On Interstellar Grain Alignment by a Magnetic Field

Abstract

The role of the galactic magnetic field in the alignment of interstellar grains believed to cause optical polarization is discussed. The angular momentum of a charged grain is shown to precess about the magnetic field on a timescale less than the relaxation time of most alignment mechanisms, thus introducing the magnetic field as an important factor in many processes other than the Davis–Greenstein mechanism. The different predictions of Davis–Greenstein alignment and alignment combined with precession are presented. Recent direct observational evidence concerning magnetic alignment raises both positive support and contradictions.