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| EPoS Contribution |
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The flip of polarization patterns in multi-wavelength observations
Michael Kueffmeier UVA/MPE, Charlottesville/Munich-Garching, US/DE | |
| Theory predicts that magnetic fields play an important role in regulating star formation by redistributing angular momentum and suppressing fragmentation. Constraining the magnetic field properties from observations is therefore extremely important. It is challenging though, as magnetic fields themselves are invisible. The abundance of dust grains can help us to circumvent this dilemma as non-spherical dust grains can align with the magnetic field, and induce polarization of light. Thanks to various observational facilities, it is possible to produce polarization maps for wavelengths ranging from tens of micrometers (SOFIA HAWC+) to more than a millimeter (ALMA Band 6). Using the radiative transfer code POLARIS, we will show synthetic polarization maps of various star-forming regions that are obtained from 3D multi-scale magnetohydrodynamical models and compare them to multi-wavelength observations. Our analysis demonstrates that it is a thorny task to interpret the resulting orientation of the polarization vectors correctly. In addition to the possibility of different alignment mechanisms of dust grains that can affect the polarization signal, even the same alignment mechanism can result in vectors that have a different orientation at short compared to long wavelength. The reason for such a 'flip' in the signal is that the polarization pattern at smaller wavelengths is more prone to be dominated by dichroic extinction, while the polarization pattern observed at larger wavelengths is typically dominated by dust reemission. Apart from the difficulties in constraining the magnetic field structure from observations, I will discuss how the flip in polarization can also be used as an additional diagnostic tool to constrain the density and dust distribution in upcoming multi-wavelength observations. | |
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| Caption: Top left: synthetic map of a bridge structure similar to IRAS 16293-2422 as observed at 53 micron (SOFIA HAWC+ Band A). The structure arises as a by-product of the formation of a protostellar multiple in zoom-in simulations that start with a GMC of (40 pc)3 in volume. Top right: synthetic map of the same region but observed at 1.3 mm (ALMA Band 6). The vectors in both panels are rotated by 90 degrees to illustrate the magnetic field structure, when the signal is dominated by dust reemission. Bottom: optical depth over wavelength for regions of different densities. The plot shows that the denser regions are optically thick for small wavelengths, but optically thin for longer wavelengths. As a consequence, the signal at small wavelength (top left panel) is dominated by dichroic extinction, which causes a flip in orientation compared to the reemission-dominated signal at larger wavelength (top right panel). | |
| Collaborators: Z. Y. Li, UVA, US J. Tan, UVA/Chalmers, US/SE C. Y. Law, Chalmers, SE S. Reissl, ITA, DE Y. L. Yang, RIKEN, JP |
Key publication
Suggested Session: Magnetic Fields |