Subsonic Mechanical Alignment of Irregular Grains

Abstract

We show that grains can be efficiently aligned by interacting with a subsonic gaseous flow. The alignment arises from grains having irregularities that scatter atoms with different efficiency in the right and left directions. The grains tend to align with long axes perpendicular to magnetic field, which corresponds to Davis-Greenstein predictions, but does not involve paramagnetic dissipation. Choosing conservative estimates of the scattering efficiency of impinging atoms and a conservative ``degree of helicity'' of grains, the alignment of helical grains is much more efficient than the Gold-type alignment processes.