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
Context. The ambiguous origin of the [C II] 158μ m 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 conten...
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| Hauptverfasser: | , , , , , , , , , , , , , |
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| Dokumenttyp: | Article (Journal) |
| Sprache: | Englisch |
| Veröffentlicht: |
10 December 2019
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| In: |
Astronomy and astrophysics
Year: 2019, Jahrgang: 632 |
| ISSN: | 1432-0746 |
| DOI: | 10.1051/0004-6361/201936303 |
| Online-Zugang: | Verlag, lizenzpflichtig, Volltext: https://doi.org/10.1051/0004-6361/201936303 Verlag, lizenzpflichtig, Volltext: https://www.aanda.org/articles/aa/abs/2019/12/aa36303-19/aa36303-19.html 
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| Verfasserangaben: | V. Lebouteiller, D. Cormier, S.C. Madden, M. Galametz, S. Hony, F. Galliano, M. Chevance, M.-Y. Lee, J. Braine, F.L. Polles, M.A. Requeña-Torres, R. Indebetouw, A. Hughes, and N. Abel |
| Zusammenfassung: | Context. The ambiguous origin of the [C II] 158μ m 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 H<sub>2<sub/> gas, and how these parameters are impacted by environmental effects such as stellar feedback.<i>Methods.<i/> 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 H<sub>2<sub/> column density traced by C<sup>+<sup/>.<i>Results.<i/> 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 H<sub>2<sub/> 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 H<sub>2<sub/> gas, whose density is typically a few 100s cm<sup>−3<sup/> and thermal pressure in the range 10<sup>3.5−5<sup/> K cm<sup>−3<sup/>, is not always in pressure equilibrium with the neutral atomic gas. The fraction of CO-dark H<sub>2<sub/> 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 <i>Herschel<i/>, 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 H<sub>2<sub/> gas heating where [C II] and [O I] emit.<i>Conclusions.<i/> 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. |
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| Beschreibung: | Gesehen am 06.03.2020 |
| Beschreibung: | Online Resource |
| ISSN: | 1432-0746 |
| DOI: | 10.1051/0004-6361/201936303 |