Paramagnetic alignment of small grains: A novel method for measuring interstellar magnetic fields

Abstract

We present a novel method to measure the strength of interstellar magnetic fields using ultraviolet (UV) polarization of starlight that is in part produced by weakly aligned, small dust grains. We begin with calculating the degrees of the paramagnetic alignment of small (size a ∼ 0.01 μm) and very small (a ∼ 0.001 μm) grains in the interstellar magnetic field due to the Davis-Greenstein relaxation and resonance relaxation. To calculate the degrees of paramagnetic alignment, we use Langevin equations and take into account various interaction processes essential for the rotational dynamics of small grains. We find that the alignment of small grains is necessary to reproduce the observed polarization in the UV, although the polarization arising from these small grains is negligible at the optical and infrared (IR) wavelengths. Based on fitting theoretical models to observed extinction and polarization curves, we find that the best-fit model for the case with the peak wavelength of polarization λmax < 0.55 μm requires a higher degree of alignment of small grains than for the typical case with λmax = 0.55 μm. We interpret the correlation between the systematic increase of the UV polarization relative to maximum polarization (i.e., of p(6 μm-1)/p max ) with for cases of low λmax by appealing to the higher degree of alignment of small grains. We utilize the correlation of the paramagnetic alignment of small grains with the magnetic field strength B to suggest a new way to measure B using the observable parameters λmax and p(6 μm-1)/p max . © 2014. The American Astronomical Society. All rights reserved..