Magnetic fields in star-forming systems (I): idealized synthetic signatures of dust polarization and Zeeman splitting in filaments

Abstract

We use the POLARIS radiative transport code to generate predictions of the two main observables directly sensitive to the magnetic field morphology and strength in filaments: dust polarization and gas Zeeman line splitting. We simulate generic gas filaments with power-law density profiles assuming two density-field strength dependencies, six different filament inclinations, and nine distinct magnetic field morphologies, including helical, toroidal, and warped magnetic field geometries. We present idealized spatially resolved dust polarization and Zeeman-derived field strengths and directions maps. Under the assumption that dust grains are aligned by radiative torques (RATs), dust polarization traces the projected plane-of-the-sky magnetic field morphology. Zeeman line splitting delivers simultaneously the intensity-weighted line-ofsight field strength and direction. We show that linear dust polarization alone is unable to uniquely constrain the 3D field morphology. We demonstrate that these ambiguities are ameliorated or resolved with the addition of the Zeeman directional information. Thus, observations of both the dust polarization and Zeeman splitting together provide the most promising means for obtaining constraints of the 3D magnetic field configuration. We find that the Zeeman-derived field strengths are at least a factor of a few below the input field strengths due to line-of-sight averaging through the filament density gradient. Future observations of both dust polarization and Zeeman splitting are essential for gaining insights into the role of magnetic fields in star and cluster forming filaments.

Could bow-shaped magnetic morphologies surround filamentary molecular clouds?

“…If we forgo the ∇ • B = 0 requirement, we can use the equations provided by Reissl et al (2018), which model the bow morphology in a slightly different manner:…”

“…Three magnetic field morphologies that can explain this direction-change of B across filamentary structures are toroidal, helical, and bow morphologies. The helical or toroidal morphologies have been investigated in a number of theoretical studies (e.g., Shibata & Matsumoto 1991;Nakamura et al 1993;Hanawa et al 1993;Matsumoto et al 1994;Fiege & Pudritz 2000a,b;Schleicher & Stutz 2018;Reissl et al 2018). Fiege & Pudritz (2000a,b) studied the fragmentation length scale, stability, density profile, and mass per length of filamentary MCs, and, based on observational constraints, they suggest that many filamentary structures are likely wrapped by helical magnetic fields.…”

“…We refer to this morphology, regardless of how it is formed, as a bow morphology. This general morphology was recently investigated in a number of theoretical studies (Gómez et al 2018;Inoue et al 2018;Reissl et al 2018), as well as observational studies (Liu et al 2018), and has been also referred to as a U-shape or a pinched field.…”

SILCC-Zoom: Polarization and depolarization in molecular clouds

“…Recently, Reissl et al (2016) have presented the freely available dust polarisation radiative transfer code PO-LARIS, which is able to calculate grain alignment efficiencies and the subsequent radiative transfer in a fully selfconsistent manner. We have already applied the code to the case of protostellar outflows showing the necessity of wavelength-dependent radiative transfer (Reissl et al 2017) and filaments showing the advantage of additional Zeeman observations (Reissl et al 2018).…”

Probing the cold magnetised Universe with SPICA-POL (B-BOP)

“…As significant degeneracies exist between different models because only the plane-of-sky magnetic field is directly accessible to dust polarimetry (cf. Reissl et al 2018;Tomisaka 2015), discriminating between these various magnetic topologies will require sensitive imaging observations of large samples of molecular filaments for which the distribution of viewing angles may be assumed to be essentially random. One advantage of the model of oblique MHD shocks (e.g., Chen & Ostriker 2014;Inoue et al 2018;Lehmann & Wardle 2016) is that it could potentially explain both how dense filaments maintain a roughly constant ∼0.1 pc width while evolving (cf.…”