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The planetary nebula (PN) PB 8 around a [WN/WC]-hybrid central star is one ofPNe with moderate abundance discrepancy factors (ADFs ~ 2-3), which could be anindication of a tiny fraction of metal-rich inclusions embedded in the nebula(bi-abundance). In this work, we have constructed photoionization models toreproduce the optical and infrared observations of the PN PB 8 using a non-LTEstellar model atmosphere ionizing source. A chemically homogeneous modelinitially used cannot predict the optical recombination lines (ORLs). However,a bi-abundance model provides a better fit to most of the observed ORLs from Nand O ions. The metal-rich inclusions in the bi-abundance model occupy 5.6percent of the total volume of the nebula, and are roughly 1.7 times cooler anddenser than the mean values of the surrounding nebula. The N/H and O/Habundance ratios in the metal-rich inclusions are ~ 1.0 and 1.7 dex larger thanthe diffuse warm nebula, respectively. To reproduce the Spitzer spectral energydistribution of PB 8, dust grains with a dust-to-gas ratio of 0.01 (by mass)were also included. It is found that the presence of metal-rich inclusions canexplain the heavy element ORLs, while a dual-dust chemistry with differentgrain species and discrete grain sizes likely produces the infrared continuumof this PN. This study demonstrates that the bi-abundance hypothesis, which wasexamined in a few PNe with large abundance discrepancies (ADFs > 10), couldalso be applied to those typical PNe with moderate abundance discrepancies.

Bi-abundance ionization structure of the Wolf-Rayet planetary nebula PB 8