Three-dimensional magnetic field imaging of protoplanetary disks using Zeeman broadening and linear polarization observations

Abstract

Magnetic fields are predicted to have a crucial impact on the structure, evolution and chemistry of protoplanetary disks. However, a direct detection of the magnetic field towards these objects has yet to be achieved. In order to characterize protoplanetary disk magnetic fields, we investigate the impact of the Zeeman effect on the (polarized) radiative transfer of emission from paramagnetic molecules excited in protoplanetary disks. While the effects of the Zeeman effect are commonly studied in the circular polarization of spectral lines, we perform a comprehensive modeling also of the Zeeman-induced broadening of spectral lines and their linear polarization. We develop simplified radiative transfer models adapted to protoplanetary disks, which we compare to full three-dimensional polarized radiative transfer simulations. We find that the radiative transfer of circular polarization is heavily affected by the expected polarity-change of the magnetic field between opposite sides of the disk. In contrast, Zeeman broadening and linear polarization are relatively unaffected by this sign change due to their quadratic dependence on the magnetic field. We can match our simplified radiative transfer models to full polarization modeling with high fidelity, which in turn allows us to prescribe straight-forward methods to extract magnetic field information from Zeeman broadening and linear polarization observations. We find that Zeeman broadening and linear polarization observations are highly advantageous methods to characterize protoplanetary disk magnetic fields as they are both sensitive probes of the magnetic field and are marginally affected by any sign change of the disk magnetic field. Applying our results to existing circular polarization observations of protoplanetary disk spectral lines suggests that the current upper limits on the toroidal magnetic field strengths have to be raised.