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 Local Standard of Rest ( LSR ) velocities between 7 and 50 kms ^ { -1 } we estimate total column densities of N ( OH ^ { + } ) \geq 2.5 \times 10 ^ { 14 } cm ^ { -2 } , N ( H _ { 2 } O ^ { + } ) \sim 6 \times 10 ^ { 13 } cm ^ { -2 } and N ( H _ { 3 } O ^ { + } ) \sim 4.0 \times 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 .