Interstellar OH⁺, H₂O⁺and H₃O⁺along the sight-line to G10.6–0.4

Abstract

We report the detection of absorption lines by the reactive ions OH + , H 2 O + and H 3 O + along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH + at 971 GHz, H 2 O + at 1115 and 607 GHz, and H 3 O + at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH + spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H 2 O + lines remains lower than 1 in the same velocity range, and the H 3 O + line shows only weak absorption. For LSR velocities between 7 and 50 km s −1 we estimate total column densities of N(OH + ) ≥ 2.5 × 10 14 cm −2 , N(H 2 O + ) ∼6 × 10 13 cm −2 and N(H 3 O + ) ∼4.0 × 10 13 cm −2 . These detections confirm the role of O + and OH + in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH + by the H 2 O + column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.

Low temperature rates for key steps of interstellar gas-phase water formation

“…The protonated water molecule H 3 O + was first detected much earlier, using the National Radio Astronomy Observatory 12m telescope toward Orion KL and OMC-1 ( 8 , 9 ). Furthermore, all of these molecular ions were found along the line of sight toward the submillimeter continuum source G10.6-0.4 (W31C) using the Herschel Heterodyne Instrument for the Far Infrared ( 10 ). Since OH + and H 2 O + are known to react exothermically with H 2 , both ions are believed to reside in environments with a low fraction of molecular to atomic hydrogen.…”

Lifetime of low vibrational levels of the metastableB̃2B2state ofH2O+<

“…The water cation is found in naturally occurring plasmas, for instance, in the Earth's ionosphere [1,2], in comet tails [3,4], and in several regions of the interstellar medium [5][6][7][8]. In these plasmas water ions participate in reactions that influence the thermal and chemical properties of the environment as formulated in several model chemistries [9][10][11][12][13].…”

Modelling interstellar physics and chemistry: implications for surface and solid-state processes

“…Gas-phase routes to these species are controversial, but we have shown that an assumed reaction involving efficient hydrogenation of nitrogen at surfaces could readily account for the NH abundance without adverse chemical consequences [28]. However, no [29]. Conventional ion-molecule gas-phase chemistry involving O + ions appears capable of accounting for the observed abundances of these species.…”