We unveil the stellar wind–driven shell of the luminous massive star-forming region of RCW 49 using SOFIA FEEDBACK observations of the [C ii] 158 μm line. The complementary data set of the 12CO and 13CO J = 3 → 2 transitions is observed by the APEX telescope and probes the dense gas toward RCW 49. Using the spatial and spectral resolution provided by the SOFIA and APEX telescopes, we disentangle the shell from a complex set of individual components of gas centered around RCW 49. We find that the shell of radius ∼6 pc is expanding at a velocity of 13 km s−1 toward the observer. Comparing our observed data with the ancillary data at X-ray, infrared, submillimeter, and radio wavelengths, we investigate the morphology of the region. The shell has a well-defined eastern arc, while the western side is blown open and venting plasma further into the west. Though the stellar cluster, which is ∼2 Myr old, gave rise to the shell, it only gained momentum relatively recently, as we calculate the shell’s expansion lifetime of ∼0.27 Myr, making the Wolf–Rayet star WR 20a a likely candidate responsible for the shell’s reacceleration.

Context. The [C ii] 158 µm far-infrared (FIR) fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [C ii] emission. Aims. Our aim is to determine whether the complex profiles observed in [ 12 C ii] are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the [ 12 C ii] and isotopic [ 13 C ii] line profiles. Methods. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin [ 13 C ii] emission lines, along with the [ 12 C ii] emission lines, with a high signalto-noise ratio (S/N). We first derived the [ 12 C ii] optical depth and the [C ii] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multicomponent velocity fit allows us to derive the physical conditions of the [C ii] gas: column density and excitation temperature. Results. We find moderate to high [ 12 C ii] optical depths in all four sources and self-absorption of [ 12 C ii] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent A v of up to 41 mag. The foreground absorption requires substantial column densities of cold and dense [C ii] gas, with an equivalent A v ranging up to about 13 mag. Conclusions. The column density of the warm background material requires multiple photon-dominated region (PDR) surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [C ii] gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins. Key words. ISM:clouds -ISM:individual objects: M43 -ISM:individual objects: M17 -photon-dominated region (PDR) -ISM:individual objects: Horsehead -ISM:individual objects: MonR2 1 At that time, the spectroscopic data were less accurate and the wavelength of the transition was assumed to fall at 157 µm instead of 158 µm.Article number, page 1 of 40 A&A proofs: manuscript no. 34380corr_2 mentum change F=2→1, F=1→0, and F=1→1. The frequencies of the fine structure transitions of both isotopes were determined by Cooksy et al. (1986). The astronomical observations are fully consistent with these frequencies, as was discussed by Ossenkopf et al. (2013), who also noted that the relative strengths of the [ 13 C ii] hyperfine satellites (s F→F , see Table 1) given by Cooksy et al. (1986) are incorrect. We summarize all the relevant [ 12 C ii] and [ 13 C ii] spectroscopic parameters in Table 1, including the velocity offsets of the [ 13 C ii] hyperfine components relative to [ 12 C ii]. The frequency separation of the hyperfi...

Aims. Revealing the 3D dynamics of H II region bubbles and their associated molecular clouds and H I envelopes is important for developing an understanding of the longstanding problem as to how stellar feedback affects the density structure and kinematics of the different phases of the interstellar medium. Methods. We employed observations of the H II region RCW 120 in the [C II] 158 μm line, observed within the Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program FEEDBACK, and in the 12CO and 13CO (3 →2) lines, obtained with the Atacama Pathfinder Experiment (APEX) to derive the physical properties of the gas in the photodissociation region (PDR) and in the molecular cloud. We used high angular resolution H I data from the Southern Galactic Plane Survey to quantify the physical properties of the cold atomic gas through H I self-absorption. The high spectral resolution of the heterodyne observations turns out to be essential in order to analyze the physical conditions, geometry, and overall structure of the sources. Two types of radiative transfer models were used to fit the observed [C II] and CO spectra. A line profile analysis with the 1D non-LTE radiative transfer code SimLine proves that the CO emission cannot stem from a spherically symmetric molecular cloud configuration. With a two-layer multicomponent model, we then quantified the amount of warm background and cold foreground gas. To fully exploit the spectral-spatial information in the CO spectra, a Gaussian mixture model was introduced that allows for grouping spectra into clusters with similar properties. Results. The CO emission arises mostly from a limb-brightened, warm molecular ring, or more specifically a torus when extrapolated in 3D. There is a deficit of CO emission along the line-of-sight toward the center of the H II region which indicates that the H II region is associated with a flattened molecular cloud. Self-absorption in the CO line may hide signatures of infalling and expanding molecular gas. The [C II] emission arises from an expanding [C II] bubble and from the PDRs in the ring/torus. A significant part of [C II] emission is absorbed in a cool (~60–100 K), low-density (<500 cm−3) atomic foreground layer with a thickness of a few parsec. Conclusions. We propose that the RCW 120 H II region formed in a flattened, filamentary, or sheet-like, molecular cloud and is now bursting out of its parental cloud. The compressed surrounding molecular layer formed a torus around the spherically expanding H II bubble. This scenario can possibly be generalized for other H II bubbles and would explain the observed “flat” structure of molecular clouds associated with H II bubbles. We suggest that the [C II] absorption observed in many star-forming regions is at least partly caused by low-density, cool, H I -envelopes surrounding the molecular clouds.

Radiative and mechanical feedback of massive stars regulates star formation and galaxy evolution. Positive feedback triggers the creation of new stars by collecting dense shells of gas, while negative feedback disrupts star formation by shredding molecular clouds. Although key to understanding star formation, their relative importance is unknown. Here, we report velocity-resolved observations from the SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program FEEDBACK of the massive star-forming region RCW 120 in the [CII] 1.9-THz fine-structure line, revealing a gas shell expanding at 15 km/s. Complementary APEX (Atacama Pathfinder Experiment) CO J = 3-2 345-GHz observations exhibit a ring structure of molecular gas, fragmented into clumps that are actively forming stars. Our observations demonstrate that triggered star formation can occur on much shorter time scales than hitherto thought (<0.15 million years), suggesting that positive feedback operates on short time periods.

We investigate the origin of self-absorption in [O i] 63 μm line emission, which is very clearly seen in approximately half of the 12 Galactic giant molecular cloud (GMC)/H ii regions observed. For this study, we observed velocity-resolved spectra of photon-dominated region (PDR) and H ii region tracers, the [O i] 63 μm, [N ii] 205 μm, and CO J  =  5–4 and 8–7 lines, with the upGREAT instrument in the 4GREAT configuration on the NASA/DLR Stratospheric Observatory For Infrared Astronomy (SOFIA). To probe the origin of the [O i] absorption and line shape and what they tell us about the physical conditions, we focus on the W3 region, for which we obtained data for eight positions along a line near the H ii region W3 A. We derive the foreground column density of low-excitation atomic oxygen to be in the range 2–7 × 1018 cm − 2 . At the position of strongest [O i] emission and greatest absorbing column density, 24% of the oxygen in the PDR is in the form of low-excitation atomic oxygen. We employ the Meudon PDR code to study the chemical and thermal structure of the PDR and to understand the large column density of neutral oxygen throughout the PDR. The reduction in the integrated intensity of the [O i] 63 μm emission is a factor of ≃2–4 in directions with strong [O i] emission. The results from our sample, if general, would significantly impact the use of the [O i] 63 μm line as a tracer of massive star formation and could play a significant role in explaining the “63 μm [O i] deficit” seen in very luminous extragalactic sources.