thesis - IRS, The Infrared Spectrograph

74 CHAPTER 5: Probing AGB nucleosynthesis via accurate Planetary Nebula abundancesNGC 6537NGC 6302NGC 7027NGC 6445NGC 2440He 2-111NGC 6741NGC 7662NGC 6543NGC 5315Figure 5.1–. HR diagram for the PNe of the sample (diamonds). The T eff have been derived with theZanstra method using the helium lines except for NGC 5315 (open diamond). The Post-AGB evolutionarytracks from Vassiliadis & Wood (1994) for Z=0.016 are also plotted for different core masses,indicated in the lower-right corner of the figure. In the lower-left the uncertainty in the luminosity dueto the error of a factor two in the distance is shown.5.2.2 HR diagramWith the data in Table 5.1 and the Hβ and helium λ4686Å fluxes the Zanstra temperatures(T Z ), radii and luminosities have been derived (see Table 5.2). As pointed out by Stasińska &Tylenda (1986) when using the Zanstra method, T Z is over-estimated in the case of hydrogenand underestimated when using helium. This is because the Zanstra method assumes thatenergies above 54.4 eV are only absorbed by helium. This is not completely true. Inaddition recombination of He 2+ sometimes produces more than one photon which can ionizehydrogen and the proportion of stellar photons with energies above 54.4 increases with T eff .Both T Z (He) and T Z (H) yield the same results for most PNe. T Z (H) fails when the nebula isthin and some photons escape. In the case of a thick nebula both methods should yield thesame result, but this is tricky because a nebula can be thick in the torus and thin in the poles.For all those reasons the T Z (He) was preferred over T Z (H).These results are shown in Fig. 5.1. For NGC 5315 no helium line is detected so thatresults using T Z (H) have been plotted. The evolutionary tracks of Vassiliadis & Wood (1994)