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

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 hydroca...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2015-03, Vol.575, p.A82
Main Authors: Cuadrado, S., Goicoechea, J. R., Pilleri, P., Cernicharo, J., Fuente, A., Joblin, C.
Format: Article
Language:English
Subjects:
astrochemistry
Carbon
Density
Fractionation
Hydrocarbons
ISM: abundances
ISM: molecules
photon-dominated region (PDR)
Polycyclic aromatic hydrocarbons
Radiative transfer
Spatial distribution
surveys
Synthesis (chemistry)
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Summary: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
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201424568