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The IRAM-30 m 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
Astronomy & Astrophysics 548 (2012) 68 - http://hal.archives-ouvertes.fr/hal-00747242
Physique/Astrophysique/Astrophysique galactique
Planète et Univers/Astrophysique/Astrophysique galactique
The IRAM-30 m line survey of the Horsehead PDR. II. First detection of the l-C3H+ hydrocarbon cation
J. 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
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 Astofis Mol, Madrid 28850
6 :  Instituto de Radio Astronomía Milimétrica
Instituto de Radio Astronomía Milimétrica
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
Context. Pure gas-phase chemistry models do not succeed in reproducing the measured abundances of small hydrocarbons in the interstellar medium. Information on key gas-phase progenitors of these molecules sheds light on this problem. Aims: We aim to constrain the chemical content of the Horsehead mane with a millimeter unbiased line survey at two positions, namely the photo-dissociation region (PDR) and the nearby shielded core. This project revealed a consistent set of eight unidentified lines toward the PDR position. We associate them to the l-C3H+ hydrocarbon cation, which enables us to constrain the chemistry of small hydrocarbons. We observed the lowest detectable J line in the millimeter domain along a cut toward the illuminating direction to constrain the spatial distribution of the l-C3H+ emission perpendicular to the photo-dissociation front. Methods: 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. 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. Results: Six out of the eight unidentified lines observable in the millimeter bands are detected with a signal-to-noise ratio from 6 to 19 toward the Horsehead PDR, while the two last ones are tentatively detected. Mostly noise appears at the same frequency toward the dense core, located less than 40'' away. Moreover, the spatial distribution of the species integrated emission has a shape similar to radical species such as HCO, and small hydrocarbons such as C2H, which show enhanced abundances toward the PDR. The observed lines can be accurately fitted with a linear rotor model, implying a 1Σ ground electronic state. The deduced rotational constant value is B = 11 244.9512 ± 0.0015 MHz, close to that of l-C3H. Conclusions: This is the first detection of the l-C3H+ hydrocarbon in the interstellar medium. Laboratory spectroscopy is underway to confirm these results. Interferometric imaging is needed to firmly constrain the small hydrocarbon chemistry in the Horsehead. Based on observations obtained with the IRAM-30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

Astronomy & Astrophysics
Articles dans des revues avec comité de lecture

surveys – photon-dominated region (PDR) – ISM: lines and bands – ISM: individual objects: Horsehead nebula – line: identification
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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