Alignment of Irregular Grains by Mechanical Torques
  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.
