Comparative chemistry of diffuse clouds - III. Sulfur-bearing molecules

Using the Plateau de Bure Interferometer and IRAM 30 m Telescope, we observed lambda2-3 mm 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) approximate to 0.5-2.0 x 10(12) cm(-2) 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+) approximate to 2 +/- 1 orX(CS) approximate to 4 x 10(-9). 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 3C 111 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.