Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
  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$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch
mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+
at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep
absorption over a broad velocity range that originates in the interstellar
matter along the line of sight to G10.6$-$0.4 as well as in the molecular gas
directly associated with that source. The OH+ spectrum reaches saturation over
most velocities corresponding to the foreground gas, while the opacity of the
H2O+ lines remains lower than 1 in the same velocity range, and the H3O+ line
shows only weak absorption. For LSR velocities between 7 and 50 kms$^{-1}$ we
estimate total column densities of $N$(OH+) $> 2.5 \times 10^{14}$ cm$^{-2}$,
$N$(H2O+) $\sim 6 \times 10^{13}$ cm$^{-2}$ and $N$(H3O+) $\sim 4.0 \times
10^{13}$ 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 H2O+ column density implies that these species predominantly trace
low-density gas with a small fraction of hydrogen in molecular form.
