Lapo Fanciullo (ASIAA)
Interstellar dust grains are generally non-spherical and their major axes tend to orient perpendicularly to magnetic field lines. The net result of this is that thermal emission from dust is polarized, and this polarization provides information not just on dust itself, but also on the orientation of interstellar magnetic fields. The study of polarized dust emission – which is found mainly at far-infrared and submillimeter wavelengths – is central to many astrophysical fields, from the study of dust itself, to CMB analysis (as a foreground), and magnetic field mapping on both Galactic and molecular cloud scales. Unfortunately, interpreting the polarization fraction in emission (P/I) is a non-trivial task, since this quantity is determined by several factors: the orientation and structure of the magnetic field, the grain alignment efficiency and the optical properties of dust itself. As an example, the well-known drop of P/I at high column density is often used to trace the loss of grain alignment in molecular clouds, but the analysis is complicated by the complex magnetic field inside these clouds, which also plays a role. The possibility that polarization efficiency may be different for dust in cloud cores (due to dust processing such as accretion and coagulation) further complicates the picture. Studying the spectral shape of dust polarization can help break this degeneracy, since the effects of magnetic field and alignment on P/I are expected to be independent of wavelength at first approximation. Returning to the example above, multi-wavelength polarimetry could be the key to studying how dust polarization cross-sections evolve in dense clouds. As part of the BISTRO project (B-fields In STar-forming Region Observations), I lead a team to combine 850 µm polarimetry from JCMT and 150–210 µm polarimetry from SOFIA to recover a polarized spectrum for Gould Belt clouds. I will show our latest results for NGC 2071 in Orion B. I will also explain the importance of using long-wavelength (λ > 500 µm) data in multi-wavelength polarimetric studies.