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

Abstract

Probability density functions (pdfs) of $^{13}CO$ 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. Although some are approximately Gaussian in form, most of the pdfs exhibit relatively broader, often nearly exponential, tails, similar to the pdfs of velocity {\em differences} and {\em derivatives} (but not the velocity field itself) found in experiments and numerical simulations of incompressible turbulence. The broad pdf tails found in the present work are also similar to those found in decades-old measurements of interstellar velocity pdfs using atomic line centroids, and to the excess wing emission recently found in individual molecular line profiles. Some possible interpretations of the observed deviations are briefly discussed, although none of these account for the nearly exponential tails.