EPoS Contribution
EPoS Contribution
Statistically significant observations of magnetic field strengths in molecular clouds

Thomas Troland
University of Kentucky, Lexington, USA
Magnetic fields certainly influence and may dominate the earliest phases of star formation. The radio frequency Zeeman effect provides the most direct measurements of magnetic field strengths in molecular clouds. I will report upon two extensive surveys of the Zeeman effect that sample magnetic field strengths at two distinctly different densities within molecular clouds. A large scale (500 hour) survey of the Zeeman effect in 18cm OH emission lines from dark cloud cores, conducted at Arecibo Observatory, has yielded data for 34 sources. This survey samples line-of-sight field strengths at densities of order 10^3 to 10^4 cm-3. The survey has yielded nine new probable detections, and data on all positions have been included in a careful statistical analysis. The results of this analysis are that the average mass-to-flux ratio in these cores is supercritical by a ratio of about 2, and the ratio of turbulent to magnetic energies is also about 2. A separate Zeeman effect survey of the N=1-0 CN emission lines, undertaken with the IRAM 30m telescope, has yielded five new detections, representative of magnetic fields in the density range 10^5 to 10^6 cm-3. These data suggest that the mass-to-flux ratio is also supercritical, in this case by a factor of 1-4, and the ratio of turbulent to magnetic energies is about unity. These two surveys provide the first statistically significant and relatively uniform data sets on magnetic field strengths in molecular clouds. They suggest that magnetic fields play an important role in cloud evolution but probably not a dominant role.