Trans-Alfvenic Motions in the Taurus Molecular Cloud

TL;DR

This study investigates magnetic field influence on gas motions in the Taurus Molecular Cloud using spectroscopic imaging, revealing varying magnetic regimes and turbulence characteristics across different density regions.

Contribution

It provides observational evidence of trans-Alfvenic flows and constrains the magnetic field density relation, advancing understanding of magnetic turbulence in molecular clouds.

Findings

Magnetic field aligned velocity anisotropy is limited to low-density regions.

Diffuse envelope shows strong magnetic fields and sub-Alfvenic turbulence.

High-density filaments exhibit super-Alfvenic motions.

Abstract

Magnetically aligned velocity anisotropy over varying physical conditions and environments within the Taurus Molecular Cloud is evaluated from analysis of wide field spectroscopic imaging of 12CO and 13CO J=1-0 emission. Such anisotropy is a result of MHD turbulence in the strong magnetic field regime and provides an indirect measure of the role of magnetic fields upon the gas. Velocity anisotropy aligned with the local, projected mean magnetic field direction is limited to fields with low surface brightness 12CO emission corresponding to regions of low visual extinction and presumably, low gas volume density. The more optically thin 13CO J=1-0 emission shows little evidence for velocity anisotropy. We compare our results with computational simulations with varying degrees of magnetic field strength and Alfvenic Mach numbers. In the diffuse, molecular envelope of the cloud, a strong magnetic field and sub-Alfvenic turbulent motions are inferred. Super-Alfvenic motions are present within the high column density filaments of the Taurus cloud. From this trans-Alfvenic flow, we constrain the scaling exponent, kappa, of the magnetic field density relation (B ~ n^kappa) to be near zero as expected for ambipolar diffusion or material loading of magnetic flux tubes.