SIMBAD references

We report the detection of absorption lines by the reactive ions OH⁺, H₂O⁺and H₃O⁺ 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 971GHz, H₂O⁺ at 1115 and 607GHz, and H₃O⁺ at 984GHz. 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 H₂O⁺ lines remains lower than 1 in the same velocity range, and the H₃O⁺line shows only weak absorption. For LSR velocities between 7 and 50km/s we estimate total column densities of N(OH⁺)≥2.5x10¹⁴cm^–2, N(H₂O⁺)∼6x10¹³cm^–2 and N(H₃O⁺)∼4.0x10¹³cm^–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 H₂O⁺ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.