Observational study of hydrocarbons in the bright photodissociation region of Messier 8

Abstract

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 H II 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) 12 m, and the Institut de Radioastronomie Millimétrique (IRAM) 30 m 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 1.3 × 1.3 pc 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 H II 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 H II region and the PDR of M8. Fragmentation of PAHs seems less likely to contribute to the formation of small hydrocarbons as the 8 μm 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) ∼5 × 104-5 × 106 cm-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.