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

Abstract

Hydrocarbons are ubiquitous in the interstellar medium, but their formation is still not well understood, depending on the physical environment they are found in. M8 is host to one of the brightest HII regions and PDRs in our galaxy. Using the APEX, and the 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 $\times$ 1.3 pc around Her 36 of various transitions of C${}_2$H and c-C${}_3$H${}_2$. We used both 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 PAHs in the formation of small hydrocarbons and to investigate their association with M8, we compared archival GLIMPSE 8 $\mu$m and the SPIRE 250 $\mu$m continuum images with the C${}_2$H emission maps. We observed a total of three rotational transitions of C${}_2$H with their hyperfine structure components and four rotational transitions of c-C${}_3$H${}_2$ with ortho and para symmetries toward M8. Fragmentation of PAHs seems less likely to contribute to the formation of small hydrocarbons as the 8 $\mu$m emission does not follow the distribution of C${}_2$H emission, which is more associated with the molecular cloud. From the quantitative analysis, we obtained abundances of $\sim$ 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 $n({\mathrm{H}}_2)$ $\sim$ 5 $\times$ 10${}^4$--5 $\times$ 10${}^6$ cm${}^{-3}$. 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.

14 pages, 10 figures

References in corpus (7)

• Early Science with SOFIA, the Stratospheric Observatory for Infrared Astronomy
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• Ethynyl (C2H) in massive star formation: Tracing the initial conditions?
• C${}_2$H observations toward the Orion Bar
• High-Resolution Infrared Imaging of Herschel 36 SE: A Showcase for the Influence of Massive Stars in Cluster Environments
• The Gaia-ESO Survey: dynamics of ionized and neutral gas in the Lagoon nebula (M8)
• Resolving the stellar components of the massive multiple system Herschel 36 with AMBER/VLTI

Cited by in corpus (2)

• ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -- I. Survey description and a first look at G9.62+0.19
• Impact of PAH photodissociation on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs