thesis - IRS, The Infrared Spectrograph

5.6. Comparison between models and observations 93we are led to consider additional He contributions from the third dredge-up and HBB duringthe TP-AGB phase.Qualitatively, the enrichment in helium and nitrogen and the simultaneous deficiency incarbon would naturally point to HBB as a possible responsible process, via the CNO-cyclereactions. Therefore, as a working hypothesis, let us assume that the stellar progenitors ofthese extremely He-rich PNe are intermediate-mass stars (M 4.5 M ⊙ ), experiencing HBBduring their AGB evolution. We will now investigate under which conditions all elementalfeatures can be reproduced.To allow an easier understanding of the following analysis, Figs. 5.9 and 5.10 show thepredicted time evolution of He, C, N, O, and Ne elemental abundances in the envelope duringthe TP-AGB phase of models experiencing HBB. Different assumptions are explored.The observed PN abundances should be compared with the last starred point along the theoreticalcurves, which marks the last event of mass ejection, and it may be then consideredrepresentative of the expected PN abundances.Constraints from oxygen and sulfur abundancesAt this point additional information comes from the marked oxygen under-abundancecompared to solar (for both High and Low values), common to the extremely He-rich PNe.We recall that the oxygen abundance in the envelope remains essentially unchanged afterthe first and second dredge-up events. The third dredge-up may potentially increase oxygen,depending on the chemical composition of the convective inter-shell that forms at thermalpulses. In any case, no oxygen depletion is expected by any of these processes. A destructionof oxygen could be caused by a very efficient HBB, that is if the ON cycle is activated andoxygen starts being transformed into nitrogen.We have explored this possibility on a 5M ⊙ TP-AGB model with original solar metallicity.To analyse the effects of a larger HBB efficiency, the mixing-length parameter α MLhas been increased, and set equal to 1.68, 2.00, and 2.50. In fact, larger values of α MLcorrespond to higher temperatures at the base of the convective envelope. In none of thethree cases have we found any hint of oxygen destruction, as indicated by the flat behaviourof the abundance curves in the bottom-left panel of Fig. 5.9 (solid and long-dashed lines, forα = 1.68 and 2.50, respectively).At this point we decided to stop further increasing α – which would have likely ledto oxygen destruction at some point – since we run into a major discrepancy. In fact,increasing the efficiency of HBB causes a systematic over-enrichment in nitrogen, as shownby the model with α ML = 2.50 (short-dashed line). We also note that a significant nitrogenproduction is accompanied by a mirror-like destruction of carbon (upper-left panel ofFig. 5.9). This is not the case of the model with α ML = 1.68 (solid line), in which HBB isalmost inoperative.From these results we can expect that, even if a destruction of oxygen is obtained forlarger values of α ML , the problem of nitrogen over-production would become even more