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A&A 417, 135-149 (2004)
DOI: 10.1051/0004-6361:20034534
Carbon budget and carbon chemistry in Photon Dominated Regions
D. Teyssier1, 2, D. Fossé2, M. Gerin2, J. Pety2, 3, A. Abergel4 and E. Roueff51 Space Research Organization Netherlands, PO Box 800, 9700 AV Groningen, The Netherlands, under an ESA external fellowship
2 Laboratoire d'Étude du Rayonnement et de la Matière, UMR 8112, CNRS, École Normale Supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
3 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St Martin d'Hères, France
4 Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
5 LUTH, UMR8102 du CNRS, Observatoire de Paris, Place J. Janssen, 92195 Meudon Cedex, France
(Received 17 October 2003 / Accepted 9 December 2003)
Abstract
We present a study of small carbon chains and rings in Photon Dominated Regions (PDRs) performed at millimetre wavelengths.
Our sample consists of the Horsehead nebula (B33), the
Oph L1688 cloud interface, and the cometary-shaped cloud IC 63. Using the IRAM 30-m telescope, the SEST and the Effelsberg
100-m telescope, we mapped the emission of C
2H,
c-C
3H
2 and C
4H, and searched for heavy hydrocarbons such as
c-C
3H,
l-C
3H,
l-C
3H
2,
l-C
4H
2 and C
6H. The large scale maps show that small hydrocarbons are present until the edge of all PDRs, which is surprising as they are
expected to be easily destroyed by UV radiation. Their spatial distribution reasonably agrees with the aromatic emission mapped
in mid-IR wavelength bands. C
2H and
c-C
3H
2 correlate remarkably well, a trend already reported in the diffuse ISM (Lucas & Liszt [CITE]). Their abundances relative
to H
2 are relatively high and comparable to the ones derived in dark clouds such as L134N or TMC-1, known as efficient carbon factories.
The heavier species are however only detected in the Horsehead nebula at a position coincident with the aromatic emission
peak around 7
m. In particular, we report the first detection of C
6H in a PDR. We have run steady-state PDR models using several gas-phase chemical networks (UMIST95 and the New Standard Model)
and conclude that both networks fail in reproducing the high abundances of some of these hydrocarbons by an order of magnitude.
The high abundance of hydrocarbons in the PDR may suggest that the photo-erosion of UV-irradiated large carbonaceous compounds
could efficiently feed the ISM with small carbon clusters or molecules. This new production mechanism of carbon chains and
rings could overcome their destruction by the UV radiation field. Dedicated theoretical and laboratory measurements are required
to understand and implement these additional chemical routes.
Key words: ISM: abundances -- astrochemistry -- ISM: individual objects: Horsehead nebula, IC 63,
Oph
Offprint request: D. Teyssier, teyssier@sron.rug.nl
SIMBAD Objects
© ESO 2004