Volume 587, March 2016
|Number of page(s)||13|
|Section||Interstellar and circumstellar matter|
|Published online||19 February 2016|
Understanding star formation in molecular clouds
III. Probability distribution functions of molecular lines in Cygnus X
Université Bordeaux, LAB, CNRS, UMR 5804,
2 I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
3 IRFU/SAp CEA/DSM, Laboratoire AIM CNRS – Université Paris Diderot, 91191 Gif-sur-Yvette, France
4 Universität Heidelberg, Zentrum für Astronomie, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
5 Maison de la simulation, CEA Saclay, 91191 Gif-sur-Yvette, France
6 Max-Planck Institut für Radioastronomie, Auf dem Hügel, 53121 Bonn, Germany
7 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
8 Joint ALMA observatory, 15782 Santiago, Chile
9 School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
10 Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
Received: 8 August 2015
Accepted: 22 November 2015
The probability distribution function of column density (N-PDF) serves as a powerful tool to characterise the various physical processes that influence the structure of molecular clouds. Studies that use extinction maps or H2 column-density maps (N) that are derived from dust show that star-forming clouds can best be characterised by lognormal PDFs for the lower N range and a power-law tail for higher N, which is commonly attributed to turbulence and self-gravity and/or pressure, respectively. While PDFs from dust cover a large dynamic range (typically N ~ 1020−24 cm-2 or Av~ 0.1−1000), PDFs obtained from molecular lines – converted into H2 column density – potentially trace more selectively different regimes of (column) densities and temperatures. They also enable us to distinguish different clouds along the line of sight through using the velocity information. We report here on PDFs that were obtained from observations of 12CO, 13CO, C18O, CS, and N2H+ in the Cygnus X North region, and make a comparison to a PDF that was derived from dust observations with the Herschel satellite. The PDF of 12CO is lognormal for Av ~ 1–30, but is cut for higher Av because of optical depth effects. The PDFs of C18O and 13CO are mostly lognormal up to Av ~ 1–15, followed by excess up to Av ~ 40. Above that value, all CO PDFs drop, which is 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 because of abundance and excitation temperature variations. If we take the dust PDF at face value, we “calibrate” the molecular line PDF of CS to that of the dust and determine an abundance [CS]/[H2] of 10-9. The slopes of the power-law tails of the CS, N2H+, and dust PDFs are −1.6, −1.4, and −2.3, respectively, and are thus consistent with free-fall collapse of filaments and clumps. A quasi static configuration of filaments and clumps can also possibly account for the observed N-PDFs, providing 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.
Key words: ISM: abundances / ISM: clouds / dust, extinction / ISM: molecules / ISM: structure
© ESO, 2016
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