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The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
Jérôme Pety1, 2, P. Gratier1, V. Guzmán1, E. Roueff3, M. Gerin4, J.R. Goicoechea5, S. Bardeau1, A. Sievers6, F. Le Petit3, J. Le Bourlot3, A. Belloche7, D. Talbi8

We present the first detection of the l-C3H+ hydrocarbon in the interstellar medium. The Horsehead WHISPER project, a millimeter unbiased line survey at two positions, namely the photo-dissociation region (PDR) and the nearby shielded core, revealed a consistent set of eight unidentified lines toward the PDR position. Six of them are detected with a signal-to-noise ratio from 6 to 19, while the two last ones are tentatively detected. Mostly noise appears at the same frequency toward the dense core, located less than 40" away. We simultaneously fit 1) the rotational and centrifugal distortion constants of a linear rotor, and 2) the Gaussian line shapes located at the eight predicted frequencies. The observed lines can be accurately fitted with a linear rotor model, implying a 1Sigma ground electronic state. The deduced rotational constant value is Be= 11244.9512 +/- 0.0015 MHz, close to that of l-C3H. We thus associate the lines to the l-C3H+ hydrocarbon cation, which enables us to constrain the chemistry of small hydrocarbons. A rotational diagram is then used to infer the excitation temperature and the column density. We finally compare the abundance to the results of the Meudon PDR photochemical model.
1 :  IRAM - Institut de RadioAstronomie Millimétrique
2 :  OP - Observatoire de Paris - Site de Paris
3 :  LUTH - Laboratoire Univers et Théories
4 :  LERMA - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique
5 :  Ctr Astrobiol CSIC INTA, Lab Astrofis Mol, Madrid 28850, Spain
6 :  IRAM
7 :  Max-Planck Institut für Radioastronomie
8 :  LUPM - Laboratoire Univers et Particules de Montpellier
Physique/Astrophysique/Astrophysique galactique

Planète et Univers/Astrophysique/Astrophysique galactique
Astrochemistry – ISM: clouds – ISM: molecules – ISM: individual objects: Horsehead nebula – Radio lines: ISM
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