Alignment of interstellar grains by mechanical torques: suprathermally rotating Gaussian random spheres

TL;DR

This paper investigates how mechanical torques from gas collisions influence the alignment of interstellar grains, showing that grain shape and drift speed significantly affect alignment efficiency, with implications for understanding interstellar magnetic fields.

Contribution

It provides detailed calculations of mechanical torques on irregular grains and analyzes their rotational dynamics, highlighting shape and drift speed effects on alignment.

Findings

Alignment efficiency depends on grain shape and drift speed.

More efficient alignment occurs with shapes having substantial mechanical torque.

Supersonic drift enhances grain alignment.

Abstract

Collisions of gas particles with a drifting grain give rise to a mechanical torque on the grain. Recent work by Lazarian & Hoang showed that mechanical torques might play a significant role in aligning helical grains along the interstellar magnetic field direction, even in the case of subsonic drift. We compute the mechanical torques on 13 different irregular grains and examine their resulting rotational dynamics, assuming steady rotation about the principal axis of greatest moment of inertia. We find that the alignment efficiency in the subsonic drift regime depends sensitively on the grain shape, with more efficient alignment for shapes with a substantial mechanical torque even in the case of no drift. The alignment is typically more efficient for supersonic drift. A more rigorous analysis of the dynamics is required to definitively appraise the role of mechanical torques in grain alignment.