  Aim: We aim to determine the abundances of CS, SiO, and SiS in a large sample
of carbon star envelopes covering a wide range of mass loss rates to
investigate the potential role that these molecules could play in the formation
of dust in the surroundings of the central AGB star. Methods: We surveyed a
sample of 25 carbon-rich AGB stars in the $\lambda$ 2 mm band, using the IRAM
30 m telescope. We performed excitation and radiative transfer calculations
based on the LVG method to model the observed lines of the molecules and to
derive their fractional abundances in the observed CSEs. Results: We detected
CS in all 25 CSEs, SiO in 24 of them, and SiS in 17 sources. We found that CS
and SiS have similar abundances in carbon star envelopes, while SiO is present
with a lower abundance. We also found a strong correlation in which the denser
the envelope, the less abundant are CS and SiO. The trend is however only
tentatively seen for SiS in the range of high mass loss rates. Furthermore, we
found a relation in which the integrated flux of the MgS dust feature at 30 um
increases as the fractional abundance of CS decreases. Conclusions: The decline
in the fractional abundance of CS with increasing density could be due to
gas-phase chemistry in the inner envelope or to adsorption onto dust grains.
The latter possibility is favored by a correlation between the CS fractional
abundance and the 30 um feature, which suggests that CS is efficiently
incorporated onto MgS dust around C-rich AGB stars. In the case of SiO, the
observed abundance depletion with increasing density is most likely caused by
an efficient incorporation onto dust grains. We conclude that CS, SiO (very
likely), and SiS (tentatively) are good candidates to act as gas-phase
precursors of dust in C-rich AGB envelopes.
