1 Introduction

The planetary nebula (PN) PB8 (PNG292.4$ +$04.1) has been the subject of some recent studies (Miller Bertolami et al., 2011; García-Rojas et al., 2009; Todt et al., 2010). The central star of PB8 has been classified as [WC5-6] by Acker & Neiner (2003), as weak emission-line star type (wels; Tylenda et al., 1993; Gesicki et al., 2006), as [WC]-PG1159 (Parthasarathy et al., 1998), and finally as [WN/WC] hybrid by Todt et al. (2010). Particularly, it is one of the rare stars, which has provoked a lot controversy about its evolutionary origin (Miller Bertolami et al., 2011). A detailed abundance analysis of the nebula by García-Rojas et al. (2009) showed abundance discrepancy factors (ADF$ \equiv$ORL/CEL) of $ 2.57$ for the O$ ^{++}$ ion and $ 1.94$ for the N$ ^{++}$ ion, which are in the range of typical ADFs observed in PNe (ADFs$ \sim$1.6-3.2; see review by Liu, 2006). The nebular morphology was described as a round nebula with inner knots or filaments by Stanghellini et al. (1993), and classified as elliptical by Gorny et al. (1997). However, a narrow-band H$ \alpha $+[NII] image of PB8 taken by Schwarz et al. (1992) shows a roughly spherical nebula with an angular diameter of about 7 arcsec (6.5 arcsec $ \times$ 6.6 arcsec reported by Tylenda et al., 2003), which is used throughout this paper.

Table 1: Journal of the Observations for PB 8.
Observatory Obs Date $ \lambda $-range(Å) FWHM(Å) Inst./Mod. Program ID/PI Exp.Time (s)
Magellan 6.5-m 2006 May 9 3350-5050, 4950-9400 0.15, 0.25 MIKE M. Peña 300,600,900
ANU 2.3-m 2010 Apr. 21 4415-5589, 5222-7070 0.83, 1.03 WiFeS 1100147, Q.A. Parker 60,1200
Spitzer 2008 Feb. 25 5.2-14.5$ \mu $m, 14-38$ \mu $m - SL, LL 40115, G. Fazio -

The ionic abundances of heavy elements derived from optical recombination lines (ORLs) have been found to be systematically higher than those derived from collisionally excited lines (CELs) in many PNe (see e.g. Liu et al., 2001; García-Rojas et al., 2009; Rola & Stasinska, 1994; Liu et al., 2006; Liu et al., 2000; Tsamis et al., 2004; Tsamis et al., 2008). To solve this problem, Liu et al. (2000) suggested a bi-abundance model in which the nebula contains two components of different abundances: cold hydrogen-deficient `metal-rich' component and diffuse warm component of `normal' abundances. The H-deficient inclusions embedded in the nebular gas of normal abundances can dominate the emissions of ORLs (Liu et al., 2004; Liu et al., 2000). The bi-abundance photoionization model of Abell 30 by Ercolano et al. (2003b) showed the possibility of such a scenario. More recently, the bi-abundance model by Yuan et al. (2011) was able to predict the ORLs in NGC 6153. Previously, the analysis of the emission-line spectrum of NGC 6153 by Liu et al. (2000) pointed to a component of the ionized gas, cold and very metal-rich. The photoionization modeling of NGC 1501 (Ercolano et al., 2004) and Abell 48 (Danehkar et al., 2014) also suggested that they may contain some cold H-deficient structures.

The aim of this paper is to construct photoionization models of PB 8, for which high quality spectroscopy has become available (García-Rojas et al., 2009), constrained by an ionizing source determined using the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres (Todt et al., 2010). To reproduce the observed ORLs, a bi-abundance model is used, which consists of a chemically homogeneous abundance distribution containing a small fraction of dense metal-rich structures. In addition, the dust properties are constrained using the Spitzer infrared (IR) continuum of the PN PB 8. The observations and modeling procedure are respectively described in Sections 2 and 3. In Section 4, we present our modeling results, while our conclusions are given in Section 5.

Ashkbiz Danehkar