thesis - IRS, The Infrared Spectrograph

5Probing AGB nucleosynthesis viaaccurate Planetary NebulaabundancesBased on:P. Marigo, J. Bernard-Salas, S.R. Pottasch, A.G.G.M. Tielens, & P.R. WesseliusASTRONOMY & ASTROPHYSICS, IN PRESS (2003)THE elemental abundances of ten planetary nebulae, derived with high accuracy includingISO and IUE spectra, are analysed with the aid of synthetic evolutionary models for theTP-AGB phase. The accuracy on the observed abundances is essential in order to make areliable comparison with the models. The advantages of the infrared spectra in achieving thisaccuracy are discussed. Model prescriptions are varied until we achieve the simultaneousreproduction of all elemental features, which allows placing important constraints on thecharacteristic 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 theoccurrence of only the third dredge-up during the TP-AGB phase, and the other showingalso the chemical signature of hot-bottom burning. The former group is reproduced by stellarmodels 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 (0.145 ≤He/H≤ 0.20) and marked oxygen deficiency, isconsistent with original sub-solar metallicity (i.e. LMC composition). Moreover, we are ableto 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 obtainedonly 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-upevents. Finally, the neon abundances of the He-rich PNe can be recovered by invoking asignificant production of 22 Ne during thermal pulses, which would imply a reduced role ofthe 22 Ne(α, n) 25 Mg reaction as neutron source to the s-process nucleosynthesis in these stars.