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.
Far-UV photons strongly affect the physical and chemical state of molecular gas in the vicinity of young massive stars. We have obtained maps of the HCO and H13CO+ ground state lines towards the Horsehead edge at 5 angular resolution with a combinati
on of IRAM PdBI and 30m observations. These maps have been complemented with IRAM-30m observations of several excited transitions at two different positions. Bright formyl radical emission delineates the illuminated edge of the nebula, with a faint emission remaining towards the shielded molecular core. Viewed from the illuminated star, the HCO emission almost coincides with the PAH and CCH emission. HCO reaches a similar abundance than HCO+ in the PDR (~1-2 x10^{-9} with respect to H2). Pure gas-phase chemistry models fail to reproduce the observed HCO abundance by ~2 orders of magnitude, except if reactions of OI with carbon radicals abundant in the PDR (i.e., CH2) play a significant role in the HCO formation. Alternatively, HCO could be produced in the PDR by non-thermal processes such as photo-processing of ice mantles and subsequent photo-desorption of either HCO or H2CO, and further gas phase photodissociation. The measured HCO/H13CO+ abundance ratio is large towards the PDR (~50), and much lower toward the gas shielded from FUV radiation (<1). We propose that high HCO abundances (>10^{-10}) together with large HCO/H13CO+ abundance ratios (>1) are sensitive diagnostics of the presence of active photochemistry induced by FUV radiation.
