Alignment of Dust with Magnetic Inclusions: Radiative Torques and Superparamagnetic Barnett and Nuclear Relaxation

TL;DR

This paper investigates how superparamagnetic inclusions in dust grains influence their internal relaxation and alignment mechanisms, revealing enhanced relaxation effects and conditions for perfect alignment with magnetic fields.

Contribution

It introduces superparamagnetic Barnett relaxation and shows how superparamagnetic inclusions modify radiative torque alignment, extending the understanding of grain alignment in astrophysics.

Findings

Superparamagnetic inclusions enhance internal relaxation of dust grains.

Grains can achieve perfect alignment with magnetic fields under certain conditions.

Higher degrees of alignment than previously predicted may indicate superparamagnetic inclusions.

Abstract

We consider grains with superparamagnetic inclusions and report two new condensed matter effects that can enhance the internal relaxation of the energy of a wobbling grain, namely, superparamagnetic Barnett relaxation, as well as, an increase of frequencies for which nuclear relaxation becomes important. This findings extends the range of grain sizes for which grains are thermally trapped, i.e. rotate thermally, in spite of the presence of uncompensated pinwheel torques. In addition, we show that the alignment of dust grains by radiative torques gets modified for superparamagnetic grains, with grains obtaining perfect alignment with respect to magnetic fields as soon as the grain gaseous randomization time gets larger than that of paramagnetic relaxation. The same conclusion is valid for the mechanical alignment of helical grains. If observations confirm that the degrees of alignment are higher than radiative torques can produce alone, this may be a proof of the presence of superparamagentic inclusions.