Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations

TL;DR

This study used Arecibo telescope observations of OH Zeeman effect to measure magnetic fields in dark cloud cores, providing statistically significant evidence of magnetic field importance and informing star formation models.

Contribution

First statistically robust measurements of magnetic field strength in dark cloud cores, supporting the energetic significance of magnetic fields in star formation.

Findings

Mass-to-flux ratio is supercritical by ~2.

Turbulent to magnetic energy ratio is ~2.

Magnetic fields are energetically significant in dark cloud cores.

Abstract

We have carried out an extensive survey of magnetic field strengths toward dark cloud cores in order to test models of star formation: ambipolar-diffusion driven or turbulence driven. The survey involved $\sim500$ hours 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 $\sim 2$, and that the ratio of turbulent to magnetic energies is also $\sim 2$. 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 $10^{3-4}$ cm$^{-3}$, and they lay the foundation for further observations that could provide a more definitive test.