We have observed five sulphur-bearing molecules in foreground diffuse molecular clouds lying along the sight-lines to five bright continuum sources .
We have used the GREAT instrument on SOFIA to observe the SH 1383 GHz ^ { 2 } \Pi _ { 3 / 2 } J = 5 / 2 \leftarrow 3 / 2 lambda doublet towards the star-forming regions W31C , G29.96–0.02 , G34.3+0.1 , W49N and W51 , detecting foreground absorption towards all five sources ; and the EMIR receivers on the IRAM 30m telescope at Pico Veleta to detect the H _ { 2 } S 1 _ { 10 } -1 _ { 01 } ( 169 GHz ) , CS J = 2 - 1 ( 98 GHz ) and SO 3 _ { 2 } -2 _ { 1 } ( 99 GHz ) transitions .
Upper limits on the H _ { 3 } S ^ { + } 1 _ { 0 } -0 _ { 0 } ( 293 GHz ) transitions were also obtained at the IRAM 30 m. In nine foreground absorption components detected towards these sources , the inferred column densities of the four detected molecules showed relatively constant 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 .
The column densities of the sulphur-bearing molecules are very well correlated amongst themselves , moderately well correlated with CH ( a surrogate tracer for H _ { 2 } ) , and poorly correlated with atomic hydrogen .
The observed SH/H _ { 2 } ratios – in the range 5 to 26 \times 10 ^ { -9 } – indicate that SH ( and other sulphur-bearing molecules ) account for \ll 1 \% of the gas-phase sulphur nuclei .
The observed abundances of sulphur-bearing molecules , however , greatly exceed those predicted by standard models of cold diffuse molecular clouds , providing further evidence for the enhancement of endothermic reaction rates by elevated temperatures or ion-neutral drift .
We have considered the observed abundance ratios in the context of shock and turbulent dissipation region ( TDR ) models .
Using the TDR model , we find that the turbulent energy available at large scale in the diffuse ISM is sufficient to explain the observed column densities of SH and CS .
Standard shock and TDR models , however , fail to reproduce the column densities of H _ { 2 } S and SO by a factor of about 10 ; more elaborate shock models – in which account is taken of the velocity drift , relative to H _ { 2 } , of SH molecules produced by the dissociative recombination of H _ { 3 } S ^ { + } – reduce this discrepancy to a factor \sim 3 .