Turbulent Velocity Structure in Molecular Clouds

We compare velocity structure observed in the Polaris Flare molecular cloud at scales ranging from 0.015 pc to 20 pc to the velocity structure of a suite of simulations of supersonic hydrodynamic and MHD turbulence computed with the ZEUS MHD code. We examine different methods of characterising the structure, including a scanning-beam size-linewidth relation, structure functions, velocity and velocity difference probability distribution functions (PDFs), and the ∆-variance wavelet transform, and use them to compare models and observations. The ∆-variance is most sensitive in detecting characteristic scales and varying scaling laws, but is limited in the observational application by its lack of intensity weighting. We compare the true velocity PDF in our models to simulated observations of velocity centroids and average line profiles in optically thin lines, and find that the line profiles reflect the true PDF better. The observed velocity structure is consistent with supersonic turbulence showing a complete spectrum from a driving scale larger than 10 pc, through an inertial range, to a dissipation scale under 0.05 pc. Ambipolar diffusion could explain this dissipation scale. Strong enough magnetic fields impose a clear anisotropy on the velocity field, reducing the velocity variance in directions perpendicular to the field.