Evidence for Dynamically Important Magnetic Fields in Molecular Clouds

TL;DR

This paper reviews observational evidence indicating magnetic fields play a crucial role in molecular cloud dynamics, affecting turbulence and gravitational contraction, with new data from NGC 2024 supporting these claims.

Contribution

It provides new observational data showing magnetic fields influence turbulence anisotropy and gravitational contraction in molecular clouds, challenging previous assumptions about mass-to-flux ratios.

Findings

Magnetic fields cause turbulence anisotropy in molecular clouds.

Magnetic fields channel gravitational contraction even in super-critical regions.

Mass-to-flux ratio increases over time during anisotropic contraction.

Abstract

Recent observational evidence that magnetic fields are dynamically important in molecular clouds, compared to self-gravity and turbulence, is reviewed and illustrated with data from the NGC 2024 region. One piece of evidence, turbulence anisotropy, was found in the diffuse envelope of a cloud (Av~1; Heyer et al. 2008); our data further suggests turbulence anisotropy in the cloud (Av >7) and even near the cloud core (Av~100). The data also shows that magnetic fields can channel gravitational contraction even for a region with super-critical N(H2)/2Blos ratio (the ratio between the observed column density and two times the line-of-sight observed field strength), a parameter which has been widely used by observers to estimate core mass-to-flux ratios. Although the mass-to-flux ratio is constant under the flux-freezing condition, we show that N(H2)/2Blos grows with time if gravitational contraction is anisotropic due to magnetic fields.