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 Zeeman effect in OH undertaken with the Australia Telescope National Facility Parkes 64 m radio telescope and the National Radio Astronomy Observatory Green Bank 43 m radio telescope. The Zeeman effect was clearly detected in the cloud associated with the H II region RCW 38, with a field strength of 38 +/- 3 muG, and possibly detected in a cloud associated with the H II region RCW 57, with a field strength of -203 +/- 24 muG. The remaining 21 measurements give formal upper limits to the magnetic field strength, with typical 1 sigma sensitivities less than 20 muG. For 22 of the molecular clouds we are also able to determine the column density of the gas in which we have made a sensitive search for the Zeeman effect. We combine these results with previous Zeeman studies of 29 molecular clouds, most of which were compiled by Crutcher, for a comparison of theoretical models with the data. This comparison implies that if the clouds can be modeled as initially spherical with uniform magnetic fields and densities that evolve to their final equilibrium state assuming flux freezing, then the typical cloud is magnetically supercritical, as was found by Crutcher. If the clouds can be modeled as highly flattened sheets threaded by uniform perpendicular fields, then the typical cloud is approximately magnetically critical, in agreement with Shu et al., but only if the true values of the field for the nondetections are close to the 3 sigma upper limits. If instead these values are significantly lower (for example, similar to the 1 sigma limits), then the typical cloud is generally magnetically supercritical. When all observations of the Zeeman effect are considered, the single-dish detection rate of the OH Zeeman effect is relatively low. This result may be due to low mean field strengths, but a more realistic explanation may be significant field structure within the beam. As an example, for clouds associated with H II regions, the molecular gas and magnetic field may be swept up into a thin shell, which results in a nonuniform field geometry and measurements of the beam-averaged field strength, which are significantly lower than the true values. This effect makes it more difficult to distinguish magnetically subcritical and supercritical clouds.