The Role of Magnetic Fields in Galactic GMCs Thomas Troland University of Kentucky, Lexington, USA
Giant Molecular Clouds (GMCs) represent the very earliest phase of star formation. Therefore, it is important to understand the processes by which they form and evolve. In particular, what is the role of magnetic fields? Theoretical arguments suggest that GMCs should be magnetically supercritical (mass-to-flux ratio of order twice critical). However, these arguments only apply to bound clouds, not clouds resulting from colliding flows. Therefore, it is important to measure field strengths in GMCs. Although extensive recent Zeeman Effect data have shown that GMC cores are supercritcal, and diffuse HI gas is subcritical, there is little information about magnetic fields in GMCs as a whole. We have carried out an extensive Arecibo Zeeman Effect survey in 18 cm OH absorption lines toward extragalactic sources. These sources lie behind galactic GMCs, and their lines-of-sight sample general regions of the GMCs, not cores specifically. We have sampled 16 lines of sight and obtained magnetic field detections or sensitive upper limits for 38 separate velocity components. These results strongly suggest that the maximum total magnetic field strength in GMCs (outside of cores) is 10 to 20 microgauss, independent of N(H). At lower N(H), equivalent to Av about 1 mag, the GMCs are nearly magnetically critical. At Av of 10-20 mag, the GMCs are supercritical by factors of 5 or more. A plausible inference is that the process by which lower density diffuse gas forms a self-gravitating GMC involves a progressive rise in the mass-to-flux ratio, from subcritical diffuse gas to increasingly supercritical molecular gas. Also, our preliminary analysis of magnetic field directions in GMCs suggests a correlation with directions derived from pulsar rotation measures. This correlation implies that magnetic field direction is preserved as diffuse gas is converted into GMCs. Therefore, magnetic fields are likely to play an important role in GMC formation, especially in the very earliest phases.
Collaborators: K. Thompson, U Kentucky, USA C. Heiles, UC Berkeley, USA