In this A&A Letter to the Editor I show that the infamous [CII]-deficit (relative to the far-IR flux) can be explained by [CII] self-absorption without the need for secondary effects
using SOFIA/upGREAT observations. We report the discovery that S144, a bubble‑shaped source embedded in the RCW79 HII region, is predominantly “filled” with ionized carbon and excited by a single O7.5–9.5 V/III star, indicating an early evolutionary stage before significant wind‑blown cavities form.
We modeled the [CII] emission with the SimLine non‑LTE radiative transfer code and decomposed it into a central fully ionized HII region and two high‑density (∼2500 cm⁻³) PDR layers. We found that the inner PDR shell expands at ∼2.6 km s⁻¹ and that the outermost shell exhibits a steep temperature and velocity gradient, leading to a high optical depth of τ ∼ 4 and self‑absorption features. We studied spatially averaged [¹²CII] and [¹³CII] F = 1–0 hyperfine spectra to derive a lower limit on the excitation temperature (Tₑₓ ≳ 54 K) and confirm significant optical depth effects.
I developed a procedure to reconstruct the missing [CII] flux by modeling the line wings and obtained correction factors ranging from 1.1 to 1.4. We examined the correlation between the corrected [CII] intensities and total FIR continuum and found that the restored data follow a linear relation without a [CII]‑deficit. We argue that alternative explanations for [CII]‑deficits, such as high dust optical depth or reduced photoelectric heating efficiency, are less likely under the moderate density and radiation field conditions of S144. We show that self‑absorption by cooler C⁺ layers along the line of sight can account for the apparent [CII]‑deficit in this PDR region. We conclude that self‑absorption must be accounted for when interpreting [CII]‑deficits in Galactic HII bubbles and that broader surveys are needed to quantify the prevalence of this effect.
Read the full A&A letter here or here.
The movies produced within this research project are listed below: