OH ZEEMAN OBSERVATIONS OF DARK CLOUDS

We have made measurements with the Green Bank 43 m telescope of the Zeeman effect in the 1665 and 1667 MHz lines of OH toward dark clouds. The typical 1 sigma sensitivity was 3 muG. The only certain detection of a magnetic field was toward B1, for which we measured a line-of-sight component Absolute value of B cos theta = - 19.1 +/- 3.9 muG. Comparison with our earlier measurement of the field toward B1 with the Arecibo telescope provided evidence for a 40% enhancement in field strength between the molecular envelope and core of the B1 cloud, which is consistent with quasi-static contraction of the cloud driven by ambipolar diffusion. Because the Zeeman effect is only sensitive to the line-of-sight component of the magnetic field, a statistical analysis of the detection and upper limits was necessary. This analysis indicated that the total (not line-of-sight) field strength was typically Absolute value of B almost-equal-to 16 muG toward the central regions of dark clouds sampled by the Green Bank beam (for which A(v) almost-equal-to 5 mag and n(H) is similar to 10(3) cm-3), which implied that the central regions were approximately magnetically critical. The data were found to be consistent with the hypotheses that (1) dark clouds are in approximate virial equilibrium between magnetic and gravitational energy and (2) the supersonic line widths observed in dark clouds are the result of MHD motions such as Alfven waves. The data were also consistent with detailed physical models of initially magnetically subcritical clouds evolving on the ambipolar diffusion time scale.