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Journal Paper

전체
Physical conditions in the gas phases of the giant HII region LMC-N 11 II. Origin of [CII] and fraction of CO-dark gas SCI
  • V. Lebouteiller; Min-Young Lee; D. Cormier; et al.
  • 2019-12-10
  • ASTRONOMY & ASTROPHYSICS 632 : A106-1~A106-30
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?