  The interstellar medium is enriched primarily by matter ejected from evolved
low and intermediate mass stars. The outflows from these stars create a
circumstellar envelope in which a rich gas-phase and dust-nucleation chemistry
takes place. We observed the nearest carbon-rich evolved star, IRC+10216, using
the PACS (55-210 {\mu}m) and SPIRE (194-672 {\mu}m) spectrometers on board
Herschel. We find several tens of lines from SiS and SiO, including lines from
the v=1 vibrational level. For SiS these transitions range up to J=124-123,
corresponding to energies around 6700K, while the highest detectable transition
is J=90-89 for SiO, which corresponds to an energy around 8400K. Both species
trace the dust formation zone of IRC+10216, and the broad energy ranges
involved in their detected transitions permit us to derive the physical
properties of the gas and the particular zone in which each species has been
formed. This allows us to check the accuracy of chemical thermodynamical
equilibrium models and the suggested depletion of SiS and SiO due to accretion
onto dust grains.
