# Alignment of Irregular Grains by Mechanical Torques

## TL;DR

This paper investigates how irregular dust grains can be aligned by mechanical torques resulting from their drift through gas, revealing conditions under which this alignment is efficient and comparable to radiative torques.

## Contribution

The study introduces a detailed analysis of mechanical torques on irregular grains, demonstrating their potential to align grains similarly to radiative torques, especially for highly irregular shapes.

## Key findings

- HIS grains can be aligned by MATs in subsonic drift conditions.
- MATs can effectively align grains in supersonic drift scenarios.
- Magnetic torques combined with MATs can achieve perfect grain alignment.

## Abstract

We study the alignment of irregular dust grains by mechanical torques due to the drift of grains through the ambient gas. We first calculate mechanical torques (MATs) resulting from specular reflection of gas atoms for seven irregular shapes: one shape of mirror symmetry, three highly irregular shapes (HIS), and three weakly irregular shapes (WIS). We find that the grain with mirror symmetry experiences negligible MATs due to its mirror-symmetry geometry. Three highly irregular shapes can produce strong MATs which exhibit some generic properties as radiative torques, while three weakly irregular shapes produce less efficient MATs. We then study grain alignment by MATs for the different angles between the drift velocity and the ambient magnetic field, for paramagnetic and superparamagnetic grains assuming efficient internal relaxation. We find that for HIS grains, MATs can align subsonically drifting grains in the same way as radiative torques, with low-J and high-J attractors. For supersonic drift, MATs can align grains with low-J and high-J attractors, analogous to radiative alignment by anisotropic radiation. We also show that the joint action of MATs and magnetic torques in grains with iron inclusions can lead to perfect MAT alignment. Our results point out the potential importance of MAT alignment for HIS grains predicted by the analytical model of Lazarian \& Hoang (2007b), although more theoretical and observational studies are required due to uncertainty in the shape of interstellar grains. We outline astrophysical environments where MAT alignment is potentially important.