Using the IRAM 30 m telescope we observed molecular absorption lines from CCH and c-C 3 H 2 produced by diffuse and translucent clouds along the lines of sight towards massive star forming regions. The same sources are surveyed with Herschel/HIFI as part of the PRISMAS guaranteed time program, for molecular absorption lines due to hydrides and carbon clusters. The background sources are massive star-forming regions (G34.3 + 0.1, G10.62 − 0.39, W51, W49N) and SgrA * . The line profiles of the CCH and c-C 3 H 2 are strikingly similar for all lines of sight, showing that the ratio of the opacities of the probed transitions, (J Ka,Kc = 2 1,2 −1 0,1 ) for c-C 3 H 2 and (J = 1−0, F = 5/2−3/2) for CCH, is nearly constant along all lines of sight, at τ CCH ∼ 1.8 × τ c-C 3 H 2 . As a consequence, the ratio of the column densities of CCH and c-C 3 H 2 is nearly constant and similar to the value derived earlier for diffuse clouds detected along lines of sight towards extragalactic continuum sources, N(CCH) = (28 ± 1.4)N(c-C 3 H 2 ) (Lucas & Liszt 2000, A&A, 358, 1069. PDR models are able to reproduce the observed CCH column densities for the range of physical conditions appropriate for the absorbing matter (n = 100−3000 cm −3 ; A V = 1−5 mag) but can neither fit the observed c-C 3 H 2 column densities nor the tight correlation between CCH and c-C 3 H 2 .

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.

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1-0 transition of OH + and the 1115 GHz 1 11 −0 00 transition of ortho-H 2 O + . The resultant spectra show absorption by ortho-H 2 O + , and strong absorption by OH + , in foreground material at velocities in the range 0 to 70 km s −1 with respect to the local standard of rest. The inferred OH + /H 2 O + abundance ratio ranges from ∼3 to ∼15, implying that the observed OH + arises in clouds of small molecular fraction, in the 2−8% range. This conclusion is confirmed by the distribution of OH + and H 2 O + in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH + /H abundance ratio of a few ×10 −8 suggests a cosmic ray ionization rate for atomic hydrogen of 0.6−2.4 × 10 −16 s −1 , in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H + 3 and other species.

We report the detection of strong absorption by interstellar hydrogen fluoride along the sight-line to the submillimeter continuum source G10.6-0.4 (W31C). We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1232.4763 GHz J = 1−0 HF transition in the upper sideband of the Band 5a receiver. The resultant spectrum shows weak HF emission from G10.6-0.4 at LSR velocities in the range -10 to -3 km s −1 , accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km s −1 . The spectrum is similar to that of the 1113.3430 GHz 1 11 −0 00 transition of para-water, although at some frequencies the HF (hydrogen fluoride) optical depth clearly exceeds that of para-H 2 O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6 × 10 14 cm −2 on the HF column density along the sight-line to G10.6-0.4. Our lower limit on the HF abundance, 6 × 10 −9 relative to hydrogen nuclei, implies that hydrogen fluoride accounts for between ∼30% and 100% of the fluorine nuclei in the gas phase along this sight-line. This observation corroborates theoretical predictions that -because the unique thermochemistry of fluorine permits the exothermic reaction of F atoms with molecular hydrogen -HF will be the dominant reservoir of interstellar fluorine under a wide range of conditions.

We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschel's HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1−0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure -as opposed to obtain a lower limit on -the column density of HF for the first time. As in previous observations of HF toward the source G10.6-0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH) − N(H 2 ) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H 2 is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H 2 column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of ∼24 km s −1 , that had not been identified in molecular absorption line studies prior to the launch of Herschel.