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The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
Pety J., Gratier P., Guzmán V., Roueff E., Gerin M. et al
Physique/Astrophysique/Astrophysique galactique
Planète et Univers/Astrophysique/Astrophysique galactique
The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
Jérôme Pety ()1, 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
1 :  IRAM - Institut de RadioAstronomie Millimétrique
CNRS : UPS2074
300 rue de la Piscine, Domaine Universitaire 38406 Saint Martin d'Hères
2 :  OP - Observatoire de Paris - Site de Paris
CNRS – INSU – Observatoire de Paris
61 Av de l'Observatoire 75014 PARIS
3 :  LUTH - Laboratoire Univers et Théories
CNRS : UMR8102 – INSU – Observatoire de Paris – Université Paris VII - Paris Diderot
5 place Jules Janssen 92195 Meudon cedex
4 :  LERMA - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique
CNRS : UMR8112 – INSU – Observatoire de Paris – Université Pierre et Marie Curie (UPMC) - Paris VI – Université de Cergy Pontoise – École normale supérieure [ENS] - Paris
61, avenue de l'Observatoire - 75014 PARIS
5 :  Ctr Astrobiol CSIC INTA, Lab Astrofis Mol, Madrid 28850, Spain
Ctr Astrobiol CSIC INTA, Lab Astrofis Mol, Madrid
6 :  IRAM
7 Avenida Pastora, Granada
7 :  Max-Planck Institut für Radioastronomie
Max-Planck Institut für Radioastronomie
Auf dem Hügel 69, 53121 Bonn
8 :  LUPM - Laboratoire Univers et Particules de Montpellier
CNRS : UMR5299 – Université Montpellier II - Sciences et techniques – IN2P3
Université de Montpellier II Place Eugène Bataillon - CC 72 34095 Montpellier Cédex 05
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.

Astrochemistry – ISM: clouds – ISM: molecules – ISM: individual objects: Horsehead nebula – Radio lines: ISM
9 pages, 7 PostScript figures. Accepted for publication in Astronomy & Astrophysics. Uses aa LaTeX macros.

This work was funded by grant ANR-09-BLAN-0231-01 from the French Agence Nationale de la Recherche as part of the SCHISM project.

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