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Fig. 12

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ADF(O++) plotted against mean nebular surface brightness (upper left panel); absolute nebular diameters (upper right panel) and excitation class as defined by Dopita & Meatheringham (1990) (lower left panel). The dashed lines represent the fits obtained by Liu et al. (2004) for a sample of 19 PNe. In the lower right panel we show the relation between the nebular surface brightness and the nebular diameter, which are clearly correlated; the line is a fit to the data.

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