From parallel to perpendicular -- On the orientation of magnetic fields in molecular clouds

Abstract

We present synthetic dust polarization maps of simulated molecular clouds (MCs) with the goal to systematically explore the origin of the relative orientation of the magnetic field ($\mathbf{B}$) with respect to the MC sub-structures identified in density ($n$; 3D) and column density ($N$; 2D). The polarization maps are generated with the radiative transfer code POLARIS, including self-consistently calculated efficiencies for radiative torque alignment. The MCs are formed in two sets of 3D MHD simulations: in (i) colliding flows (CF), and (ii) the SILCC-Zoom simulations. In 3D, for the CF simulations with an initial field strength below $\sim$5 $\mu$G, $\mathbf{B}$ is oriented parallel or randomly with respect to the $n$-structures. For CF runs with stronger initial fields and all SILCC-Zoom simulations, which have an initial field strength of 3 $\mu$G, a flip from parallel to perpendicular orientation occurs at high densities of $n_{\text{trans}}$ $\simeq$ 10${}^2$ - 10${}^3$ cm${}^{-3}$. We suggest that this flip happens if the MC's mass-to-flux ratio, $\mu$, is close to or below the critical value of 1. This corresponds to a field strength around 3 - 5 $\mu$G. In 2D, we use the Projected Rayleigh Statistics (PRS) to study the orientation of $\mathbf{B}$. If present, the flip in orientation occurs at $N_{\text{trans}}$ $\simeq$ 10${}^{21-21.5}$ cm${}^{-2}$, similar to the observed transition value from sub- to supercritical magnetic fields in the ISM. However, projection effects can reduce the power of the PRS method: Depending on the MC or LOS, the projected maps of the SILCC-Zoom simulations do not always show the flip, although expected from the 3D morphology. Such projection effects can explain the variety of recently observed field configurations, in particular within a single MC. Finally, we do not find a correlation between the observed orientation of $\mathbf{B}$ and the $N$-PDF.

arXiv comment20 pages, 12 figures, accepted for publication in MNRAS

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