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

¹ 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.