Magnetic fields in molecular clouds: Limitations of the analysis of Zeeman observations

Abstract

Context. Observations of Zeeman split spectral lines represent an important approach to derive the structure and strength of magnetic fields in molecular clouds. In contrast to the uncertainty of the spectral line observation itself, the uncertainty of the analysis method to derive the magnetic field strength from these observations is not been well characterized so far. Aims. We investigate the impact of several physical quantities on the uncertainty of the analysis method, which is used to derive the line-of-sight (LOS) magnetic field strength from Zeeman split spectral lines. These quantities are the density, temperature, velocity, and the magnetic field strength. Methods. We simulate the Zeeman splitting of the 1665 MHz OH line with the 3D radiative transfer (RT) extension ZRAD. This extension is based on the line RT code Mol3D (Ober et al. 2015) and has been developed for the POLArized RadIation Simulator POLARIS . Results. Observations of the OH Zeeman effect in typical molecular clouds are not significantly affected by the uncertainty of the analysis method. However, some observations obtained a magnetic field strength of more than ∼300 µG, which may result in an uncertainty of the analysis method of >10%. We derived an approximation to quantify the range of parameters in which the analysis method works sufficiently accurate and provide factors to convert our results to other spectral lines and species as well. We applied these conversion factors to CN and found that observations of the CN Zeeman effect in typical molecular clouds are neither significantly affected by the uncertainty of the analysis method. In addition, we found that the density has almost no impact on the uncertainty of the analysis method, unless it reaches values higher than those typically found in molecular clouds (n H 10 7 cm −3 ). Furthermore, the uncertainty of the analysis method increases, if both the gas velocity and the magnetic field show significant variations along the line-of-sight. However, this increase should be small in Zeeman observations of most molecular clouds considering typical velocities of ∼1 km/s.

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

“…We performed RT simulations in order to identify the characteristic observables that may help to distinguish between different field morphologies. Here, we used sophisticated state-ofthe-art RT simulations within the framework of POLARIS (Reissl et al 2016;Brauer et al 2017) in order to the derive synthetic dust polarization pattern and Zeeman LOS magnetic field measurements for the magnetic field configurations presented in Fig. 1.…”

“…For a v turb 1000 km/s all the lines would show the decrease in B || close to the center that is characteristic of a toroidal magnetic field morphology (see e.g. Brauer et al 2017). Therefore, the strong increase of a toroidal field close to the center would be better traced and almost no drop would be seen.…”

“…If the magnetic field strength becomes too high in comparison to the line width, the analysis method introduced in Sect. 3.3 no longer applies (this case is extensively modeled and discussed in Brauer et al 2017). The strongest effect is for OH and HI the most considering the low gas temperature in the center of the filament (see Fig.…”

“…As for our adopted magnetic field configurations we implement numerous suggestions from observations and theoretical works, focusing on five distinct magnetic field morphologies associated with the evolution of filaments and star formation. We then pre-dict their idealized observable signatures by making use of the radiative transfer (RT) code POLARIS (Reissl et al 2016;Brauer et al 2017) 1 . The POLARIS code is the first of its kind capable of simulating dust polarization on the basis of state of the art grain alignment physics in combination with line RT, including the Zeeman effect.…”

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

“…is the wavelength independent rotation measure, e is the electron charge, n el is the density of free electrons, and B || = B cos ϑ is the magnitude of the magnetic field component along the LOS (Burn 1966;Rybicki & Lightman 1979;Huang & Shcherbakov 2011). For the RM calculations we utilize the distribution of n el provided by the MHD simulation.…”