On the alignment of interstellar dust

Abstract

Two alignment mechanisms have been investigated by means of a Monte Carlo model. In the mechanism suggested by Gold, needles or flakes which have a sufficiently high velocity with respect to the gas around them are aligned in consequence of the anisotropy of the angular momentum acquired by a grain in collision with gas atoms. Spheroids exhibited, in the Monte Carlo runs, the expected alignment. The quantitative dependence of both angular momentum alignment and axis alignment on grain shape and velocity has been established with an accuracy of 5 to 10 percent. The maintenance of grain velocity by radiation pressure and the effect of a magnetic constraint on the trajectory of a charged grain are discussed. With paramagnetic relaxation incorporated, the Monte Carlo model exhibits Davis-Greenstein alignment. The results for different grain shapes, grain temperatures, and magnetic viscosities are compared with the theoretical predictions of Jones and Spitzer. Because the lowfrequency magnetic viscosity in any paramagnetic system with dipole-dipole coupling only is independent of the dipole strength and concentration, protons are as effectives as electrons. The viscosity arising from diamagnetic relaxation in graphite is estimated to be much too small for effective alignment of graphite flakes.