Testing magnetic field models and dust grain alignment with Optical and FIR polarization

Licentiatavhandling, 2026

Magnetic fields play a key role in the interstellar medium (ISM), regulating gas dynamics and star formation. Because they cannot be observed directly, their properties are inferred from indirect tracers such as polarized optical starlight and far-infrared thermal dust emission, both of which arise from dust grains aligned with the magnetic field. The reliability of these tracers therefore depends on the efficiency of grain alignment. In the first part of this thesis, we investigated Radiative Torque Alignment (RAT), the leading mechanism for dust grain alignment. Specifically, we tested the prediction that anisotropic radiation from embedded protostars enhances grain alignment. Using 214 μm SOFIA/HAWC+ polarimetric observations of the high-mass star-forming region DR21, we did a statistical analysis of the polarization fraction, polarization angle dispersion, and alignment efficiency. We detected enhanced polarization near DR21(OH) at intensities where depolarization is typically expected. An analytical model of a centrally heated protostellar envelope reproduced our observed trends. Our results provide one of the first pieces of observational evidence for enhanced grain alignment near embedded protostars and confirm previous findings that polarized dust emission can reliably trace magnetic fields in dense molecular clouds. In the second part of this thesis, we investigate magnetic fields on Galactic scales using optical starlight polarization. We focus on the Radcliffe Wave (RW), a ∼ 3 kpc long structure in the local spiral arm that challenges existing Galactic magnetic field (GMF) models. We introduce a novel method to probe the 3D magnetic field structure by combining starlight polarization measurements with 3D dust maps. Using archival data together with new optical polarization observations from RoboPol, we tested existing GMF models as well as a new model based on the Radcliffe Wave spine geometry. Synthetic polarization maps were generated via radiative transfer calculations and statistically compared to the observations. Our preliminary results indicate that the current GMF models are unable to reproduce the observed starlight polarization towards the RW. This suggests that the GMF models need to be revised to include effects from local structures.