Infrared Polarimetry of Interstellar Dust
Grains in the diffuse interstellar medium, and in at least some parts of dense molecular clouds, are aligned with their short axis preferentially along the direction of the local magnetic field. At longer infrared wavelengths radiation can be linearly polarized through the emission from such dust grains (dichroic emission), whilst at shorter wavelengths the radiation becomes polarized through dichroic absorption. The linear birefringence of dust grains can lead to the production of circular polarization. Enhanced linear polarization, relative to the continuum, occurs across solid-state features at ~3–5μm, attributed to ices, and at ~10μm and ~20μm, attributed to silicate grains. Continuum polarimetry and spectropolarimetry of the features can provide important diagnostics of the grain properties with significant advantages over total flux photometry or spectroscopy. Polarization produced by dichroic absorption and emission also can be used to determine the geometry of magnetic fields. In the near-infrared, typically for wavelengths up to a few microns, scattering of radiation from dust grains close to a radiation source can produce both linear and circular polarization. Polarimetry, especially over a wide wavelength range, can help to determine the properties of dust grains in the environs of stars ranging from young stellar objects that are undergoing accretion and mass outflow, to evolved stars that are losing mass. In the case of star-forming regions it is possible that dust grains play a key role in the origins of homochirality on the Earth.