1 Introduction

Planetary nebulae (PNe) are ionized hydrogen-rich shells, which are generated by strong mass-loss (superwinds) from low- to intermediate-mass progenitor star at the end of the asymptotic giant branch (AGB) phase. Ultraviolet radiation from hot degenerate cores fully ionizes the ejected shells during the post-AGB phase, and produces photo-ionized nebulae. Multiwavelength emission-line imaging observations illustrate how PNe appear in different excitation regimes (see e.g. Sahai et al., 2007; Sahai et al., 2011; Miranda et al., 1999). As the degenerate core evolves from the AGB phase toward the white dwarf phase, it becomes hotter and its radiation pressure drives the stellar wind, leading to hydrodynamic shaping of the surrounding nebular shell. Morphological studies of PNe provide important clues about the mass-loss process at the end of the AGB phase and the PN evolution during the post-AGB phase (see e.g. Schönberner et al., 2005a; Kwok, 2010; Stanghellini et al., 1993; Balick & Frank, 2002; Schönberner et al., 2005b; Steffen et al., 2013).

NGC 5189 ($ =$PN G307.2$ -$03.4$ =$Hen 2-94$ =$VV 65$ =$ Sa 2-95) is a complex PN with multiple point-symmetric outflows or knots. The angular dimensions of NGC 5189 were measured to be about $ 163.4\hbox{$^{\prime\prime}$}\times 108.2\hbox{$^{\prime\prime}$}$ ($ 0.43$pc $ \times 0.29$ pc using $ D=546$pc Stanghellini et al., 2008) at the 10% level of the H$ \alpha $ peak surface brightness (Tylenda et al., 2003). More than a half century ago, Evans & Thackeray (1950) first described NGC 5189 as a planetary nebula or a massive nebula with remarkable knotted structures with no bright central star. Phillips & Reay (1983) later suggested that it may contain multiple pairs of knots revealed in the narrow-band images of the [NII], H$ \alpha $, and [OIII] emission lines. Moreover, Fabry-Perot imaging [NII] observations of NGC 5189 indicates that the nebula contains a dense ring expanding at 25 kms$ ^{-1}$ from the central star, with inclination of $ 78^{\circ}$ relative to the line of sight (Reay et al., 1984). Gonçalves et al. (2001) classified NGC 5189 as a bipolar PN with multiple pairs of knots inside the main structure with velocities similar to those of the surrounding gas. More recently, Sabin et al. (2012) suggested that NGC 5189 is a quadrupolar PN containing a dense and cold infrared torus, and multiple point-symmetric structures.

Table: Hubble Observations of NGC 5189 taken on 2012 July 6 (Program 12812, PI: Z. Levay).

Exposure (s) Instrument Filter $ \lambda_{\rm peak}$(Å) $ \Delta\lambda$(Å) Note

4200 WFC3/UVIS F673N 6731 42 [SII]

3900 WFC3/UVIS F657N 6573 41 H$ \alpha $+[NII]

8400 WFC3/UVIS F502N 5013 27 [OIII]

360 WFC3/UVIS F814W 8353 657 continuum

300 WFC3/UVIS F606W 5956 663 continuum

The central star (CSPN) of NGC 5189 has been studied by a number of authors. It has been classified as a stellar type with broad OVI and less broad HeII emission lines (Blanco et al., 1968), an “OVI sequence” object (Smith & Aller, 1969), [WC2] spectral type (Mendez & Niemela, 1982; Heap, 1982), [WO] (Polcaro et al., 1997), and [WO1] spectral type (Crowther et al., 1998). The CIV P-Cygni feature from the IUE UV spectrum shows a terminal wind velocity of $ -1540$ kms$ ^{-1}$ (Feibelman, 1997), while the line width from the optical spectrum suggests a maximum wind velocity of $ -2800\pm200$ kms$ ^{-1}$ (Polcaro et al., 1997). Recently, Keller et al. (2014) derived a terminal wind velocity of $ -2500\pm250$ kms$ ^{-1}$ and a stellar temperature of 165kK from from far-UV NeVII and OVI P-Cygni profiles in the FUSE spectrum of NGC 5189.

More recently, Manick et al. (2015) discovered significant periodic variability which is associated with a binary having an orbital period of $ 4.04\pm 0.1$ days and a companion mass of $ \geq 0.5$ M$ _{\odot}$ or $ 0.84$ M$ _{\odot}$ (at the orbital inclination of $ 40^{\circ}$). The complex morphology of NGC 5189 dominated by multiple, low-ionization structures (LISs) follows the trend of post common-envelope PNe outlined by Miszalski et al. (2009). Interactions between the [WR] star of NGC 5189 and its companion may be responsible for its fast stellar [WR] winds (Manick et al., 2015).

In this work, we use high-resolution Hubble Space Telescope images to analyze the spatially resolved inner structures of NGC5189 ( $ 120\hbox {$^{\prime \prime }$}\times 90\hbox {$^{\prime \prime }$}$, or $ 0.32$pc $ \times 0.24$ pc using a distance of $ 546$pc adopted from Stanghellini et al., 2008) with a diagnostic classification for excitation regions. This paper is organized as follows. In Section 2, we describe the observations and data analysis. In Section 3, we present the results, including newly identified low-ionization envelopes within the nebula. In Section 4, we discuss our results, and in Section 5 we summarize the conclusions of this study.

Ashkbiz Danehkar