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 they are found in. Messier 8 (M8) is host to one of the brightest Hii regions and photodissociation regions (PDRs) in our galaxy. With the observed C 2 H and c-C 3 H 2 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 C 2 H and C 3 H 2 . 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 C 2 H emission maps.Results. We observed a total of three rotational transitions of C 2 H with their hyperfine structure components and four rotational transitions of C 3 H 2 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 8 µm emission does not follow the distribution of C 2 H 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 C 2 H and c-C 3 H 2 respectively, and volume densities of the hydrocarbon emitting gas in the range n(H 2 ) ∼ 5 × 10 4 -5 × 10 6 cm −3 . Conclusions. The observed column densities of C 2 H and c-C 3 H 2 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.

ATLASGAL-selected massive clumps in the inner Galaxy

“…Besides, the c-C 3 H 2 abundances in Mon R2 were found to be higher close to the UV-exposed PDR than the C 2 H abundances (Pilleri et al 2013). The formation sequences mentioned above cannot easily result in the high c-C 3 H 2 abundances, and reproducing observed c-C 3 H 2 values with PDR models has been difficult (e.g., c-C 3 H 2 abundances in M8, Tiwari et al 2019). Another possible way to produce c-C 3 H 2 is grain-surface formation via photons.…”

“…Although those small hydrocarbons are well-known PDR tracers, their abundances vary from one PDR to the other (e.g., Pety et al 2005;Mookerjea et al 2012;Pilleri et al 2013;Cuadrado et al 2015;Tiwari et al 2019). By investigating their abundances for ATLASGAL clumps, we may gain insight into the origin of the detected small hydrocarbons (i.e., C 2 H and c-C 3 H 2 ).…”

Ethynyl around the HII regions S255 and S257

“…In the Orion Bar PDR, close to the Orion nebula, high abundance of small hydrocarbons was explained by high-temperature chemical reactions involving excited hydrogen molecules (Agúndez et al 2010;Cuadrado et al 2015). Also, no discrepancies were found between the observed abundances of C 2 H and the results of the astrochemical modelling in the M8 PDR by Tiwari et al (2019). Murga et al (2020) showed that an effective formation of C 2 H from acetylene molecules (C 2 H 2 ) occurs as a result of photo-destruction of PAHs in the Orion Bar PDR, while in the Horsehead this process is practically insignificant in comparison to the formation of C 2 H via gas-phase reactions starting from ionised carbon C + .…”

Chemical compositions in the vicinity of protostars in Ophiuchus

“…UV radiation, if sufficiently intense, produces photondominated regions (PDRs). In PDR chemistry, small hydrocarbons such as c-C 3 H 2 can be abundant in less shielded regions (e.g., A v ∼ 1.43 mag; Tiwari et al 2019), while CH 3 OH formation is efficient in more shielded regions (A v > 2 mag; Esplugues et al 2019). In order to investigate the effects of UV irradiation from the nearby Herbig Be star on Oph-emb9, we derive the N (c-C 3 H 2 )/N (CH 3 OH) ratio at each position as summarized in Table 4.…”