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Herschel/HIFI Discovery of HCL^+ in the Interstellar Medium

De Luca, M. and Gupta, H. and Neufeld, D. and Gerin, M. and Teyssier, D. and Drouin, B. J. and Pearson, J. C. and Lis, D. C. and Monje, R. and Phillips, T. G. and Goicoechea, J. R. and Godard, B. and Falgarone, E. and Coutens, A. and Bell, T. A. (2012) Herschel/HIFI Discovery of HCL^+ in the Interstellar Medium. Astrophysical Journal Letters, 751 (2). Art. No. L37. ISSN 2041-8205. doi:10.1088/2041-8205/751/2/L37.

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The radical ion HCI^+, a key intermediate in the chlorine chemistry of the interstellar gas, has been identified for the first time in the interstellar medium with the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared. The ground-state rotational transition of H^(35)CI^+, ^2Π_(3/2)J = 5/2-3/2, showing Λ-doubling and hyperfine structure, is detected in absorption toward the Galactic star-forming regions W31C (G10.6-0.4) and W49N. The complex interstellar absorption features are modeled by convolving in velocity space the opacity profiles of other molecular tracers toward the same sources with the fine and hyperfine structure of HCI^+. This structure is derived from a combined analysis of optical data from the literature and new laboratory measurements of pure rotational transitions, reported in the accompanying Letter by Gupta et al. The models reproduce well the interstellar absorption, and the frequencies inferred from the astronomical observations are in exact agreement with those calculated using spectroscopic constants derived from the laboratory data. The detection of H^(37)CI^+ toward W31C, with a column density consistent with the expected ^(35)CI/^(37)CI isotopic ratio, provides additional evidence for the identification. A comparison with the chemically related molecules HCI and H_2CI^+ yields an abundance ratio of unity with both species (HCI^+ : H_2CI^+ : HCI ~ l). These observations also yield the unexpected result that HCI^+ accounts for 3%-5% of the gas-phase chlorine toward W49N and W31C, values several times larger than the maximum fraction (~1%) predicted by chemical models.

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Lis, D. C.0000-0002-0500-4700
Additional Information:© 2012 American Astronomical Society. Received 2011 August 30; accepted 2012 April 6; published 2012 May 16. We are indebted to I. Avruch, R. Higgins, and the OBerlin group for helpful discussions on spectroscopy and support on data reduction. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United States (NASA) under the leadership of SRON, Netherlands Institute for Space Research, Groningen, The Netherlands, and with major contributions from Germany, France and the US. Consortium members are Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland: NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico 11 Only linear baselines have been removed to minimize unjustified extrapolations of standing waves into the frequency range of interest. 12 The observations do not cover the whole absorption pattern of the H37Cl+ transition; therefore, the integrated opacity is derived from the model. 13 Assuming 35Cl/37Cl = 3.1. di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronomico Nacional (IGN), Centro de Astrobiologia; Sweden: Chalmers University of Technology—MC2, RSS & GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm University—Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: CalTech, JPL, NHSC. Support for this work was provided by the Centre National de Recherche Spatiale (CNES), by the SCHISM project (grant ANR-09-BLAN-0231-01), by NASA through an award issued by JPL/Caltech, and by the Spanish MICINN (grants AYA2009-07304 and CSD2009-00038).
Funding AgencyGrant Number
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Agence Nationale pour la Recherche (ANR)ANR-09-BLAN-0231-01
Ministerio de Ciencia e Innovación (MCINN)AYA2009-07304
Ministerio de Ciencia e Innovación (MCINN)CDS2009-00038
Subject Keywords:astrochemistry; ISM: abundances; ISM: molecules; line: identification; molecular processes; radio lines: ISM
Issue or Number:2
Record Number:CaltechAUTHORS:20120620-093630191
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Official Citation:Herschel/HIFI Discovery of HCl+ in the Interstellar Medium M. De Luca et al. 2012 ApJ 751 L37
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:31974
Deposited By: Ruth Sustaita
Deposited On:20 Jun 2012 17:16
Last Modified:09 Nov 2021 20:02

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