In Fig. 10 we compared the values of the effective temperature and luminosity obtained from our two models listed in Table 6 to evolutionary tracks of hydrogen-burning and helium-burning models calculated by Blöcker (1995). We compared the post-AGB age of these different models with the dynamical age of the ring found in §3. The kinematic analysis indicates that the nebula was ejected about 23400-26300 yr ago. The post-AGB age of the hydrogen-burning model (left-hand panel in Fig. 10) is considerably shorter than the nebula's age, suggesting that the helium-burning model (VLTP; right-hand panel in Fig. 10) may be favoured to explain the age.
The physical parameters of the two A-type stars also yield a further constraint. The stellar evolutionary tracks of the rotating models for solar metallicity calculated by Ekström et al. (2012) imply that the A-type stars, both with masses close to and K, have ages of Myr. We see that they are in the evolutionary phase of the “blue hook”; a very short-lived phase just before the Hertzsprung gap. Interestingly, the initial mass of found for the ionizing source has the same age. As previously suggested by Exter et al. (2010), the PN progenitor with an initial mass slightly greater than can be coeval with the A-type stars, and it recently left the AGB phase. But, they adopted the system age of about 520 Myr according to the Y evolutionary tracks (Yi et al., 2003; Demarque et al., 2004).
The effective temperature and stellar luminosity obtained for both models correspond to the progenitor mass of . However, the strong nitrogen enrichment seen in the nebula is inconsistent with this initial mass, so another mixing process rather than the hot-bottom burning (HBB) occurs at substantially lower initial masses than the stellar evolutionary theory suggests for AGB-phase (Karakas et al., 2009; Herwig, 2005; Karakas & Lattanzio, 2007). The stellar models developed by Karakas & Lattanzio (2007) indicate that HBB occurs in intermediate-mass AGB stars with the initial mass of for the metallicity of ; and for -. However, they found that a low-metallicity AGB star () with the progenitor mass of can also experience HBB. Our determination of the argon abundance in SuWt 2 (see Table6) indicates that it does not belong to the low-metallicity stellar population; thus, another non-canonical mixing process made the abundance pattern of this PN.
The stellar evolution also depends on the chemical composition of the progenitor, namely the helium content () and the metallicity (), as well as the efficiency of convection (see e.g. Salaris & Cassisi, 2005). More helium increases the H-burning efficiency, and more metallicity makes the stellar structure fainter and cooler. Any change in the outer layer convection affects the effective temperature. There are other non-canonical physical processes such as rotation, magnetic field and mass-loss during Roche lobe overflow (RLOF) in a binary system, which significantly affect stellar evolution. Ekström et al. (2012) calculated a grid of stellar evolutionary tracks with rotation, and found that N/H at the surface in rotating models is higher than non-rotating models in the stellar evolutionary tracks until the end of the central hydrogen- and helium-burning phases prior to the AGB stage. The Modules for Experiments in Stellar Astrophysics (MESA) code developed by Paxton et al. (2013); Paxton et al. (2011) indicates that an increase in the rotation rate (or angular momentum) enhances the mass-loss rate. The rotationally induced and magnetically induced mixing processes certainly influence the evolution of intermediate-mass stars, which need further studies by MESA. The mass-loss in a binary or even triple system is much more complicated than a single rotating star, and many non-canonical physical parameters are involved (see e.g. BINSTAR code by Siess, 2006; Siess et al., 2013). Chen & Han (2002) used the Cambridge stellar evolution (STARS) code developed by Eggleton (1972); Eggleton (1971); Eggleton (1973) to study numerically evolution of Population I binaries, and produced a helium-rich outer layer. Similarly, Benvenuto & De Vito (2003); Benvenuto & De Vito (2005) developed a helium white dwarf from a low mass progenitor in a close binary system. A helium enrichment in the our layer can considerably influence other elements through the helium-burning mixing process.