Free access article

A&A 435, 885-899 (2005)
DOI: 10.1051/0004-6361:20041170

Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case

J. Pety^{1, 2}, D. Teyssier^{3, 4}, D. Fossé¹, M. Gerin¹, E. Roueff⁵, A. Abergel⁶, E. Habart⁷ and J. Cernicharo<sup>3

1</sup>  LERMA, UMR 8112, CNRS, Observatoire de Paris and Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
    e-mail: [fosse;gerin]@lra.ens.fr
²  IRAM, 300 rue de la Piscine, 38406 Grenoble Cedex, France
    e-mail: pety@iram.fr
³  Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
    e-mail: [cerni;teyssier]@damir.iem.csic.es
⁴  Space Research Organization Netherlands, PO Box 800, 9700 AV Groningen, The Netherlands
⁵  LUTH UMR 8102, CNRS and Observatoire de Paris, Place J. Janssen 92195 Meudon Cedex, France
    e-mail: evelyne.roueff@obspm.fr
⁶  IAS, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
    e-mail: abergel@ias.u-psud.fr
⁷  Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
    e-mail: habart@arcetri.astro.it

(Received 27 April 2004 / Accepted 11 January 2005 )

Abstract
We present maps at high spatial and spectral resolution in emission lines of CCH, c-C₃H₂, C₄H, ¹²CO and C¹⁸O of the edge of the Horsehead nebula obtained with the IRAM 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 CCH, c-C₃H₂ 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 amounts.

Key words: ISM: clouds -- ISM: molecules -- ISM: individual object: Horsehead nebula -- radio lines: ISM

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© ESO 2005

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