Properties of dust grains

(Hough, Aitken, Chrysostomou)

Dust grains are involved in most astrophysical situations, and hence their study, i.e. size distribution, structure and shape, and their chemical composition remains one of the fundamental research areas in astronomy. Most importantly, it has long been recognized that dust grains play a key role in astrochemistry, acting as 'catalysts' in the formation of molecules - including perhaps amino acids. For some years, CAR has had a particular interest in the alignment of dust grains, using both broad band polarization observations and spectropolarimetric observations of solid state features. That dust grains can align has been widely used in determining magnetic field structures. This relies on knowing the way that grains align relative to the ambient magnetic field. For many years it has been assumed that the short axis of the dust grains precess around the magnetic field giving polarization that is parallel to the magnetic field in absorption and perpendicular to the magnetic field for polarized emission. Whilst theoretical studies of the alignment process suggest this will be the case, at least in most circumstances, there appears to be no complete theory and the fact that grains align in cold dense clouds, where they should be in thermal equilibrium with the gas, has continued to test the theories.

Recent highlights include:

• Spectropolarimetry of the solid CO feature at 4.67 microns along the line of sight to Elias 16, a field star background to the Taurus Dark Cloud shows a clear increase in polarization across the feature with the peak of polarization shifted in wavelength relative to the peak of absorption (Hough et al. 2007). This shows that dust grains in dense, cold environments (temperatures ~20K or less) can align and produce polarization by dichroic absorption. For a grain model consisting of a core with a single mantle, the polarization feature is best modelled by a thick CO mantle, possibly including 10% water-ice, with the volume ratio of mantle to bare grain of ~5. Radiative torques could be responsible for the grain alignment provided the grains radius is at least 0.5 microns. This would require the grain cores to have a radius of at least 0.3 microns, much larger than grain sizes in the diffuse ISM. Sizes of this order seem reasonable on the basis of independent evidence for grain growth by coagulation, as well as mantle formation, inside dense clouds. These observations showed that an embedded star is not required to provide a radiation source, with the polarization efficiency (ratio of polarization to optical depth) being similar to that for the solid CO feature along the line of sight to the embedded YSO W33A (Chrysostomou et al 1996).
• Near infrared polarization of field stars background to dense interstellar clouds and embedded YSOs has been used to determine the efficiency of grain alignment in different environments (Whittet et al. 2007). The polarization along the line of sight to background stars yields a dependence on visual extinction well-represented by a power-law; grains along the line of sight to YSOs are generally better aligned compared with field stars with comparable extinction; and mantle formation has little or no effect on the efficiency of grain alignment. Of current alignment mechanisms, that based on radiative torques appear best to explain the data.
• Spectropolarimetric observations of the aliphatic hydrocarbon 3.4 micron feature, along the line of sight to GCS 3-II and GCS 3-IV toward the Galactic centre, show that polarization is not detected to a limit of 0.06%+/-0.13% (GCS 3-II) and 0.15%+/-0.31% (GCS 3-IV). This is well below the lowest available prediction of polarization on the basis of a core-mantle model, and we conclude that the hydrocarbons in the diffuse ISM do not reside on the same grains as the silicates, and likely form a separate population of small grains.
• We investigated the dependence of circular polarimetry on fractionation of ices along a line of sight. It was shown that the form of the spectrum, together with the sign of the position angle shift, indicates where along the line of sight the icy material lies. More specifically, a coincidence between the sign of the position angle displacement in the ice feature, measured north through east, and that of the circular polarization ice feature means that the icy grains are overlaid by bare grains. Some preliminary circular polarimetry of BN was found to have this characteristic. A similar situation exists in the only two other cases for which there are relevant observations.
• Earlier work included the most extensive measurements of the optical and near-infrared polarization of stars made with HATPOL, with many stars subsequently used as polarization standards by a number of observatories (Whittet et al. 1992). Most of the spectropolarimetric measurements of the 3µm ice-feature have been made by Hough and collaborators (Hough et al. 1988, Hough et al. 1989, Holloway et al. 2002), and Aitken has made the most extensive study of the 10µm silicate feature, summarized in an atlas paper (Smith et al. 2002).