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The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: The Orion Bar PDR

AuthorsCuadrado, S.; Goicoechea, Javier R. ; Pilleri, P.; Cernicharo, José ; Fuente, A.; Joblin, C.
KeywordsISM: abundances
ISM: molecules
Photon-dominated region (PDR)
Issue DateMar-2015
PublisherEDP Sciences
CitationAstronomy and Astrophysics 575: A82 (2015)
Abstract[Context] Carbon chemistry plays a pivotal role in the interstellar medium (ISM) but even the synthesis of the simplest hydrocarbons and how they relate to polycyclic aromatic hydrocarbons (PAHs) and grains is not well understood.
[Aims] We study the spatial distribution and chemistry of small hydrocarbons in the Orion Bar photodissociation region (PDR), a prototypical environment in which to investigate molecular gas irradiated by strong UV fields.
[Methods] We used the IRAM 30 m telescope to carry out a millimetre line survey towards the Orion Bar edge, complemented with ∼2′ × 2′ maps of the C2H and c-C3H2 emission. We analyse the excitation of the detected hydrocarbons and constrain the physical conditions of the emitting regions with non-LTE radiative transfer models. We compare the inferred column densities with updated gas-phase photochemical models including 13CCH and C13CH isotopomer fractionation.
[Results] Approximately 40% of the lines in the survey arise from hydrocarbons (C2H, C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H, and l-H2C3 in decreasing order of abundance). We detect new lines from l-C3H+ and improve its rotational spectroscopic constants. Anions or deuterated hydrocarbons are not detected, but we provide accurate upper limit abundances: [C2D]/[C2H] < 0.2%, [C2H−]/[C2H] < 0.007%, and [C4H−]/[C4H] < 0.05%.
[Conclusions] Our models can reasonably match the observed column densities of most hydrocarbons (within factors of <3). Since the observed spatial distribution of the C2H and c-C3H2 emission is similar but does not follow the PAH emission, we conclude that, in high UV-flux PDRs, photodestruction of PAHs is not a necessary requirement to explain the observed abundances of the smallest hydrocarbons. Instead, gas-phase endothermic reactions (or with barriers) between C+, radicals, and H2 enhance the formation of simple hydrocarbons. Observations and models suggest that the [C2H]/[c-C3H2] ratio (~32 at the PDR edge) decreases with the UV field attenuation. The observed low cyclic-to-linear C3H column density ratio (≤3) is consistent with a high electron abundance (xe) PDR environment. In fact, the poorly constrained xe gradient influences much of the hydrocarbon chemistry in the more UV-shielded gas. The inferred hot rotational temperatures for C4H and l-C3H+ also suggest that radiative IR pumping affects their excitation. We propose that reactions of C2H isotopologues with 13C+ and H atoms can explain the observed [C13CH]/[13CCH] = 1.4 ± 0.1 fractionation level.
Publisher version (URL)https://doi.org/10.1051/0004-6361/201424568
Identifiersdoi: 10.1051/0004-6361/201424568
e-issn: 1432-0746
issn: 0004-6361
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