Mapping Excitation in the Inner Regions of the Planetary Nebula NGC 5189 using HST WFC3 Imaging

Abstract: The planetary nebula (PN) NGC 5189 around a Wolf-Rayet [WO] central star demonstrates one of the most remarkable complex morphologies among PNe with many multi-scale structures, showing evidence of multiple outbursts from an AGB progenitor. In this study we use multi-wavelength Hubble Space Telescope Wide Field Camera 3 (WFC3) observations to study the morphology of the inner 0.3 pc $\times$ 0.2 pc region surrounding the central binary that appears to be a relic of a more recent outburst of the progenitor AGB star. We applied diagnostic diagrams based on emission line ratios of H$\alpha$ $\lambda$6563, [O III] $\lambda$5007, and [S II] $\lambda\lambda$6717,6731 images to identify the location and morphology of low-ionization structures within the inner nebula. We distinguished two inner, low-ionization envelopes from the ionized gas, within a radius of 55 arcsec ($\sim$ 0.15 pc) extending from the central star: a large envelope expanding toward the northeast, and its smaller counterpart envelope in the opposite direction toward the southwest of the nebula. These low-ionization envelopes are surrounded by a highly-ionized gaseous environment. We believe that these low-ionization expanding envelopes are a result of a powerful outburst from the post-AGB star that created shocked wind regions as they propagate through the previously expelled material along a symmetric axis. Our diagnostic mapping using high-angular resolution line emission imaging can provide a novel approach to detection of low-ionization regions in other PNe, especially those showing a complex multi-scale morphology.