, all of which examine the grain rotational dynamics. In 1999, Lazarian & Draine introduced the important concept of thermal flipping, in which internal relaxation processes induce the grain body to flip while its angular momentum remains fixed. Through detailed numerical simulations, we study the role of thermal flipping on the grain dynamics during periods of relatively slow rotation, known as `crossovers', for the special case of a spheroidal grain with a non-uniform mass distribution. Lazarian & Draine proposed that rapid flipping during a crossover would lead to `thermal trapping', in which a systematic torque, fixed relative to the grain body, would time average to zero, delaying spin-up to larger rotational speeds. We find that the time-averaged systematic torque is not zero during the crossover and that thermal trapping is not prevalent. As an application, we examine whether the classic Davis-Greenstein alignment mechanism is viable, for grains residing in the cold neutral medium and lacking superparamagnetic inclusions. We find that Davis-Greenstein alignment is not hindered by thermal trapping, but argue that it is, nevertheless, too inefficient to yield the alignment of large grains responsible for optical and infrared starlight polarization. Davis-Greenstein alignment of small grains could potentially contribute to the observed ultraviolet polarization. The theoretical and computational tools developed here can also be applied to analyses of alignment via radiative torques and rotational disruption of grains.