6.1 Comparison of the emission-line fluxes

Table 9 compares the flux intensities predicted by the best-fitting model with those from the observations. Columns 2 and 3 present the dereddened fluxes of our observations and those from Todt et al. (2013). The predicted emission-line fluxes are given in Column 4, relative to the intrinsic dereddened H$\beta$ flux, on a scale where $I($H$\beta)$=100. The most emission-line fluxes presented are in reasonable agreement with the observations. However, we notice that the [O II]$\lambda$7319 and $\lambda$7330 doublets are overestimated by a factor of 3, which can be due to the recombination contribution. Our photoionization code incorporates the recombination term to the statistical equilibrium equations. However, the recombination contribution are less than 30 per cent for the values of $T_{\rm e}$ and $N_{\rm e}$ found from the plasma diagnostics. Therefore, the discrepancy between our model and observed intensities of these lines can be due to inhomogeneous condensations such as clumps and/or colder small-scale structures embedded in the global structure. It can also be due to the measurement errors of these weak lines. The [O II] $\lambda\lambda$3726,3729 doublet predicted by the model is around 25 per cent lower, which can be explained by either the recombination contribution or the flux calibration error. There is a notable discrepancy in the predicted [N II]$\lambda$5755 auroral line, being higher by a factor of $\sim3$. It can be due to the errors in the flux measurement of the [N II]$\lambda$5755 line. The predicted [Ar III]$\lambda$7751 line is also 30 per cent lower, while [Ar III]$\lambda$7136 is about 20 per cent higher. The [Ar III]$\lambda$7751 line usually is blended with the telluric line, so the observed intensity of these line can be overestimated. It is the same for [S III]$\lambda$9069, which is typically affected by the atmospheric absorption band.

Table 11: Integrated ionic abundance ratios for He, C, N, O, Ne, S and Ar, derived from model ionic fractions and compared to those from the empirical analysis.

Ionic ratio	Observed	Model
He$^+$/H$^+$	0.124	0.116
C$^{2+}$/H$^+$	2.16($-3$)	2.45($-3$)
N$^+$/H$^+$	1.42($-5$)	1.26($-5$)
O$^+$/H$^+$	5.25($-5$)	3.63($-5$)
O$^{2+}$/H$^+$	1.06($-4$)	9.76($-5$)
Ne$^{2+}$/H$^+$	4.26($-5$)	3.62($-5$)
S$^+$/H$^+$	3.98($-7$)	5.20($-7$)
S$^{2+}$/H$^+$	5.58($-6$)	4.19($-6$)
Ar$^{2+}$/H$^+$	9.87($-7$)	1.01($-6$)