Bi-abundance ionization structure of the Wolf-Rayet planetary nebula PB
  8
  The planetary nebula (PN) PB 8 around a [WN/WC]-hybrid central star is one of
PNe with moderate abundance discrepancy factors (ADFs ~ 2-3), which could be an
indication of a tiny fraction of metal-rich inclusions embedded in the nebula
(bi-abundance). In this work, we have constructed photoionization models to
reproduce the optical and infrared observations of the PN PB 8 using a non-LTE
stellar model atmosphere ionizing source. A chemically homogeneous model
initially used cannot predict the optical recombination lines (ORLs). However,
a bi-abundance model provides a better fit to most of the observed ORLs from N
and O ions. The metal-rich inclusions in the bi-abundance model occupy 5.6
percent of the total volume of the nebula, and are roughly 1.7 times cooler and
denser than the mean values of the surrounding nebula. The N/H and O/H
abundance ratios in the metal-rich inclusions are ~ 1.0 and 1.7 dex larger than
the diffuse warm nebula, respectively. To reproduce the Spitzer spectral energy
distribution 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 can
explain the heavy element ORLs, while a dual-dust chemistry with different
grain species and discrete grain sizes likely produces the infrared continuum
of this PN. This study demonstrates that the bi-abundance hypothesis, which was
examined in a few PNe with large abundance discrepancies (ADFs > 10), could
also be applied to those typical PNe with moderate abundance discrepancies.
