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

Abstract

We study the spatial distribution and chemistry of small hydrocarbons in the Orion Bar PDR. We used the IRAM-30m telescope to carry out a millimetre line survey towards the Orion Bar edge, complemented with ~2'x2' 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. ~40% of the lines in the survey arise from hydrocarbons (C2H, C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H and l-H2C3). We detect new lines from l-C3H+ and improve its rotational spectroscopic constants. Anions or deuterated hydrocarbons are not detected: [C2D]/[C2H]<0.2%, [C2H-]/[C2H]<0.007% and [C4H-]/[C4H]<0.05%. Our gas-phase models can reasonably match the observed column densities of most hydrocarbons (within factors <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. We propose that reactions of C2H isotopologues with 13C+ and H atoms can explain the observed [C13CH]/[13CCH]=1.4(0.1) fractionation level.

30 pages, 23 figures, 15 tables. Accepted for publication in A&A (English edited, abstract abridged)

References in corpus (14)

• The distance to the Orion Nebula
• Photoprocesses in protoplanetary disks
• A 2 mm spectral line survey of the starburst galaxy NGC 253
• On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
• Discovery of Interstellar Propylene (CH_2CHCH_3): Missing Links in Interstellar Gas-Phase Chemistry
• Low sulfur depletion in the Horsehead PDR
• Search for anions in molecular sources: C4H- detection in L1527
• The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+: The effect of high electron density and vibrationally excited H2 in a warm PDR surface
• Formation of simple organic molecules in inner T Tauri disks
• The penetration of FUV radiation into molecular clouds
• On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs
• HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds
• Revised spectroscopic parameters of SH${}^{+}$ from ALMA and IRAM 30 m observations
• A CSO Search for $l$-C${}_3$H${}^{+}$: Detection in the Orion Bar PDR

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