Annu. Rev. Astron. Astrophys. 1990. 28: 37-70 Copyright © 1990 by Annual Reviews. All rights reserved

5.2 Polarization in Spectral Features

Inner-cloud dust provides information about diffuse dust through the polarization in the 3.08-µm ice band and the 9.7- and 18-µm silicate bands. If the polarization arises from aligned grains, the profiles of polarization, as compared with the corresponding extinction, are potentially important diagnostics for both the shapes and types of grains (DL84; 105). If the sizes and optical properties of interstellar grains are such that the light wave acts as a uniform electromagnetic field across the particle (the ``Rayleigh approximation''), the extinction and polarization cross sections are both simple functions of the optical constants of the material. The fundamental Kramers-Krönig relations force a relationship between the optical constants and, therefore, the extinction and polarization. When there is a strong frequency variation of the optical constants, as across a spectral band, the polarization peaks at longer wavelengths than does the extinction. Indeed, the silicate polarization peaks at about 10.5 µm, whereas the extinction peak is at 9.7 µm (1, 2). The amount of shift depends upon the shapes of the grains as well as upon the optical constants. If the band is strong enough, there is a polarization reversal at wavelengths on the short side of the maximum polarization. The presence of coatings also affects the shape of the polarization relative to the extinction, even if the coatings have a weak wavelength dependence of optical constants across the band.

Polarization can be produced by scattering in the NIR as well as by extinction by aligned grains. Such scattering, commonly observed in reflection nebulae around sources in dense, star-forming regions (95, 129, 130), shows that grains in very dense regions have grown to sizes of at least the order of one µm, partly by acquiring the ice coatings producing the 3.08-µm band.

In the Beckin-Neugebauer object (BN) object in Orion (see ref. 93 for references), the linear polarization is strong (16%) in the 3.08-µm ice band, as opposed to ~ 10% in the neighboring continuum. The polarization is ~ 15% in the 9.7-µm silicate feature but only ~ 1% in the continuum. The position angle is constant across the bands and is the same as in the nebula in general, as expected from grain alignment but not from scattering. However, there is a reflection nebula around BN (see references in ref. 129) that complicates the interpretation of the polarization. By assuming that the polarization in BN is entirely from aligned grains, Lee and Draine (93) found grains are oblate (disk-shaped) rather than prolate, with modest (2:1) axial ratios. One consequence of oblate grains is that the degree of alignment required is dropped by a factor of two, which is the ratio of the mean polarizing power of oblate and prolate grains.

A potentially powerful diagnostic for grains is the comparison of the polarizations in the 9.7-µm and 18-µm bands, for which many of the geometrical uncertainties, such as angles of the magnetic field relative to the line of sight to the source, cancel because the same particles produce both hands.