thesis - IRS, The Infrared Spectrograph

5.7. Summary and conclusions 103Figure 5.12–. The same as in Fig. 5.11, but expressing the abundance data with different combinationsof elemental ratios.5.7 Summary and conclusionsIn this study a sample of PNe with accurately determined elemental abundances, is usedto derive interesting information about the evolution of the stellar progenitors, and setconstraints on the nucleosynthesis and mixing processes characterising their previousevolution. To this aim, synthetic TP-AGB models are calculated to reproduce the data byvarying the parameters: initial stellar mass and metallicity, molecular opacities, dredge-upand HBB efficiency, and chemical composition of the convective inter-shell developed atthermal pulses.The clear segregation of the abundance data in two sub-samples, particularly evident inthe O/H – He/H diagram, has led us to discuss them separately. And indeed, our investigationsuggests two different interpretative scenarios. The final results that best reproduce theobserved data are summarised in Figs. 5.11 and 5.12.From the analysis of the group of PNe with low He content (He/H< 0.145) and solar-likeoxygen abundances, we conclude that:• The stellar progenitors are low- and intermediate-mass stars with original solar-likechemical composition and initial masses spanning the range 0.9 − 4.0 M ⊙ .• The oxygen abundances are consistent with the recent determination for the Sun byAllende Prieto et al. (2001), that is lower than previous estimates (see e.g. Anders