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Abstract Photodissociation regions (PDRs) are key to understanding the feedback processes that shape interstellar matter in galaxies. One important type of PDR is the interface between Hiiregions and molecular clouds, where far-ultraviolet radiation from massive stars heats gas and dissociates molecules. Photochemical models predict that as metallicity decreases, the C/CO transition occurs at greater depths in the PDR compared to the H/H₂transition, increasing the extent of CO-dark H₂gas in low-metallicity environments. This prediction has been difficult to test outside the Milky Way due to the lack of high-spatial-resolution observations tracing H₂and CO. This study examines a low-metallicity PDR in the N13 region of the Small Magellanic Cloud (SMC), where we spatially resolve the ionization front, the H₂dissociation front, and the C/CO transition using¹²COJ= 2−1, 3−2, and [CI] 1–0 observations from the Atacama Large Millimeter/submillimeter Array and near-infrared spectroscopy of the H₂2.12 1–0 S(1) vibrational line, and H recombination lines from the James Webb Space Telescope. Our analysis shows that the separation between the H/H₂and C/CO boundaries is approximately 0.043 ± 0.013(stat.) ± 0.0036(syst.) pc (equivalent to $0\stackrel{″}{.}146\pm 0\stackrel{″}{.}042\left(\mathrm{stat.}\right)\pm 0\stackrel{″}{.}012\left(\mathrm{syst.}\right)$at the SMC’s distance of 62 kpc), defining the spatial extent of the CO-dark H₂region. Compared to our plane-parallel PDR models, we find that a constant-pressure model matches the observed structure better than a constant-density one. Overall, we find that the PDR model does well at predicting the extent of the CO-dark H₂layer in N13. This study represents the first resolved benchmark for low-metallicity PDRs. 

Abstract We presentCloudymodeling of infrared emission lines in the Wolf–Rayet (WR) nebula N76 caused by one of the most luminous and hottest WR stars in the low metallicity Small Magellanic Cloud. We use spatially resolved mid-infrared Spitzer/InfRared Spectrograph and far-infrared Herschel/PACS spectroscopy to establish the physical conditions of the ionized gas. The spatially resolved distribution of the emission allows us to constrain properties much more accurately than using spatially integrated quantities. We construct models with a range of constant hydrogen densities between n_H= 4–10 cm⁻³and a stellar wind-blown cavity of 10 pc, which reproduces the intensity and shape of most ionized gas emission lines, including the high ionization lines [Oiv] and [Nev], as well as [Siii], [Siv], [Oiii], and [Neiii]. Our models suggest that the majority of [Siii] emission (91%) is produced at the edge of the Hiiregion around the transition between ionized and atomic gas while very little of the [Cii] (<5%) is associated with the ionized gas. The physical conditions of N76 are characterized by a hot HII region with a maximum electron temperature ofT_e∼ 24,000 K, electron densities that range fromn_e∼ 4 to 12 cm⁻³, and high ionization parameters of $\log \left(U\right)\sim -1.15\mathrm{to}-1.77.$By analyzing a low-metallicity WR nebula with a single ionization source, this work gives valuable insights into the impact WR stars have on the galaxy-integrated ionized gas properties in nearby dwarf galaxies. 

Context.One of the main problems in astrochemistry is determining the amount of sulfur in volatiles and refractories in the interstellar medium. The detection of the main sulfur reservoirs (icy H₂S and atomic gas) has been challenging, and estimates are based on the reliability of models to account for the abundances of species containing less than 1% of the total sulfur. The high sensitivity of theJames WebbSpace Telescope provides an unprecedented opportunity to estimate the sulfur abundance through the observation of the [SI] 25.249 µm line. Aims.Our aim is to determine the amount of sulfur in the ionized and warm molecular phases toward the Orion Bar as a template to investigate sulfur depletion in the transition between the ionized gas and the molecular cloud in HII regions. Methods.We used the [S III] 18.7 µm, [S IV] 10.5 µm, and [S l] 25.249 µm lines to estimate the amount of sulfur in the ionized and molecular gas along the Orion Bar. For the theoretical part, we used an upgraded version of the Meudon photodissociation region (PDR) code to model the observations. New inelastic collision rates of neutral atomic sulfur with ortho-and para- molecular hydrogen were calculated to predict the line intensities. Results.The [S III] 18.7 µm and [S IV] 10.5 µm lines are detected over the imaged region with a shallow increase (by a factor of 4) toward the HII region. This suggests that their emissions are partially coming from the Orion Veil. We estimate a moderate sulfur depletion, by a factor of ~2, in the ionized gas. The corrugated interface between the molecular and atomic phases gives rise to several edge-on dissociation fronts we refer to as DF1, DF2, and DF3. The [S l] 25.249 µm line is only detected toward DF2 and DF3, the dissociation fronts located farthest from the HII region. This is the first ever detection of the [S l] 25.249 µm line in a PDR. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the [S l] 25.249 µm line is coming from warm (>40 K) molecular gas located atA_V~1–5 mag from the ionization front. Moreover, the intensity of the [S l] 25.249 µm line is only accounted for if we assume the presence of undepleted sulfur. Conclusions.Our data show that sulfur remains undepleted along the ionic, atomic, and molecular gas in the Orion Bar. This is consistent with recent findings that suggest that sulfur depletion is low in massive star-forming regions because of the interaction of the UV photons coming from the newly formed stars with the interstellar matter. 

Abstract Observations of CH⁺are used to trace the physical properties of diffuse clouds, but this requires an accurate understanding of the underlying CH⁺chemistry. Until this work, the most uncertain reaction in that chemistry was dissociative recombination (DR) of CH⁺. Using an electron–ion merged-beams experiment at the Cryogenic Storage Ring, we have determined the DR rate coefficient of the CH⁺electronic, vibrational, and rotational ground state applicable for different diffuse cloud conditions. Our results reduce the previously unrecognized order-of-magnitude uncertainty in the CH⁺DR rate coefficient to ∼20% and are applicable at all temperatures relevant to diffuse clouds, ranging from quiescent gas to gas locally heated by processes such as shocks and turbulence. Based on a simple chemical network, we find that DR can be an important destruction mechanism at temperatures relevant to quiescent gas. As the temperature increases locally, DR can continue to be important up to temperatures of ∼600 K, if there is also a corresponding increase in the electron fraction of the gas. Our new CH⁺DR rate-coefficient data will increase the reliability of future studies of diffuse cloud physical properties via CH⁺abundance observations.