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T.L. Bourke, P.C. Myers (Harvard-Smithsonian Center for Astrophysics), G. Robinson (School of Physics, University College, Canberra, Australia), A.R. Hyland (Southern Cross University, Australia)
We present the results of a new survey of 23 molecular clouds for the Zeeman effect in OH undertaken with the ATNF Parkes 64-m radio telescope and the NRAO Green Bank 43-m radio telescope. The Zeeman effect was clearly detected in a cloud associated with the southern H II region RCW 38, with a field strength of 38 ±2 \muG, and possibly detected in a cloud associated with the southern H II region RCW 57, with a field strength of -203 ±25 \muG. For 22 of the molecular clouds we are able to determine also the column density of the gas in which we have measured the Zeeman effect. We combine the results for these 22 Zeeman measurements with 29 previous measurements, most of which were compiled by Crutcher (1999), for a comparison of theoretical models with the data. This comparison shows that a model of a parent spherical molecular cloud with uniform density that evolves to its final equilibrium state assuming flux-freezing implies that most of the clouds observed are supercritical and therefore unstable to gravitational collapse. A model cloud that is highly flattened (sheet-like) along the field lines, with flux-freezing, results in values of the mass-to-magnetic flux ratio which are very close to unity for the observed clouds, provided that the large number of 3\sigma upper limits to the field strength are close to their true values. If the true values of the field strength are closer to the 1\sigma limits then most of the clouds, assuming the sheet-like geometry, are also marginally supercritical.
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