thesis - IRS, The Infrared Spectrograph

70 CHAPTER 5: Probing AGB nucleosynthesis via accurate Planetary Nebula abundances5.1 IntroductionPlanetary Nebulae (PNe) are assumed to consist of the gas ejected via stellar winds bylow- and intermediate-mass stars (having initial masses 0.9 ≤ M/M ⊙ ≤ M up , withM up ∼ 5 − 8M ⊙ depending on model details) during their last evolutionary stages, theso-called Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase.PNe offer potentially a good possibility to test the results of stellar nucleosynthesis. Thiscan be done in a reliable way by comparing the predicted abundances of the gas ejected closeto the end of the AGB phase with the observed abundances because the expelled hot gas remainsunaffected by interaction with the ISM or with previous shell ejection. Furthermore theionized gas surrounding the central star shows lines of many elements from which accurateabundances can be derived. Also by the ejection of the outer layers PNe contribute to theenrichment of the interstellar medium (ISM) and therefore, a knowledge of these processesis essential to better understand the chemical composition of the Galaxy.PN elemental abundances represent the cumulative record of all nucleosynthetic andmixing processes that may have changed the original composition of the gas since the epochof stellar formation. In fact, stellar evolution models predict the occurrence of severalepisodes in which the envelope chemical composition is altered by mixing with nuclearproducts synthesised in inner regions and brought up to the surface by convective motions(e.g. Iben & Renzini 1983; Forestini & Charbonnel 1997; Girardi et al. 2000). Thesedredge-up events usually take place when a star reaches its Hayashi line and develops anextended convective envelope, either during the ascent on the Red Giant Branch (RGB; thefirst dredge-up), or later on the early AGB (the second dredge-up). Then, the subsequentTP-AGB evolution is characterised by a rich nucleosynthesis whose products may berecurrently exposed to the surface synchronised with thermal pulses (the third dredge-up), orconvected upward from the deepest envelope layers of the most massive stars (hot-bottomburning, hereinafter also HBB).As a consequence, the surface abundances of several elements (e.g. H, He, C, N,O, Ne, Mg) may be significantly altered, to an extent that crucially depends on stellarparameters (i.e. mass and metallicity) various incompletely understood physical processes(e.g. convection, mass loss), and model input prescriptions (e.g. nuclear reaction rates,opacities, etc.). In this sense, the interpretation of the elemental patterns observed in PNeshould give a good insight into the evolutionary and nucleosynthetic properties of the stellarprogenitors, thus putting constraints on these processes.For instance, the enrichment in C exhibited by some PNe should give a measure of theefficiency of the third dredge-up, still a matter of debate. Conversely, the deficiency in Cshown by some PNe with N overabundance could be interpreted as the imprint of HBB,implying rather massive TP-AGB stellar progenitors (with initial masses ≥ 4 M ⊙ ). The Hecontent should measure the cumulative effect produced by the first, possibly second and thirddredge-up, and HBB. The O abundance may help to constrain the chemical compositionof the convective inter-shell developed at thermal pulses, as well as the efficiency of HBB,depending on whether this element is found to be preserved, enhanced or depleted in thenebular composition. Finally, the Ne abundance in PNe could give important information