The Density Variance Mach Number Relation in the Taurus Molecular Cloud

TL;DR

This study estimates the density variance Mach number relation parameter in the Taurus molecular cloud using observational data, confirming theoretical predictions and revealing spectral features related to turbulence and anisotropy.

Contribution

It provides the first observational estimate of the parameter b in the density variance Mach number relation using column density data from Taurus.

Findings

Estimated b ~ 0.48 consistent with numerical models

Identified a spectral break at ~1pc scale in Taurus

Observed spectral anisotropy and similarity between 13CO and dust extinction spectra

Abstract

Supersonic turbulence in molecular clouds is a key agent in generating density enhancements that may subsequently go on to form stars. The stronger the turbulence - the higher the Mach number - the more extreme the density fluctuations are expected to be. Numerical models predict an increase in density variance with rms Mach number of the form: sigma^{2}_{rho/rho_{0}} = b^{2}M^{2}, where b is a numerically-estimated parameter, and this prediction forms the basis of a large number of analytic models of star formation. We provide an estimate of the parameter b from 13CO J=1-0 spectral line imaging observations and extinction mapping of the Taurus molecular cloud, using a recently developed technique that needs information contained solely in the projected column density field to calculate sigma^{2}_{rho/rho_{0}}. We find b ~ 0.48, which is consistent with typical numerical estimates, and is characteristic of turbulent driving that includes a mixture of solenoidal and compressive modes. More conservatively, we constrain b to lie in the range 0.3-0.8, depending on the influence of sub-resolution structure and the role of diffuse atomic material in the column density budget. We also report a break in the Taurus column density power spectrum at a scale of ~1pc, and find that the break is associated with anisotropy in the power spectrum. The break is observed in both 13CO and dust extinction power spectra, which, remarkably, are effectively identical despite detailed spatial differences between the 13CO and dust extinction maps. [ abridged ]