In our recent A&A Letter, we studied ionized carbon emission (C⁺, [C II]) at 158 μm in S144, a C⁺ bubble on the southeastern edge of the ring-shaped star-forming region RCW79. S144 hosts a compact H II region ionized by a single O7.5–9.5 V/III star. Using SOFIA/upGREAT maps with high angular and spectral resolution, we identified the first bubble that remains mostly filled with C⁺ gas – an indicator of an exceptionally early evolutionary stage. All previously characterized C⁺ bubbles exhibit shell-like rings with central cavities carved by stellar winds, marking more advanced phases.

We also uncovered an apparent shortfall in C⁺ emission relative to S144’s far-infrared luminosity: the so-called “C⁺-deficit,” long attributed to increased dust heating, cooling via other fine-structure lines, or intense radiation fields. Our analysis shows instead that cooler C⁺ gas in the bubble’s outer layers self-absorbs the emission from its warm interior, creating the illusion of a deficit.

By correcting for C⁺ self-absorption, we refine our view of how carbon governs gas cooling in star-forming regions and improve the interpretation of [C II] observations across both Galactic and extragalactic environments.

Paperlink: https://doi.org/10.1051/0004-6361/202453445