Evolution of AGB stars at varying surface C/O ratio: the crucial effect of molecular opacities

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, hence surface C/O ratio, varying with time. It turns out that in order to correctly model the properties of carbon stars (with C/O>1) the use of solar-scaled opacities - the standard choice in present stellar evolution models - should be relaxed and replaced with that of opacities properly coupled to the actual surface abundances. Only in this way we could be able to reproduce basic observables of carbon stars, like their effective temperatures, C/O ratios, and near-infrared colours.