Formation and destruction of SiS in space
  The presence of SiS in space seems to be restricted to a few selected types
of astronomical environments.
  It is long known to be present in circumstellar envelopes around evolved
stars and it has also been detected in a handful of star-forming regions with
evidence of outflows, like Sgr\,B2, Orion\,KL and more recently L1157-B1.
  The kinetics of reactions involving SiS is very poorly known and here we
revisit the chemistry of SiS in space by studying some potentially important
reactions of formation and destruction of this molecule. We calculated {\it ab
initio} potential energy surfaces of the SiOS system and computed rate
coefficients in the temperature range 50-2500 K for the reaction of destruction
of SiS, in collisions with atomic O, and of its formation, through the reaction
between Si and SO. We find that both reactions are rapid, with rate
coefficients of a few times 10$^{-10}$ cm$^3$ s$^{-1}$, almost independent of
temperature.
  In the reaction between Si and SO, SiO production is 5-7 times more efficient
than SiS formation.
  The reaction of SiS with O atoms can play an important role in destroying SiS
in envelopes around evolved stars. We built a simple chemical model of a
postshock gas to study the chemistry of SiS in protostellar outflows and we
found that SiS forms with a lower abundance and later than SiO, that SiS is
efficiently destroyed through reaction with O, and that the main SiS-forming
reactions are Si + SO and Si + SO$_2$.
