thesis - IRS, The Infrared Spectrograph

5.7. Summary and conclusions 105A significant part of the helium enrichment of these PNe should be ascribed to a largenumber of deep dredge-up events that precede the occurrence of the so-called “degeneratepulses”, according to the designation introduced by Frost et al. (1998). Theadditional requirement emerging from our study – that such dredge-up events shouldnot only be extremely deep but also carry a small amount of carbon – is not fully confirmedby theoretical analyses (i.e. Frost et al. 1998), though a positive indication inthis sense is given by the work of Vassiliadis & Wood (1993).• A significant production of 22 Ne – via α-captures starting from 14 N – should takeplace in these stars to reproduce the observed Ne/H values of the He-rich PNe, underthe hypothesis they descend from intermediate-mass stars with initial LMC chemicalcomposition. As direct consequence, this would imply a reduced role of the 22 Ne(α,n) 25 Mg reaction in providing neutrons for the slow-neutron capture nucleosyntesis thatis expected to occur during thermal pulses.We also note that the invoked inefficiency of the 22 Ne(α, n) 25 Mg channel seems consistentwith the expected low synthesis of carbon at thermal pulses in the most massiveAGB stars experiencing very deep dredge-up (see former point). In fact, as reported byVassiliadis & Wood (1993) the deep dredge-up quickly extinguishes the helium burningshell. As a consequence this could prevent both a significant production of carbon viathe triple-α reaction and the attainment of the high temperatures required for the fullactivation of the 22 Ne(α, n) 25 Mg reaction. The confirmation of this two-fold aspectdeserves detailed calculations of thermal pulses.AcknowledgementsP.M. acknowledges the SRON National Institute for Space Research (Groningen) for hospitality andfinancial support during her visit in October 2001, and the Italian Ministry of Education, University andResearch (MIUR) for the work carried out at the Astronomy Department in Padova. J. Bernard-Salasthanks Annette Ferguson for valuable discussions and suggestions as well as the Università di Padovafor hospitality. We are grateful to Dr. John Lattanzio for the careful reading of this manuscript, and forproviding comments and suggestions that result in an improvement of the paper.