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We have observed five sulphur-bearing molecules in foreground diffusemolecular clouds lying along the sight-lines to five bright continuum sources.We have used the GREAT instrument on SOFIA to observe the 1383 GHz $^2\Pi_{3/2}J=5/2-3/2$ transitions of SH towards the star-forming regions W31C,G29.96-0.02, G34.3+0.1, W49N and W51, detecting foreground absorption towardsall five sources; and the EMIR receivers on the IRAM 30m telescope at PicoVeleta to detect the H$_2$S 1(10)-1(01), CS J=2-1 and SO 3(2)-2(1) transitions.In nine foreground absorption components detected towards these sources, theinferred column densities of the four detected molecules showed relativelyconstant ratios, with N(SH)/N(H$_2$S) in the range 1.1 - 3.0, N(CS)/N(H$_2$S)in the range 0.32 - 0.61, and N(SO)/N(H$_2$S) in the range 0.08 - 0.30. Theobserved SH/H$_2$ ratios - in the range (0.5-2.6) $\times 10^{-8}$ - indicatethat SH (and other sulphur-bearing molecules) account for << 1% of thegas-phase sulphur nuclei. The observed abundances of sulphur-bearing molecules,however, greatly exceed those predicted by standard models of cold diffusemolecular clouds, providing further evidence for the enhancement of endothermicreaction rates by elevated temperatures or ion-neutral drift. We haveconsidered the observed abundance ratios in the context of shock and turbulentdissipation region (TDR) models. Using the TDR model, we find that theturbulent energy available at large scale in the diffuse ISM is sufficient toexplain the observed column densities of SH and CS. Standard shock and TDRmodels, however, fail to reproduce the column densities of H$_2$S and SO by afactor of about 10; more elaborate shock models - in which account is taken ofthe velocity drift, relative to H$_2$, of SH molecules produced by thedissociative recombination of H$_3$S$^+$ - reduce this discrepancy to a factor~ 3.

Sulphur-bearing molecules in diffuse molecular clouds: new results from SOFIA/GREAT and the IRAM 30 m telescope