2025A&A...697L...2K

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.