Dust properties in the circumstellar shells of evolved stars: Observational constraints from ISO and Spitzer infrared spectroscopy

We present the results of a systematic analysis of the solid state features identified in the circumstellar environments of a large sample of evolved stars with ISO/SWS and Spitzer/IRS spectroscopy. The sample includes several hundred stars with a wide variety of progenitor masses evolving from the early AGB phase to the PN stage. Our observations are used to propose an evolutionary scheme in which the results obtained can be interpreted as a consequence of the nucleosynthesis processes that take place in this short phase of the stellar evolution, in particular the third dredge-up and hot bottom burning, which in turn are also strongly modulated by the stellar metallicity.