Mechanical alignment of prolate cosmic-dust grains: Cross section alignment

This paper deals with the quantitative analysis of a recently introduced mechanism of mechanical alignment of suprathermally rotating cosmic-dust grains. These grains, whose rapid rotation is mostly due to H-2 formation, are insensitive to stochastic torques arising from gas-grain collisions and can be aligned either by gaseous torques during short periods of crossovers or by the so-called cross section mechanism, arising from the variations in the rate at which atoms arrive at the grain surface. In our previous publication, it was shown that the cross section mechanism is efficient for oblate grains. In the present paper, we address alignment of prolate spheroidal dust grains, derive expressions for the measure of alignment for such grains, and study how this measure depends on the angle between the interstellar magnetic field and the gaseous flow, and on the grain eccentricity. Our comparison of the present results with those obtained in our earlier study shows that the efficiency of cross section alignment for oblate grains is approximately an order of magnitude higher than for prolate grains.