A Bubble in a Bubble [C II]-deficit caused by self-absorption in an ionized carbon filled bubble in RCW79

Recent spectroscopic observations of the [C II] 158  m fine structure line of ionized carbon (C+), using the Stratospheric Observatory for Infrared Astronomy (SOFIA), have revealed expanding [C II] shells in Galactic HII regions. Here, we report the discovery of abubble (source S144 in the GLIMPSE survey), associated with a compact HII region (cH IIR), excited by a single O7.5􀀀9.5V/III star, that is still mostly “filled” with C+. S144 is prominent in [C II] and 8  m emission without a central void, indicating a very early evolutionary state in which the stellar wind has not yet blown material away, as it is the case for more evolved HII regions. Using the SimLine non-LTE radiative transfer code, the [C II] emission can be modeled to originate in three regions. First, a central HII region with little C+ in the fully ionized phase, followed by two layers with a gas density around 2500 cm􀀀3 of partially photo-dissociated gas. The second layer is a slowly expanding [C II] shell with an expansion velocity of  2:6 km/s that corresponds approximately to a bright ring at 8  m. The outermost layer exhibits a temperature and velocity gradient that produces the observed self-absorption features in the optically thick [C II] line (    4). The self-absorption leads to an apparent deficit in [C II] emission and a low ratioof [C II] to total far-infrared (FIR) emission. We developed a procedure to approximate the missing [C II] flux by fitting a Gaussian line to the [C II] line wings, considering the optical depth and the hyper-fine structure emission of [13CII]. With the corrected [C II] intensities, we find a linear correlation between [C II] and FIR without a [C II]-deficit. This example demonstrates that at least some of the [C II]-deficit found in Galactic HII 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 where [C II]-deficits may be explained by this process.