6.2 Ionization and thermal structure

The volume-averaged fractional ionic abundances are listed in Table 10. We note that hydrogen and helium are singly-ionized. We see that the O$ ^{+}$/O ratio is higher than the N$ ^{+}$/N ratio by a factor of 1.34, which is dissimilar to what is generally assumed in the $ icf$ method. However, the O$ ^{2+}$/O ratio is nearly a factor of 1.16 larger than the Ne$ ^{2+}$/Ne ratio, in agreement with the general assumption for $ icf$(Ne). We see that only 19 per cent of the total nitrogen in the nebula is in the form of N$ ^{+}$. However, the total oxygen largely exists as O$ ^{2+}$ with 70 per cent and then O$ ^{+}$ with 26 per cent.

The elemental abundances we used for the photoionization model returns ionic abundances listed in Table 11, are comparable to those from the empirical analysis derived in Section 4. The ionic abundances derived from the observations do not show major discrepancies in He$ ^{+}$/H$ ^{+}$, C$ ^{2+}$/H$ ^{+}$, N$ ^{+}$/H$ ^{+}$, O$ ^{2+}$/H$ ^{+}$, Ne$ ^{2+}$/H$ ^{+}$ and Ar$ ^{2+}$/H$ ^+$; differences remain below 18 per cent. However, the predicted and empirical values of O$ ^{+}$/H$ ^{+}$, S$ ^{+}$/H$ ^+$ and S$ ^{2+}$/H$ ^+$ have discrepancies of about 45, 31 and 33 per cent, respectively.

Fig. 7(bottom) shows plots of the ionization structure of helium, carbon, oxygen, argon (left-hand panel), nitrogen, neon and sulphur (right-hand panel) as a function of radius along the equatorial direction. As seen, ionization layers have a clear ionization sequence from the highly ionized inner parts to the outer regions. Helium is 97 percent singly-ionized over the shell, while oxygen is 26 percent singly ionized and 70 percent doubly ionized. Carbon and nitrogen are about $ \sim20$ percent singly ionized $ \sim80$ percent doubly ionized. The distribution of N$ ^{+}$ is in full agreement with the IFU map, given in Fig4. Comparison between the He$ ^{+}$, O$ ^{2+}$ and S$ ^{+}$ ionic abundance maps obtained from our IFU observations and the ionic fractions predicted by our photoionization model also show excellent agreement.

Table 12: Mean electron temperatures (K) weighted by ionic species for the whole nebula obtained from the photoionization model.
H 9044 10194          
He 9027 10189 10248        
C 9593 9741 10236 10212 10209 10150 10150
N 8598 9911 10243 10212 10209 10150 10150
O 9002 10107 10237 10241 10211 10150 10150
Ne 8672 10065 10229 10225 10150 10150 10150
S 9386 9388 10226 10208 10207 10205 10150
Ar 8294 9101 10193 10216 10205 10150 10150

Table 12 lists mean temperatures weighted by the ionic abundances. Both [N II] and [O III] doublets, as well as HeI lines arise from the same ionization zones, so they should have roughly similar values. The ionic temperatures increasing towards higher ionization stages could also have some implications for the mean temperatures averaged over the entire nebula. However, there is a large discrepancy by a factor of 2 between our model and ORL empirical value of $ T_{\rm e}$(HeI$ )$. This could be due to some temperature fluctuations in the nebula (Peimbert, 1971; Peimbert, 1967). The temperature fluctuations lead to overestimating the electron temperature deduced from CELs. This can lead to the discrepancies in abundances determined from CELs and ORLs (see e.g. Liu et al., 2000). Nevertheless, the temperature discrepancy can also be produced by bi-abundance models (Liu, 2003; Liu et al., 2004a), containing some cold hydrogen-deficient material, highly enriched in helium and heavy elements, embedded in the diffuse warm nebular gas of normal abundances. The existence and origin of such inclusions are still unknown. It is unclear whether there is any link between the assumed H-poor inclusions in PNe and the H-deficient CSs.

Ashkbiz Danehkar