HERSCHEL/HIFI DISCOVERY OF HCL{sup +} IN THE INTERSTELLAR MEDIUM

Journal Article · Fri Jun 01 00:00:00 EDT 2012 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/751/2/L37· OSTI ID:22047782

De Luca, M; Gerin, M; Falgarone, E ^[1]; Gupta, H; Drouin, B J; Pearson, J C ^[2]; Neufeld, D ^[3]; Teyssier, D ^[4]; Lis, D C; Monje, R; Phillips, T G ^[5]; Goicoechea, J R; Godard, B; Bell, T A ^[6]; Coutens, A ^[7]

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1. LERMA-LRA, UMR 8112 du CNRS, Observatoire de Paris, Ecole Normale Superieure, UPMC and UCP, 24 rue Lhomond, 75231 Paris Cedex 05 (France)
2. Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109 (United States)
3. Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States)
4. European Space Astronomy Centre, ESA, P.O. Box 78, E-28691 Villanueva de la Canada, Madrid (Spain)
5. California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125 (United States)
6. Centro de Astrobiologia (CSIC/INTA), Ctra. de Torrejon a Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid (Spain)
7. Universite de Toulouse, UPS-OMP, IRAP, Toulouse (France)

The radical ion HCl{sup +}, 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{sup 35}Cl{sup +}, {sup 2}{Pi}{sub 3/2} J = 5/2-3/2, showing {Lambda}-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 HCl{sup +}. 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{sup 37}Cl{sup +} toward W31C, with a column density consistent with the expected {sup 35}Cl/{sup 37}Cl isotopic ratio, provides additional evidence for the identification. A comparison with the chemically related molecules HCl and H{sub 2}Cl{sup +} yields an abundance ratio of unity with both species (HCl{sup +} : H{sub 2}Cl{sup +} : HCl {approx} 1). These observations also yield the unexpected result that HCl{sup +} accounts for 3%-5% of the gas-phase chlorine toward W49N and W31C, values several times larger than the maximum fraction ({approx}1%) predicted by chemical models.

• OSTI ID: 22047782
• Journal Information: Astrophysical Journal Letters, Vol. 751, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

74 ATOMIC AND MOLECULAR PHYSICS
79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABSORPTION
ASTRONOMY
ASTROPHYSICS
CHLORINE
CHLORINE 35
CHLORINE 37
COMPARATIVE EVALUATIONS
COSMOCHEMISTRY
ELEMENT ABUNDANCE
GROUND STATES
HYDROCHLORIC ACID
HYPERFINE STRUCTURE
INTERSTELLAR SPACE
ISOTOPE RATIO
MOLECULAR IONS
MOLECULES
OPACITY
RADICALS
STARS