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HARSHAL GUPTA, JOHN C. PEARSON, SHANSHAN YU, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109; PAUL RIMMER, ERIC HERBST, Departments of Physics, Chemistry, and Astronomy, The Ohio State University, Columbus, OH 43210; EDWIN A. BERGIN, Department of Astronomy, University of Michigan, Ann Arbor, MI 48109; AND THE HEXOS TEAM, HTTP://WWW.HEXOS.ORG/TEAM.PHP.
The reactive molecular ions, OH+, H2O+, and H3O+, key probes of the oxygen chemistry of the interstellar gas, have been observed toward Orion KL with the Heterodyne Instrument for Far Infrared on board the Herschel Space Observatory . All three N = 1 - 0 fine-structure transitions of OH+ at 909, 971, and 1033 GHz and both fine-structure components of the doublet ortho -H2O+ 111 - 000 transition at 1115 and 1139 GHz were detected, and an upper limit was obtained for H3O+. OH+ and H2O+ are observed purely in absorption, showing a narrow component at the source velocity of 9~km~s-1, and a broad blue shifted absorption similar to that reported recently for HF and para -H218O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH+ and H2O+ for the 9~km~s-1 component of 9 \pm 3 × 1012 cm-2 and 7 \pm 2 × 1012 cm-2, and those in the outflow of 1.9 \pm 0.7 × 1013 cm-2 and 1.0 \pm 0.3 × 1013 cm-2. Upper limits of 2.4 × 1012 cm-2 and 8.7 × 1012 cm-2 were derived for the column densities of ortho and para -H3O+ from transitions near 985 and 1657 GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. A higher gas density, despite the assumption of a large ionization rate, may explain the comparatively low column densities of OH+ and H2O+.