Aims. We present POLARIS (POLArized RadIation Simulator), a newly developed three-dimensional Monte-Carlo radiative transfer code. POLARIS was designed to calculate dust temperature, polarization maps, and spectral energy distributions. It is optimized to handle data that results from sophisticated magneto-hydrodynamic simulations. The main purpose of the code is to prepare and analyze multi-wavelength continuum polarization measurements in the context of magnetic field studies in the interstellar medium. An exemplary application is the investigation of the role of magnetic fields in star formation processes. Methods. We combine currently discussed state-of-the-art grain alignment theories with existing dust heating and polarization algorithms. We test the POLARIS code on multiple scales in complex astrophysical systems that are associated with different stages of star formation. POLARIS uses the full spectrum of dust polarization mechanisms to trace the underlying magnetic field morphology. Results. Resulting temperature distributions are consistent with the density and position of radiation sources resulting from magnetohydrodynamic (MHD) -collapse simulations. The calculated layers of aligned dust grains in the considered cirumstellar disk models are in excellent agreement with theoretical predictions. Finally, we compute unique patterns in synthetic multi-wavelength polarization maps that are dependent on applied dust-model and grain-alignment theory in analytical cloud models.

Observations indicate that molecular clouds are strongly magnetized, and that magnetic fields influence the formation of stars. A key observation supporting the conclusion that molecular clouds are significantly magnetized is that the orientation of their internal structure is closely related to that of the magnetic field. At low column densities the structure aligns parallel with the field, whereas at higher column densities, the gas structure is typically oriented perpendicular to magnetic fields, with a transition at visual extinctions A V 3 mag. Here we use far-infrared polarimetric observations from the HAWC+ polarimeter on SOFIA to report the discovery of a further transition in relative orientation, i.e., a return to parallel alignment at A V 21 mag in parts of the Serpens South cloud. This transition appears to be caused by gas flow and indicates that magnetic supercriticality sets in near A V 21 mag, allowing grav

We present synthetic dust polarization maps of simulated molecular clouds with the goal to systematically explore the origin of the relative orientation of the magnetic field ($\mathbf {B}$) with respect to the cloud sub-structure identified in density (n; 3D) and column density (N; 2D). The polarization maps are generated with the radiative transfer code POLARIS, which includes self-consistently calculated efficiencies for radiative torque alignment. The molecular clouds are formed in two sets of 3D magneto-hydrodynamical simulations: (i) in colliding flows (CF), and (ii) in the SILCC-Zoom simulations. In 3D, for the CF simulations with an initial field strength below ∼5 μG, $\mathbf {B}$ is oriented either parallel or randomly with respect to the n-structures. For CF runs with stronger initial fields as well as all SILCC-Zoom simulations, which have an initial field strength of 3 μG, a flip from parallel to perpendicular orientation occurs at high densities of $n_\rm {trans}$ ≃ 102 – 103 cm−3. We suggest that this flip happens if the cloud’s mass-to-flux ratio, μ, is close to or below the critical value of 1. This corresponds to a field strength around 3 – 5 μG, close to the Galactic average. In 2D, we use the method of Projected Rayleigh Statistics (PRS) to study the relative orientation of $\mathbf {B}$. If present, the flip in orientation occurs in the projected maps at $N_\rm {trans}$ ≃ 1021 − 21.5 cm−2. This value is similar to the observed transition value from sub- to supercritical magnetic fields in the interstellar medium. However, projection effects can strongly reduce the predictive power of the PRS method: Depending on the considered cloud or line-of-sight, the projected maps of the SILCC-Zoom simulations do not always show the flip, although it is expected given the 3D morphology. Such projection effects can explain the variety of recently observed field configurations, in particular within a single cloud. Finally, we do not find a correlation between the observed orientation of $\mathbf {B}$ and the N-PDF.

We present first results for Faraday rotation of compact polarized sources (1 to 2 GHz continuum) in The HI/OH/Recombination line (THOR) survey of the inner Galaxy. In the Galactic longitude range 39 • < < 52 • , we find rotation measures in the range −310 rad m −2 ≤ RM ≤ +4219 rad m −2 , with the highest values concentrated within a degree of = 48 • at the Sagittarius arm tangent. Most of the high RM s arise in diffuse plasma, along lines of sight that do not intersect H II regions. For > 49 • , RM drops off rapidly, while at < 47 • , the mean RM is higher with a larger standard deviation than at > 49 • . We attribute the RM structure to the compressed diffuse Warm Ionized Medium in the spiral arm, upstream of the major star formation regions. The Sagittarius arm acts as a significant Faraday screen inside the Galaxy. This has implications for models of the Galactic magnetic field and the expected amount of Faraday rotation of Fast Radio Bursts from their host galaxies. We emphasize the importance of sensitivity to high Faraday depth in future polarization surveys.