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.

arXiv comment30 pages, 23 figures, 15 tables. Accepted for publication in A&A (English edited, abstract abridged)

References in corpus (55)

• A computer program for fast non-LTE analysis of interstellar line spectra
• The distance to the Orion Nebula
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Water, O2 and Ice in Molecular Clouds
• Are PAHs precursors of small hydrocarbons in Photo--Dissociation Regions? The Horsehead case
• Photoprocesses in protoplanetary disks
• A 2 mm spectral line survey of the starburst galaxy NGC 253
• Spatially Resolved Chemistry in Nearby Galaxies I. The Center of IC 342
• 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
• A line confusion limited millimeter survey of Orion KL (I): sulfur carbon chains
• Surface chemistry in the interstellar medium - I - H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms
• Low sulfur depletion in the Horsehead PDR
• A study of deuterated water in the low-mass protostar IRAS16293-2422
• The chemistry of vibrationally excited H2 in the interstellar medium
• The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
• Molecular Carbon Chains and Rings in TMC-1
• Evaporating Very Small Grains as tracers of the UV radiation field in Photo-dissociation Regions
• Herschel/HIFI observations of [CII] and [13CII] in PDRs
• Carbon budget and carbon chemistry in Photon Dominated Regions
• The far-IR spectrum of Sagittarius B2 region: Extended molecular absorption, photodissociation and photoionization
• First Astronomical Detection of the Cumulene Carbon Chain Molecule H_2C_6 in TMC1
• 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
• Photon-Dominated Chemistry in the Nucleus of M82: Widespread HOC+ emission in the inner 650 pc disk
• The penetration of FUV radiation into molecular clouds
• A lambda = 1.3 mm and 2 mm molecular line survey towards M82
• H_2 Pure Rotational Lines in the Orion Bar
• Evolution of dust in the Orion Bar with Herschel: I. Radiative transfer modelling
• The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds
• OH emission from warm and dense gas in the Orion Bar PDR
• CCH in prestellar cores
• High-Accuracy Quartic Force Field Calculations for the Spectroscopic Constants and Vibrational Frequencies of 1 ^1A' l-C3H-: A Possible Link to Lines Observed in the Horsehead Nebula PDR
• Molecular survey of CRL618
• Spatially Resolved Chemistry in Nearby Galaxies II. The Nuclear Bar in Maffei 2
• Tearing the Veil: interaction of the Orion Nebula with its neutral environment
• Spatial variation of the cooling lines in the Orion Bar from Herschel/PACS
• Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations
• The abundance of C3H2 and other small hydrocarbons in the diffuse interstellar medium
• On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs
• Carbon Isotope and Isotopomer Fractionation in Cold Dense Cloud Cores
• Spatially extended OH+ emission from the Orion Bar and Ridge
• Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs
• Detection of OH${}^{+}$ and H${}_2$O${}^{+}$ towards Orion~KL
• Spatial distribution of small hydrocarbons in the neighborhood of the Ultra Compact HII region Monoceros R2
• HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds
• Herschel Search for O2 Toward the Orion Bar
• Reactions forming C(0,+)n=2,10, Cn=2,4H(0,+) and C3H(0,+) in the gas phase: semi empirical branching ratios
• A Search for l-C3H+ and l-C3H in Sgr B2(N), Sgr B2(OH), and the Dark Cloud TMC-1
• Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023
• Kinematics of the ionized-to-neutral interfaces in Monoceros R2
• The water abundance behind interstellar shocks: results from $Herschel$/PACS and $Spitzer$/IRS observations of H${}_2$O, CO, and H${}_2$
• 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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• Which molecule traces what: chemical diagnostics of protostellar sources
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• Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon