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Paper: Polarization of Starlight
Volume: 309, Astrophysics of Dust
Page: 65
Authors: Whittet, D.C.B.
Abstract: The polarization of starlight is universally attributed to directional extinction arising when dust grains along the line of sight are aligned in the presence of a magnetic field. This phenomenon provides a valuable tool both for studying properties of the dust and for charting the galactic magnetic field. Current observational constraints at all wavelengths are consistent with a picture in which only the larger grains in the size range responsible for extinction are efficient polarizers. Small grains are much less well aligned and/or much less anisotropic compared with large grains. The polarizing grains include silicates (with or without ice mantles), but there is no compelling evidence that they include appreciable quantities of carbonaceous material. The degree of alignment is highly sensitive to ambient physical conditions as well as to particle size. The polarization per unit extinction declines with increasing optical depth within dark clouds, hindering attempts to map the intracloud magnetic field. However, studies of interstellar ice absorption features in the spectra of deeply embedded YSOs indicate that ice-mantled grains within the cores of dense molecular clouds can be surprisingly efficient polarizers. This suggests that the local radiation field may be a key factor in maintaining alignment.
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