thesis - IRS, The Infrared Spectrograph

92 CHAPTER 5: Probing AGB nucleosynthesis via accurate Planetary Nebula abundancesFigure 5.8–. The same as in Fig. 5.7, but expressing the abundance data with different combinationsof elemental ratios.would be flatter 2 than the present one (connected triangles in Fig. 5.11). But this casecould not be rejected either since corresponding predictions are still confined withinthe observed domain.5.6.4 PNe with extremely high He abundanceBelow we discuss the PNe exhibiting the largest enrichment in helium, i.e. with 0.14 He/H 0.20 (see e.g. Fig. 5.3). These objects share other chemical properties, namely:• Marked carbon deficiency compared to the solar value;• Sizeable enhancement of nitrogen, but not exceeding the upper values observed in thePNe with lower He/H;• Significant depletion of oxygen, which makes these PNe appear as a distinct groupwith respect to the PNe with lower He/H abundances;• Possible, but still not compelling, hint of over-abundance of neon compared to the solarvalue.By looking at Fig. 5.5, we conclude that model predictions after the first and seconddredge-up processes cannot account for the extremely high He/H values of these PNe. Then,2 In any case Ne/H should keep a slightly increasing trend, not becoming exactly horizontal since, even if Ne=const., the hydrogen content H decreases as a consequence of dredge-up events.