Molecular abundances in the inner layers of IRC +10216
  Observations towards IRC +10216 of CS, SiO, SiS, NaCl, KCl, AlCl, AlF, and
NaCN have been carried out with the IRAM 30-m telescope in the 80-357.5 GHz
frequency range. A large number of rotational transitions covering a wide range
of energy levels, including highly excited vibrational states, are detected in
emission and serve to trace different regions of the envelope. Radiative
transfer calculations based on the LVG formalism have been performed to derive
molecular abundances from the innermost out to the outer layers. The excitation
calculations include infrared pumping to excited vibrational states and
inelastic collisions, for which up-to-date rate coefficients for rotational
and, in some cases, ro-vibrational transitions are used. We find that in the
inner layers CS, SiO, and SiS have abundances relative to H$_2$ of 4e-6,
1.8e-7, and 3e-6, respectively, and that CS and SiS have significant lower
abundances in the outer envelope, which implies that they actively contribute
to the formation of dust. Moreover, in the inner layers, the amount of sulfur
and silicon in gas phase molecules is only 27 % for S and 5.6 % for Si,
implying that these elements have already condensed onto grains, most likely in
the form of MgS and SiC. Metal-bearing molecules lock up a relatively small
fraction of metals, although our results indicate that NaCl, KCl, AlCl, AlF,
and NaCN, despite their refractory character, are not significantly depleted in
the cold outer layers. In these regions a few percent of the metals Na, K, and
Al survive in the gas phase, either in atomic or molecular form, and are
therefore available to participate in the gas phase chemistry in the outer
envelope.
