Self-Consistent Analysis of OH Zeeman Observations

TL;DR

This paper reanalyzes OH Zeeman observations of molecular clouds, confirming the original findings that core regions have smaller mass-to-flux ratios than envelopes, supporting ambipolar diffusion models under certain assumptions.

Contribution

It demonstrates that the original data are consistent with a nearly uniform magnetic field assumption, validating previous conclusions and critiquing alternative models.

Findings

Data are consistent with nearly uniform magnetic fields in clouds.

Original conclusions about core-envelope flux ratios remain valid.

Alternative magnetic field models are inconsistent with the data.

Abstract

Crutcher, Hakobian, and Troland (2009) used OH Zeeman observations of four nearby molecular dark clouds to show that the ratio of mass to magnetic flux was smaller in the ~0.1 pc cores than in the ~1 pc envelopes, in contradiction to the prediction of ambipolar diffusion driven core formation. A crucial assumption was that the magnetic field direction is nearly the same in the envelope and core regions of each cloud. Mouschovias and Tassis (2009) have argued that the data are not consistent with this assumption, and presented a new analysis that changes the conclusions of the study. Here we show that the data are in fact consistent with the nearly uniform field direction assumption; hence, the original study is internally self-consistent and the conclusions are valid under the assumptions that were made. We also show that the Mouschovias and Tassis model of magnetic fields in cloud envelopes is inconsistent with their own analysis of the data. However, the data do not rule out a more complex field configuration that future observations may discern.