Published online by Cambridge University Press: 14 August 2015
Polarization at infrared wavelengths has been detected from a number of different objects within the Galaxy. These include young sources associated with molecular clouds and H II regions, cool stars with thick circumstellar shells, bi-polar nebulae, and normal stars suffering interstellar polarization. Typical levels of polarization detected at 2.2 μm are up to 25% for the molecular cloud sources, less than −5% for the cool stars, around 30% for some bi-polar nebulae and less than −2% for interstellar polarization. For the latter three types of source the origin of the polarization is basically understood: it results from scattering of stellar radiation off small particles in the surrounding shell or nebula in the cool stars and bi-polar nebulae and by transfer of flux through a foreground medium of aligned dust grains for the interstellar polarization. The phenomenon of large infrared polarization in the young stellar and pre-stellar sources is less well understood, and it is to this problem that we address ourselves in this review.