Issue |
A&A
Volume 697, May 2025
|
|
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Article Number | L2 | |
Number of page(s) | 11 | |
Section | Letters to the Editor | |
DOI | https://doi.org/10.1051/0004-6361/202453445 | |
Published online | 06 May 2025 |
Letter to the Editor
[C II]-deficit caused by self-absorption in an ionized carbon-filled bubble in RCW79
1
I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
2
SOFIA Science Center, USRA, NASA Ames Research Center, Moffett Field, CA 94 045, USA
3
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA
4
Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
5
Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
6
Institut Universitaire de France, 1 Rue Descartes, Paris, France
7
Physikalischer Verein, Gesellschaft für Bildung und Wissenschaft, Robert-Mayer-Str. 2, 60325 Frankfurt, Germany
8
University of Maryland, Department of Astronomy, College Park, MD 20742-2421, USA
9
Leiden Observatory, PO Box 9513 2300 RA Leiden, The Netherlands
⋆ Corresponding author: keilmann@ph1.uni-koeln.de
Received:
13
December
2024
Accepted:
8
April
2025
Recent spectroscopic observations of the [C II] 158 μm fine-structure line of ionized carbon (C+), using the Stratospheric Observatory for Infrared Astronomy (SOFIA), have revealed expanding [C II] shells in Galactic H II regions. We report the discovery of a bubble-shaped source (S144 in RCW79 in the GLIMPSE survey), associated with a compact H II region, excited by a single O7.5–9.5V/III star, which is consistent with a scenario that the bubble is still mostly “filled” with C+. This indicates most likely a very early evolutionary state, in which the stellar wind has not yet blown material away as it has in more evolved H II regions. Using the SimLine non-local thermodynamic equilibrium radiative transfer code, the [C II] emission can be modeled to originate from three regions: first, a central H II region with little C+ in the fully ionized phase, followed by two layers with a gas density around 2500 cm−3 of partially photodissociated gas. From these two layers, the second layer is a slowly expanding [C II] shell with an expansion velocity of ∼2.6 km s−1 that corresponds approximately to a bright ring at 8 μm. The outermost layer exhibits a temperature and velocity gradient that produces the observed self-absorption features in the optically thick [C II] line (τ ∼ 4), leading to an apparent deficit in [C II] emission and a low ratio of [C II] to total far-infrared (FIR) emission. We developed a procedure to reconstruct the missing [C II] flux and find a linear correlation between [C II] and FIR without a [C II]-deficit after incorporating the missing [C II] flux. This example demonstrates that at least some of the [C II]-deficit found in Galactic H II bubbles can be attributed to self-absorption, although a broader sample of these objects needs to be studied to fully constrain the range of conditions in which [C II]-deficits can be explained by this process.
Key words: ISM: bubbles / evolution / HII regions / ISM: kinematics and dynamics / photon-dominated region (PDR) / infrared: ISM
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://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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