Polarization Of Starlight By Interstellar Dust

Abstract

This paper reviews observations of interstellar linear polarization over the spectral range 0.12-12 μm and discusses their implications for models of interstellar extinction and grain alignment. The wavelength dependence of polarization in the visible is well described by the empirical Serkowski formula p(λ)/P _max = exp[-Kln²(λ_max/λ)], where λ_max is the wavelength of maximum polarization. Deviations from Serkowski behavior occur in the infrared, where both extinction and polarization converge to a common functional form, well described by a power law p(λ) ∝ λ^-1.8.±0.2 Current observational constraints at all wavelengths are consistent with a picture in which only relatively large grains containing silicates are efficiently aligned. There is no general evidence for polarization associated with either the mid-ultraviolet bump or the far-ultraviolet rise in the interstellar extinction curve. The efficiency of alignment depends on both grain size and prevailing physical conditions. In dark clouds, a systematic long-ward shift in λ_max is accompanied by a reduction in UV continuum polarization, explicable in terms of a fall in the relative number of small grains as a function of cloud density. The polarization per unit extinction produced by the large grains also declines in the inner regions of dense clouds, indicating that the efficiency of the alignment mechanism decreases with increasing density.