  It is generally considered, as a rule of thumb, that carbon monoxide forms
very early in envelopes of AGB stars, and that it consumes most of the carbon,
or most of the oxygen, depending on whether the photosphere is oxygen-rich or
carbon-rich, respectively. \rm This work focuses on the latter case, with the
purpose of quantifying the remaining fraction of gaseous carbon which is then
available for forming carbonaceous grains. Since AGB stars are (probably) the
main providers of cosmic carbon grains, this residual fraction is essential in
establishing the validity of current grain models. Here, we use a kinetic
treatment to follow the chemical evolution of circumstellar shells towards
steady state. It is shown that the residual fraction depends essentially on the
atomic ratio of pristine gaseous carbon and oxygen, and on the cross-section
for CH formation by collision of C and H atoms. It lies between 55 and 144 C
atoms per million H atoms, depending on the values adopted for unknown reaction
rates and cosmic C abundance. This is much larger than predicted by the rule of
thumb recalled above.
  The present results depend strongly on the rate of the reaction C+H-->CH: far
from thermodynamic equilibrium (which is the case here), CO cannot be formed if
this rate is as low as generally assumed. We have, therefore, estimated this
rate by chemically modelling the reaction and found it indeed much higher, and
high enough to yield CO abundances compatible with observations. An accurate
experimental rate determination is highly desirable.
