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

TL;DR

This study analyzes molecular line PDFs in Cygnus X, revealing how different tracers reflect turbulence, gravity, and depletion effects, and compares them with dust PDFs to understand star formation processes.

Contribution

It provides the first detailed comparison of molecular line PDFs with dust PDFs in Cygnus X, calibrates molecular abundances, and interprets the PDFs in terms of cloud dynamics and collapse.

Findings

12CO PDF is lognormal up to Av~30, then cut off due to optical depth.

C18O and 13CO PDFs are mostly lognormal, with excess and depletion at high Av.

CS and N2H+ PDFs follow power-law distributions, consistent with free-fall collapse.

Abstract

Column density (N) PDFs serve as a powerful tool to characterize the physical processes that influence the structure of molecular clouds. Star-forming clouds can best be characterized by lognormal PDFs for the lower N range and a power-law tail for higher N, commonly attributed to turbulence and self-gravity and/or pressure, respectively. We report here on PDFs obtained from observations of 12CO, 13CO, C18O, CS, and N2H+ in the Cygnus X North region and compare to a PDF derived from dust observations with the Herschel satellite. The PDF of 12CO is lognormal for Av~1-30, but is cut for higher Av due to optical depth effects. The PDFs of C18O and 13CO are mostly lognormal up for Av~1-15, followed by excess up to Av~40. Above that value, all CO PDFs drop, most likely due to depletion. The high density tracers CS and N2H+ exhibit only a power law distribution between Av~15 and 400, respectively. The PDF from dust is lognormal for Av~3-15 and has a power-law tail up to Av~500. Absolute values for the molecular line column densities are, however, rather uncertain due to abundance and excitation temperature variations. Taken the dust PDF face value, we 'calibrate' the molecular line PDF of CS to the one of the dust and determined an abundance [CS]/[H2] of 10^-9. The slopes of the power-law tails of the CS, N2H+, and dust PDFs are consistent with free-fall collapse of filaments and clumps. A quasi static configuration of filaments and clumps can possibly also account for the observed N-PDFs, as long as they have a sufficiently condensed density structure and external ram pressure by gas accretion is provided. The somehow flatter slopes of N2H+ and CS can reflect an abundance change and/or subthermal excitation at low column densities.