Barnett relaxation in non-symmetric grains

TL;DR

This paper investigates Barnett relaxation in non-symmetric grains, developing models for dissipation and diffusion, and demonstrates that thermal flipping occurs in non-symmetric grains but not in symmetric ones, impacting grain alignment theories.

Contribution

It provides new approximate expressions for Barnett relaxation rates and clarifies the conditions under which thermal flipping can occur in non-symmetric grains.

Findings

Thermal flipping occurs in non-symmetric grains.

Approximate models for dissipation and diffusion are developed.

Symmetric grains do not undergo thermal flipping.

Abstract

Barnett relaxation, first described by Purcell in 1979, appears to play a major role in the alignment of grains with the interstellar magnetic field. In 1999, Lazarian and Draine proposed that Barnett relaxation and its relative, nuclear relaxation, can induce grains to flip. If this thermal flipping is rapid, then the dynamical effect of torques that are fixed relative to the grain body can be greatly reduced. To date, detailed studies of Barnett relaxation have been confined to grains exhibiting dynamic symmetry. In 2003, Weingartner argued that internal relaxation cannot induce flips in any grains, whether they exhibit dynamic symmetry or not. In this work, we develop approximate expressions for the dissipation rate and diffusion coefficient for Barnett relaxation. We revisit the issue of internally induced thermal flipping, finding that it cannot occur for grains with dynamic symmetry but does occur for grains lacking dynamic symmetry.