  We investigate the effects of molecular opacities on the evolution of TP-AGB
stars that experience the third dredge-up, i.e. with surface abundances of
carbon and oxygen varying with time. To this aim, a routine is constructed to
derive the molecular concentrations through dissociation equilibrium
calculations, and estimate the opacities due to H2, H2O, OH, C2, CN, and CO for
any given density, temperature, and chemical composition of the gas. Then,
synthetic TP-AGB models with dredge-up are calculated by either 1) adopting the
newly developed routine, or 2) interpolating between fixed opacity tables for
solar chemical composition, which is the usual prescription in AGB evolution
calculations. The comparison between the two cases shows that the change in the
dominant opacity sources, as the C/O ratio grows from below to above unity,
crucially affects the evolution of the effective temperature, i.e. causing a
notable cooling of the carbon-rich models (with C/O>1). From the comparison
with observational data, it turns out that TP-AGB models with variable
molecular opacities are able to reproduce the observed range of effective
temperatures, mass-loss rates, and wind expansion velocities of C-type giants
in the solar neighbourhood, otherwise failed if assuming fixed molecular
opacities for solar-scaled mixtures. Finally, we mention other important
evolutionary and observational effects that result from the adoption of the
variable opacities, such as: i) significant shortening of the C-star phase due
to the earlier onset of the super-wind; ii) consequent reduction of the carbon
yields; iii) reproduction of the observed range of near-infrared colours of
C-stars.
