Probing AGB nucleosynthesis via accurate Planetary Nebula abundances

The elemental abundances of ten planetary nebulae, derived with high accuracy including ISO and IUE spectra, are analysed with the aid of synthetic evolutionary models for the TP-AGB phase. The accuracy on the observed abundances is essential in order to make a reliable comparison with the models. The advantages of the infrared spectra in achieving this accuracy are discussed. Model prescriptions are varied until we achieve the simultaneous reproduction of all elemental features, which allows placing important constraints on the characteristic masses and nucleosynthetic processes experienced by the stellar progenitors. First of all, it is possible to separate the sample into two groups of PNe, one indicating the occurrence of only the third dredge-up during the TP-AGB phase, and the other showing also the chemical signature of hot-bottom burning. The former group is reproduced by stellar models with variable molecular opacities (see Marigo \cite{Marigo2002}), adopting initial solar metallicity, and typical efficiency of the third dredge-up, lambda ~ 0.3-0.4. The latter group of PNe, with extremely high He content (0.15 <=He/H <=0.20) and marked oxygen deficiency, is consistent with original sub-solar metallicity (i.e. LMC composition). Moreover, we are able to explain quantitatively both the N/H-He/H correlation and the N/H-C/H anti-correlation, thus solving the discrepancy pointed out long ago by Becker & Iben (\cite{Becker1980}). This is obtained only under the hypothesis that intermediate-mass TP-AGB progenitors (M >~ 4.5-5.0 Msun) with LMC composition have suffered a number of very efficient, carbon-poor, dredge-up events. Finally, the neon abundances of the He-rich PNe can be recovered by invoking a significant production of 22Ne during thermal pulses, which would imply a reduced role of the 22Ne(alpha , n)25Mg reaction as neutron source to the s-process nucleosynthesis in these stars.