Photoprocessing of H2S on dust grains: building S chains in translucent
  clouds and comets
  Context. Sulfur is used as a tracer of the evolution from interstellar clouds
to stellar systems. However, most of the expected sulfur in molecular clouds
remains undetected. Sulfur disappears from the gas phase in two steps. One
first depletion occurs during the translucent phase, reducing the gas phase
sulfur by 7-40 times, while the following freeze-out step occurs in molecular
clouds, reducing it by another order of magnitude. This long-standing dilemma
awaits an explanation. Aims. The aim of this study is to understand under which
form the missing sulfur is hiding in molecular clouds. Depletion onto dust
grains is considered. Methods. Experimental simulations mimicking H2S ice
UV-photoprocessing in molecular clouds were conducted. The ice was monitored
using infrared spectroscopy and the desorbing molecules were measured by
quadrupole mass spectrometry. Theoretical Monte Carlo simulations were
performed for interpretation of the experimental results and extrapolation to
astrophysical conditions. Results. H2S2 formation was observed during
irradiation at 8 K. Molecules H2Sx with x > 2 were also identified and found to
desorb during warm-up, along with S2 to S4 species. Larger Sx molecules up to
S8 are refractory at room temperature and remained on the substrate forming a
residue. Monte Carlo simulations were able to reproduce the molecules desorbing
during warming up, and found that residues are chains or sulfur consisting of
6-7 atoms. Conclusions. We propose that S+ in translucent clouds contributes
notoriously to S depletion in denser regions by forming long S-chains on dust
in few times 10^4 years. We suggest that the S2 to S4 molecules observed in
comets are not produced by fragmentation of these large chains. Instead, they
probably come either from UV-photoprocessing of H2S-bearing ice produced in
molecular clouds or from short S chains formed during the translucent cloud
phase
