Exponential Tails in the Centroid Velocity Distributions of Star-forming Regions

Probability density functions (pdfs) of ¹³CO emission line centroid (line-of-sight, intensity-weighted average) velocities are presented for several densely sampled molecular clouds as quantitative descriptors of their underlying dynamics. Some are found to be approximately Gaussian in form, but most of the pdfs exhibit relatively broader, often nearly exponential, tails, similar to the behavior found for pdfs of velocity differences and derivatives (but not the velocity field itself) in experiments and numerical simulations of incompressible turbulence. Other nonturbulent processes can also give rise to non-Gaussian pdfs, although we know of no interpretations (turbulent or otherwise) that can unambiguously account for the nearly exponential tails. The centroid pdfs presented here are less sensitive to systematic motions, optical depth effects, and secondary thermal components than are individual molecular line profiles, which have also been used as pdf estimators and which have also been recently found to exhibit excess wing emission similar to the broad pdf tails found here.