Figure 5.12–. The same as in Fig. 5.11, but expressing the abundance data with different combinations of elemental ratios.
5.7 Summary and conclusions
In this study a sample of PNe with accurately determined elemental abundances, is used to derive interesting information about the evolution of the stellar progenitors, and set constraints on the nucleosynthesis and mixing processes characterising their previous evolution. To this aim, synthetic TP-AGB models are calculated to reproduce the data by varying the parameters: initial stellar mass and metallicity, molecular opacities, dredge-up and HBB efficiency, and chemical composition of the convective inter-shell developed at thermal pulses.
The clear segregation of the abundance data in two sub-samples, particularly evident in the O/H – He/H diagram, has led us to discuss them separately. And indeed, our investigation suggests two different interpretative scenarios. The final results that best reproduce the observed data are summarised in Figs. 5.11 and 5.12.
From the analysis of the group of PNe with low He content (He/H< 0.15) and solar-like oxygen abundances, we conclude that:
• The stellar progenitors are low- and intermediate-mass stars with original solar-like chemical composition and initial masses spanning the range0.9 − 4.0 M .
• The oxygen abundances are consistent with the recent determination for the Sun by Allende Prieto et al. (2001), that is lower than previous estimates (see e.g. Anders
104 CHAPTER5: Probing AGB nucleosynthesis via accurate Planetary Nebula abundances
& Grevesse 1989) by almost 0.2 dex. For a few PNe there may be a limited oxygen enrichment, possibly associated with dredge-up during the TP-AGB phase.
• There is clear evidence of carbon enrichment in some PNe that also exhibit C/O> 1, suggesting that they evolved from carbon stars experiencing the third dredge-up during the TP-AGB phase.
• Measured carbon abundances are well reproduced by TP-AGB models with dredge-up efficienciesλ ∼ 0.3 − 0.4, and adopting variable molecular opacities in place of the usual solar-scaled opacity tables (see Marigo 2002). The introduction of variable opacities prevents the likely over-enrichment of carbon by shortening the duration of the carbon-star phase, and causing an earlier shut-down of the third dredge-up due to the cooling of the envelope structure (Marigo 2003).
• The degree of nitrogen enrichment is consistent with the expectations from the first and second dredge-up events, occurred prior to the TP-AGB phase. The efficiency of HBB in intermediate-mass stars with solar-metallicity should be modest.
• Helium abundances are well accounted for by considering the whole contribution of all dredge-up processes (i.e. first, and possibly second and third).
From the study of the extremely helium-rich (0.15 ≤He/H≤ 0.20) and oxygen-poor PNe we can conclude the following:
• The stellar progenitors should be intermediate-mass stars (4−5 M ) experiencing both the third dredge-up and HBB during their TP-AGB evolution.
• The PN oxygen abundances are consistent with a sub-solar initial stellar metallicity.
In fact, under the hypothesis of solar metallicity we are forced to invoke a significant oxygen destruction via very efficient HBB, which violates other chemical constraints, e.g. causing a large over-production of nitrogen. Instead, models with assumed initial LMC composition provide a fairly good agreement with the data.
• The first two assumptions are needed in the models to reproduce the observed abun-dances. This leads to a controversy which is important to point out. The combination of low metallicity with intermediate-mass progenitors is peculiar, since these stars are probably recently formed from gas with interstellar abundances. Although there are several indications that these PNe are of lower metallicity (see Sec. 5.6.4), this could also suggest that perhaps another physical process has not fully been taken into ac-count. This issue should be further investigated.
• The long-standing problem – initially formulated by Becker & Iben (1980) – of ac-counting, simultaneously and quantitatively, for the observed N/O–He/H correlation and the C/O–N/O anti-correlation seems to be solved by assuming that the third dredge-up i) is very efficient, and ii) brings dredge-up to the surface material containing only a small amount of primary carbon synthesised during thermal pulses. A good agreement with the observed data is obtained by adoptingλ ∼ 0.9 and Xcsh(12C) ∼ 0.02 − 0.03.
The former indication on the dredge-up efficiency, derived empirically, is supported on theoretical grounds by full TP-AGB calculations of intermediate-mass stars, i.e. Vas-siliadis & Wood (1993), and more recently Frost et al. (1998) and Siess et al. (2002).
5.7. Summary and conclusions 105
A significant part of the helium enrichment of these PNe should be ascribed to a large number of deep dredge-up events that precede the occurrence of the so-called “de-generate pulses”, according to the designation introduced by Frost et al. (1998). The additional requirement emerging from our study – that such dredge-up events should not only be extremely deep but also carry a small amount of carbon – is not fully con-firmed by theoretical analyses (i.e. Frost et al. 1998), though a positive indication in this sense is given by the work of Vassiliadis & Wood (1993).
• A significant production of 22Ne – via α-captures starting from14N – should take place in these stars to reproduce the observed Ne/H values of the He-rich PNe, under the hypothesis they descend from intermediate-mass stars with initial LMC chemical composition. As direct consequence, this would imply a reduced role of the22Ne(α, n)25Mg reaction in providing neutrons for the slow-neutron capture nucleosyntesis that is expected to occur during thermal pulses.
We also note that the invoked inefficiency of the22Ne(α, n)25Mg channel seems con-sistent with the expected low synthesis of carbon at thermal pulses in the most massive AGB stars experiencing very deep dredge-up (see former point). In fact, as reported by Vassiliadis & Wood (1993) the deep dredge-up quickly extinguishes the helium burning shell. As a consequence this could prevent both a significant production of carbon via the triple-α reaction and the attainment of the high temperatures required for the full activation of the22Ne(α, n)25Mg reaction. The confirmation of this two-fold aspect deserves detailed calculations of thermal pulses.
Acknowledgements
P.M. acknowledges the SRON National Institute for Space Research (Groningen) for hospitality and financial support during her visit in October 2001, and the Italian Ministry of Education, University and Research (MIUR) for the work carried out at the Astronomy Department in Padova. J. Bernard-Salas thanks Annette Ferguson for valuable discussions and suggestions as well as the Universit `a di Padova for hospitality. We are grateful to Dr. John Lattanzio for the careful reading of this manuscript, and for providing comments and suggestions that result in an improvement of the paper.
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