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Are PAHs precursors of small hydrocarbons in Photo-Dissociation Regions? The Horsehead case

AuthorsPety, Jérôme; Teyssier, D.; Fosse, David; Gerin, M.; Roueff, E.; Abergel, A.; Habart, E.; Cernicharo, José
KeywordsISM clouds
Individual object (Horsehead nebula)
Radio lines: ISM
Issue Date17-Jan-2005
PublisherEDP Sciences
Astronomy & Astrophysics 435, 885-899 (2005)
AbstractWe present maps at high spatial and spectral resolution in emission lines of C2H, c-C3H2, C4H, 12CO and C18O of the edge of the Horsehead nebula obtained with the Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H2 ro-vibrational lines and in the PAH mid-infrared bands. C18O peaks farther away from the cloud edge. Our observations demonstrate that C2H, cC3H2 and C4H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well shielded molecular cores. PDR models (i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H2 peaks and (ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amount.
DescriptionIn press in Astronomy & Astrophysics. Uses aa LaTeX macros.
Appears in Collections:(CFMAC-IEM) Artículos
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