New OH Zeeman Measurements of Magnetic Field Strengths in Molecular Clouds

We present the results of a new survey of 23 molecular clouds for the Zeemaneffect in OH undertaken with the ATNF Parkes 64-m radio telescope and the NRAOGreen Bank 43-m radio telescope. The Zeeman effect was clearly detected in thecloud associated with the HII region RCW 38, with a field strength of 38+/-3micro-Gauss, and possibly detected in a cloud associated with the HII regionRCW 57, with a field strength of -203+/-24 micro-Gauss. The remaining 21measurements give formal upper limits to the magnetic field strength, withtypical 1-sigma sensitivities <20 micro-Gauss. For 22 of the molecular cloudswe are also able to determine thecolumn density of the gas in which we havemade a sensitive search for the Zeeman effect. We combine these results withprevious Zeeman studies of 29 molecular clouds, most of which were compiled byCrutcher (1999), for a comparsion of theoretical models with the data. Thiscomparison implies that if the clouds can be modeled as initially sphericalwith uniform magnetic fields and densities that evolve to their finalequilibrium state assuming flux-freezing then the typical cloud is magneticallysupercritical, as was found by Crutcher (1999). If the clouds can be modeled ashighly flattened sheets threaded by uniform perpendicular fields, then thetypical cloud is approximately magnetically critical, in agreement with Shu etal. (1999), but only if the true values of the field for the non-detections areclose to the 3-sigma upper limits. If instead these values are significantlylower (for example, similar to the 1-sigma limits), then the typical cloud isgenerally magnetically supercritical.Comment: 39 pages, 7 figures. Accepted for publication in Ap