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| Paper: |
Alignment of Dust by Radiative Torque: Recent Developments |
| Volume: |
449, Astronomical Polarimetry 2008: Science from Small to Large Telescopes |
| Page: |
116 |
| Authors: |
Lazarian, A.; Hoang, T. |
| Abstract: |
Alignment of dust by radiative torques (RATs) has proven to be the most promising mechanism to explain alignment in various astrophysical environments, from comet atmospheres to circumstellar accretion disks, molecular clouds, and diffuse interstellar gas. Recent years have been marked by intensive attempts to provide proper theoretical treatment of the alignment process. We discuss some of the major advances, which include, first of all, formulating of the analytical model of RATs. This model was shown to reproduce well the torques acting on actual irregular dust grains and allowed studies of the parameter space for which the alignment happens with long axes perpendicular and parallel to the magnetic field. Such a study resulted in an important conclusion that, without any paramagnetic relaxation, the RAT alignment always happens for interstellar grains with long axes perpendicular to the magnetic field. Incidentally, this conclusion is not true for the alignment of large (i.e. a > 10–4 cm) grains that are present, e.g. in accretion disks, as these grains can demonstrate both the alignment with long axes parallel and perpendicular to the magnetic field, although the alignment with long grain axes perpendicular to the magnetic field is preferential in many cases, which can also be identified with the analytical model. Additional recent advances include the description of effects of gaseous bombardment and pinwheel torques on grains aligned by RATs. Very counterintuitively, the gaseous bombardment was shown in some cases to increase the degree of alignment by knocking out grains from the positions of imperfect alignment when the grains rotate slowly to more stable positions of perfect alignment where grains rotate fast. In terms of pinwheel torques, important revisions have been made in the Lazarian & Draine model of grain flipping and thermal trapping. Those, however, do not change the major conclusion that very small grains (i.e. a < 3× 10–6 cm) should be marginally aligned. Recent work made the RAT alignment a predictive theory which is ready for quantitative modeling of astrophysical polarization. The right timing of this work is not only due to renewed efforts in terms of astrophysical polarimetry, but also due to the interest to the polarized foreground arising from the attempts to measure CMB polarization. In particular, we predict that the microwave emission from the Zodiacal dust presents an important contaminant, which should be included into foreground polarization templates. |
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