Volume 632, December 2019
|Number of page(s)||30|
|Section||Interstellar and circumstellar matter|
|Published online||10 December 2019|
Physical conditions in the gas phases of the giant H II region LMC-N 11
II. Origin of [C II] and fraction of CO-dark gas★
AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité,
2 Department of Physics and Astronomy, University of North Carolina, 290 Phillips Hall CB 3255, Chapel Hill, NC 27599, USA
3 Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
4 Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
5 Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, 34055 Daejeon, Republic of Korea
6 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
7 Laboratoire d’Astrophysique de Bordeaux, University of Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
8 LERMA, Observatoire de Paris, PSL Research Univiversity, CNRS, Sorbonne Université, 75014 Paris, France
9 Department of Astronomy, University of Maryland, College Park, ND 20742-2421, USA
10 Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
11 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
12 IRAP, Université de Toulouse, CNRS, UPS, CNES, 31400 Toulouse, France
13 University of Cincinnati, Clermont College, 4200 Clermont College Drive, Batavia, OH 45103, USA
Accepted: 17 October 2019
Context. The ambiguous origin of the [C II] 158μm line in the interstellar medium complicates its use for diagnostics concerning the star-formation rate and physical conditions in photodissociation regions.
Aims. We investigate the origin of [C II] in order to measure the total molecular gas content, the fraction of CO-dark H2 gas, and how these parameters are impacted by environmental effects such as stellar feedback.
Methods. We observed the giant H II region N 11 in the Large Magellanic Cloud with SOFIA/GREAT. The [C II] line is resolved in velocity and compared to H I and CO, using a Bayesian approach to decompose the line profiles. A simple model accounting for collisions in the neutral atomic and molecular gas was used in order to derive the H2 column density traced by C+.
Results. The profile of [C II] most closely resembles that of CO, but the integrated [C II] line width lies between that of CO and that of H I. Using various methods, we find that [C II] mostly originates from the neutral gas. We show that [C II] mostly traces the CO-dark H2 gas but there is evidence of a weak contribution from neutral atomic gas preferentially in the faintest components (as opposed to components with low [C II]/CO or low CO column density). Most of the molecular gas is CO-dark. The CO-dark H2 gas, whose density is typically a few 100s cm−3 and thermal pressure in the range 103.5−5 K cm−3, is not always in pressure equilibrium with the neutral atomic gas. The fraction of CO-dark H2 gas decreases with increasing CO column density, with a slope that seems to depend on the impinging radiation field from nearby massive stars. Finally we extend previous measurements of the photoelectric-effect heating efficiency, which we find is constant across regions probed with Herschel, with [C II] and [O I] being the main coolants in faint and diffuse, and bright and compact regions, respectively, and with polycyclic aromatic hydrocarbon emission tracing the CO-dark H2 gas heating where [C II] and [O I] emit.
Conclusions. We present an innovative spectral decomposition method that allows statistical trends to be derived for the molecular gas content using CO, [C II], and H I profiles. Our study highlights the importance of velocity-resolved photodissociation region (PDR) diagnostics and higher spatial resolution for H I observations as future steps.
Key words: ISM: general / photon-dominated region / Magellanic Clouds / submillimeter: ISM / infrared: ISM / galaxies: star formation
The reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/632/A106
© V. Lebouteiller et al. 2019
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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