Probing AGB nucleosynthesis via accurate Planetary Nebula abundances

Abstract

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 2002), adopting initial solar metallicity, and typical efficiency of the third dredge-up, λ ∼ 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 (1980). This is obtained only under the hypothesis that intermediate- mass TP-AGB progenitors (M > 4.5−5.0 M� ) 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 22 Ne during thermal pulses, which would imply a reduced role of the 22 Ne(α, n) 25 Mg reaction as neutron source to the Planetary Nebulae (PNe) are assumed to consist of the gas ejected via stellar winds by low- and intermediate-mass stars (having initial masses 0.9 ≤ M/M� ≤ Mup, with Mup ∼ 5−8 Mdepending on model details) during their last evo- lutionary stages, the so-called Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase. PNe offer potentially a good possibility to test the results of stellar nucleosynthesis. This can be done in a reliable way by comparing the predicted abundances of the gas ejected close to the end of the AGB phase with the observed abundances because the expelled hot gas remains unaffected by interac- tion with the ISM or with previous shell ejection. Furthermore the ionized gas surrounding the central star shows lines of many elements from which accurate abundances can be de- rived. Also by the ejection of the outer layers PNe contribute to the enrichment of the interstellar medium (ISM) and therefore,