Paper:	Polarization Observations of Molecular Clouds
Volume:	343, Astronomical Polarimetry: Current Status and Future Directions
Page:	321
Authors:	Whittet, D.C.B.
Abstract:	Polarization arises from alignment of interstellar dust grains in the presence of a magnetic field. However, the efficiency with which polarization is produced per unit dust column varies strongly with environment, presumably because the alignment mechanism is sensitive to local physical conditions. Moreover, the detailed physics of alignment is not well understood. The reliability with which observations of polarization trace magnetic field structure within dense molecular clouds is therefore controversial. Observations relevant to studies of both alignment and growth of dust within molecular clouds are discussed in this review. Grains responsible for polarization within dense clouds are shown to have undergone significant growth by coagulation. In the TMC–1 clump, grains appear to be coagulating and growing icy mantles simultaneously, whereas in ρ Oph coagulation dominates. Ice-mantled grains are aligned within externally-heated clouds, but with low efficiency unless a local embedded source of luminosity is present. This result strongly suggests that spin-up by radiative torques is the dominant mechanism leading to alignment of grains within molecular clouds.

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