Tracing cosmic magnetic fields using molecules

Doctoral thesis, 2021

Understanding the magnetic field strength and morphology of astrophysical regions is of great importance to understand their dynamics. There exist a number of methods astronomers can employ to trace magnetic field structures, and each have their own limitations. This thesis focuses on tracing magnetic field using molecules.

A promising technique to trace the magnetic field morphology around evolved stars, or on the smallest scales of star forming regions, is (sub-)millimeter spectral line polarization observations. Line (linear) polarization can either arise in association with maser radiative transfer, or alternatively, molecular lines polarize through the Goldreich-Kylafis effect. In both cases, the polarization angle traces the magnetic field with a 90-degree ambiguity. In order to remove this ambiguity, and to estimate the observational viability of particular line polarization measurements, polarized line radiative transfer needs to be employed. This thesis contributes to this field in that it presents a three-dimensional polarized line radiative transfer tool: PORTAL. PORTAL simulates the emergence of thermal molecular line polarization in astrophysical objects of arbitrary geometry and magnetic field morphology. Also, this thesis introduces a novel polarization mechanism: collisional polarization. Which provides the possibility of directly detecting ambipolar diffusion in disks through the polarization of molecular ions.

Some molecules occur as masers. Masers occur naturally in specific astrophysical regions, which are often associated with highly dynamical events. Their emission is characterized by narrow lines and high brightness temperatures, and is often associated with polarization. The polarization of masers contains information on the magnetic field strength and direction of the regions they occur in. Many maser polarization observations have been performed over the last 30 years. However, one requires versatile maser polarization models that can aide in the interpretation of these observations. This thesis contributes to the study of maser polarization by presenting a modeling program called CHAMP (CHAracterizing Maser Polarization) that simulates the polarization of masers of arbitrarily high maser saturation and high angular momentum.

Methanol masers occur exclusively in association with high-mass star forming regions. They trace specific regions there, and may teach us about the magnetic field structures in the densest regions. There have been many polarization observations of methanol, but proper interpretation of them has not been possible because the molecular properties associated with its magnetic field interactions have been unknown. This thesis presents the first quantum chemical models of methanols magnetic field interactions. With them, we re-interpret the many previous methanol maser polarization observations and conclude that magnetic fields are dynamically important to the process of high-mass star formation.