Expected far infrared polarisation of H II regions and the nuclei of galaxies

The expected polarisation of far-infrared sources is discussed and it is shown that the observed degree of polarisation generally depends on two factors: the availability of a mechanism that can produce polarisation, and the gain in polarisation which absorption or stimulated emission can produce within a source. That stimulated emission could produce sizeable gain changes in compact dust clouds such as those associated with H II regions and possibly also in dense regions exhibiting OH and H2O masers. Whether such a gain can actually become positive (maser-like behaviour) is not clear, but this would in any case depend on the chemical structure of the grain material and the spectral range of interest