Paramagnetic alignment of fractal grains

Abstract

Paramagnetic alignment of fractal suprathermally rotating grains is discussed. It is shown that if the concentration of H$_{2}$formation sites is low and resurfacing is active, fractal structure of grains enhances their alignment. Studying the influence of grain surface physics and chemistry on the alignment we found that there exist two critical values of grain sizes, and the alignment of smaller grains is expected to decrease as compared to the predictions of the Purcell's theory (1979). One of the critical sizes is due to intensified poisoning of active sites, while the other is a result of a finite value of the imaginary part of magnetic susceptibility in the limit of high angular velocities. On the contrary, if active sites completely cover grain surface, suprathermal rotation, and therefore efficient alignment, is possible only for a limited range of grain sizes.