The alignment of interstellar grains

The alignment of interstellar grains by superparamagnetic (SPM) inclusions is discussed. Observations require that the polarizing grains be a dielectric; it is assumed that these are silicates. It is also assumed that grains are conglomerations of individual particles, that some of the particles are SPM (as in the Orgueil meteorite), and that a grain is aligned if and only if it contains one or more SPM particles. With the particle size distribution constrained by fitting the extinction, the entire wavelength dependence law for average polarization is recovered with just one free parameter: the average size of a grain containing an SPM particle.