Understanding star formation in molecular clouds III. Probability distribution functions of molecular lines in Cygnus X

摘要: Column density (N) PDFs serve as a powerful tool to characterize the physical processes that influence structure of molecular clouds. Star-forming clouds can best be characterized by lognormal for lower N range and power-law tail higher N, commonly attributed turbulence self-gravity and/or pressure, respectively. We report here on obtained from observations 12CO, 13CO, C18O, CS, N2H+ in Cygnus X North region compare PDF derived dust with Herschel satellite. The 12CO is Av~1-30, but cut Av due optical depth effects. C18O 13CO are mostly up Av~1-15, followed excess Av~40. Above value, all CO drop, most likely depletion. high tracers CS exhibit only power law distribution between Av~15 400, Av~2-15 has Av~500. Absolute values line column densities are, however, rather uncertain abundance excitation temperature variations. Taken face we 'calibrate' one determined an [CS]/[H2] 10^-9. slopes tails N2H+, -1.6, -1.4, -1.9, respectively, thus consistent free-fall collapse filaments clumps. A quasi static configuration clumps possibly also account observed N-pdfs, long they have sufficiently condensed external ram pressure gas accretion provided. somehow flatter reflect change subthermal at low densities.