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

Abstract: 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.