  The spatial distribution of molecules in AGB circumstellar envelopes is
regulated by different processes. In the outer layers all molecules are
destroyed due to the interaction with interstellar ultraviolet photons. Here we
aim to characterize in a coherent and uniform way the radial extent of three
molecules (SiO, CS, and SiS) in envelopes around AGB stars of O- and C-rich
character, and to study their dependence with mass loss rate. To that purpose,
we used the Yebes 40m and IRAM 30m telescopes to observe 7 M-type and 7 C-type
AGB envelopes covering a wide range of mass loss rates (1e-7 - 1e-5 Msun/yr) in
lines of SiO, CS, and SiS spanning a range of upper level energies of 2-130 K.
We carried out excitation and radiative transfer calculations over a wide
parameter space to characterize the molecular abundance and radial extent. A
chi2 analysis indicates that the abundance is well constrained while the radial
extent is more difficult to constrain. The radial extent increases with
increasing envelope density, in agreement with previous observational findings.
At high envelope densities, Mdot/vexp > 1e-6 (Msun/yr)/(km/s), the radial
extent of SiO, CS, and SiS are similar, while at low envelope densities,
Mdot/vexp < 1e-7 (Msun/yr)/(km/s), the radial extent differ among the three
molecules, in agreement with theoretical expectations based on destruction due
to photodissociation. At low envelope densities we find a sequence of
increasing radial extent, SiS -> CS -> SiO. We also find a tentative dependence
of the radial extent with the chemical type (O- or C-rich) of the star for SiO
and CS. Interferometric observations and further investigation of the
photodissociation of SiO, CS, and SiS should allow to clarify the situation on
the relative photodissociation radius of SiO, CS, and SiS in AGB envelopes and
the dependence with envelope density and C/O ratio.
