Magnetic fields in dark cloud cores: Arecibo OH Zeeman observations

We have carried out an extensive survey of magnetic field strengths toward dark cloud cores in order to test ambipolar-diffusion-driven and turbulence-driven models of star formation. The survey involved similar to 500 hr of observing with the Arecibo telescope in order to make sensitive OH Zeeman observations toward 34 dark cloud cores. Nine new probable detections were achieved at the 2.5 sigma level; the certainty of the detections varies from solid to marginal, so we discuss each probable detection separately. However, our analysis includes all the measurements and does not depend on whether each position has a detection or just a sensitive measurement. Rather, the analysis establishes mean (or median) values over the set of observed cores for relevant astrophysical quantities. The results are that the mass-to-flux ratio is supercritical by similar to 2, and that the ratio of turbulent to magnetic energies is also similar to 2. These results are compatible with both models of star formation. However, these OH Zeeman observations do establish for the first time on a statistically sound basis the energetic importance of magnetic fields in dark cloud cores at densities of order 10(3)-10(4) cm(-3), and they lay the foundation for further observations that could provide a more definitive test.