Centroid Velocity Increments as a Probe of the Turbulent Velocity Field in Interstellar Molecular Clouds

Summary

We present a comparison of histograms (or PDFs) of CO (2−1) line centroid velocity increments in the ρ Ophiuchi and ζ Ophiuchi molecular clouds with those computed for spectra synthesized from a three-dimensional, compressible, but non-star forming and non-gravitating hydrodynamic simulation. Histograms of centroid velocity increments in the two molecular clouds show non-Gaussian wings, similar to those found in histograms of velocity increments and derivatives in experimental studies of laboratory and atmospheric flows, as well as numerical simulations of turbulence. The magnitude of these wings increases monotonically with decreasing separation down to the angular resolution of the data. This behavior is consistent with that found in the phase of the simulation which has most of the properties of incompressible turbulence. This is consistent with the proposition that ISM velocity structure is vorticity dominated like that of the turbulent simulation. The ρ Ophiuchi molecular cloud contains some active star formation, as indicated by the presence of infrared sources and molecular outflows. As a result shocks may have important effects on the velocity field structure and molecular line shapes in this region. However, the ζ Ophiuchi cloud represents a quiescent region without ongoing star formation and should be a good laboratory for studies of interstellar turbulence.

Introduction

Early spectroscopic observations of interstellar lines of HI, OH, and CO have revealed that observed line widths (or velocity dispersions) in interstellar clouds are larger than thermal line widths expected for these low-temperature regions (see e.g. Myers 1997 and references therein).