2019A&A...626A..28T

Aims. Hydrocarbons are ubiquitous in the interstellar medium, but their formation is still not well understood, depending on the physical environment in which they are found. Messier 8 (M8) is host to one of the brightest HII regions and photodissociation regions (PDRs) in our galaxy. With the observed C₂H and c-C₃H₂ data toward M8, we aim at obtaining their densities and abundances and to shed some light on their formation mechanism.
Methods. Using the Atacama Pathfinder Experiment (APEX) 12m, and the Institut de Radioastronomie Millimetrique (IRAM) 30m telescopes, we performed a line survey toward Herschel 36 (Her 36), which is the main ionizing stellar system in M8, and an imaging survey within 1.3x1.3pc around Her 36 of various transitions of C₂H and c-C₃H₂. We used both local thermodynamic equilibrium (LTE) and non-LTE methods to determine the physical conditions of the emitting gas along with the column densities and abundances of the observed species, which we compared with (updated) gas-phase photochemical PDR models. In order to examine the role of polycyclic aromatic hydrocarbons (PAHs) in the formation of small hydrocarbons and to investigate their association with the HII region, the PDR and the molecular cloud, we compared archival Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 8 µm and the Spectral and Photometric Imaging Receiver (SPIRE) 250 µm continuum images with the C₂H emission maps.
Results. We observed a total of three rotational transitions of C₂H with their hyperfine structure components and four rotational transitions of c-C₃H₂ with ortho and para symmetries toward the HII region and the PDR of M8. Fragmentation of PAHs seems less likely to contribute to the formation of small hydrocarbons as the 8µm emission does not follow the distribution of C₂H emission, which is more associated with the molecular cloud toward the north west of Her 36. From the quantitative analysis, we obtained abundances of ∼10^–8 and 10^–9 for C₂H and c-C₃H₂ respectively, and volume densities of the hydrocarbon emitting gas in the range n(H₂)∼5x10⁴-5x10⁶cm^–3.
Conclusions. The observed column densities of C₂H and c-C₃H₂ are reproduced reasonably well by our PDR models. This supports the idea that in high-UV flux PDRs, gas-phase chemistry is sufficient to explain hydrocarbon abundances.