Comparative Chemistry of Diffuse Clouds III: Sulfur-bearing Molecules

Using the Plateau de Bure Interferometer and IRAM 30m Telescope, we observed λ2-3mm absorption lines of CS, SO, SO2, H2S and HCS + from some of the diffuse clouds which occult our well-studied sample of compact extragalactic mm-wave continuum sources. Our observations of SO, H2S and HCS + represent the first detections of these species in diffuse clouds; SO2 was not detected at all. We find a typical value N(CS) ≈ 0.5− 2.0× 10 cm which is actually much smaller than the values derived previously, along very different lines of sight, from interpretation of CS J=2-1 emission lines. CS forms somewhat sluggishly and is occasionally absent even in features with appreciable N(HCO). But for lines of sight where CS is found, N(CS)/N(HCO) ≈ 2 ± 1 or X(CS) ≈ 4×10. N(CS) correlates well and varies about linearly with N(HCN) (N(CS)/N(HCN) = 0.7±0.3) though, again, lines of sight with appreciable N(HCN) occasionally lack CS. CS correlates well with H2S (N(CS)/N(H2S) = 6± 1) and marginally with SO (N(CS)/N(SO) = 1.7± 0.8). For the two high column density features observed toward 3C111 we find N(CS)/N(HCS) = 13.3±1.0. CS can easily be shown to form from the observed amounts of HCS via electron recombination in cool, quiescent gas but the obvious gas-phase routes to formation of HCS fail by factors of 25 or more.