Observational study of the role of magnetic fields in star formation

It has become clear in recent years that magnetic fields play an important and perhaps crucial role in the process of star formation. For this reason, measurements of magnetic field strengths and morphology are an important component in ongoing attempts to understand star formation. The goal of this thesis is to significantly extend the available data on the strength and morphology of magnetic fields in star-forming regions using the Zeeman effect. The Very Large Array telescope observations described here include: (1) A Zeeman study of H I and OH gas toward the star-forming region M17; (2) A study of the W49 complex using the H I Zeeman effect; (3) A Zeeman study of OH (1720 MHz) masers toward five galactic SNRs. Some of the results of these Zeeman observations include: (1) An H I component with Blos up to --700 muG has been detected toward the M17 H II region/M17 SW interface. The velocity and line width of this component closely match those of molecular emission lines from the M17 SW cloud. OH Zeeman observations toward M17 reveal a similar Blos in this region as well as three additional Blos detection regions along the NW edge of the interface. We also find that the M17 SW molecular cloud core is magnetically supercritical but close to dynamic equilibrium. (2) Significant Blos of 60 to 300 muG were detected toward W49A in H I components at 4 and ∼7 km s--1. The W49A Blos show a significant increase in field strength with higher resolution especially for the ∼4 km s--1 H I component. Based on comparisons with molecular data toward W49A, the ∼4 km s--1 H I component appears to be directly associated with the W49A H II region ring, while the ∼7 km s--1 H I component originates from a halo surrounding W49A. From these results, we estimate that W49A North is magnetically supercritical, and unstable to overall gravitational collapse. (3) We detected significant magnetic fields between 0.2 to 2 mG in ten supernova remnant -- OH (1720 MHz) masers. From comparison of these field values with theoretical maser studies we conclude that the thermal Zeeman equation overestimates &vbm0;B&ar;&vbm0; by less than a factor of five. We suggest that the ambient magnetic fields were compressed via the SNR shock to the observed values and that the magnetic pressure (10--7--10--9 erg cm--3) far exceeds the thermal pressure of the hot gas interior to the remnant. These studies have added a number of new insights into the role that magnetic fields play in the process of star formation.