Comparative chemistry of diffuse clouds

Using the Plateau de Bure Interferometer and IRAM 30 m Telescope, we observed λ2–3 mm absorption lines of CS, SO, SO2, S 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, S 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) – which is actually much smaller than the values derived previously, along very different lines of sight, from interpretation of CS –1 emission lines. CS forms somewhat sluggishly and is occasionally absent even in features with appreciable N(). But for lines of sight where CS is found, N(CS)/N() or X(CS) . N(CS) correlates well and varies about linearly with N(HCN) (N(CS)/N(HCN) = ) though, again, lines of sight with appreciable N(HCN) occasionally lack CS. CS correlates well with S (N(CS)/N(S) ) and marginally with SO (N(CS)/N(SO) = ). For the two high column density features observed toward 3C 111 we find N(CS)/N(HCS+) = 13.3 . 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.