EXPONENTIAL TAILS IN THE CENTROID VELOCITY DISTRIBUTIONS OF STAR-FORMING REGIONS

Probability density functions (pdfs) of (CO)-C-13 emission line centro id (line-of-sight, intensity-weighted average) velocities are presente d for several densely sampled molecular clouds as quantitative descrip tors of their underlying dynamics. Some are found to be approximately Gaussian in form, but most of the pdfs exhibit relatively broader, oft en nearly exponential, tails, similar to the behavior found for pdfs o f velocity differences and derivatives (but not the velocity field its elf) in experiments and numerical simulations of incompressible turbul ence. 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 c entroid pdfs presented here are less sensitive to systematic motions, optical depth effects, and secondary thermal components than are indiv idual molecular line profiles, which have also been used as pdf estima tors and which have also been recently found to exhibit excess wing em ission similar to the broad pdf tails found here.