Kinematics and physical structure of Ou 5: a bipolar planetary nebula with extreme abundance discrepancy and an eclipsing binary core

ABSTRACT We present new deep imaging and high-resolution spectroscopy of the extreme-abundance-discrepancy planetary nebula Ou 5, together with photoionization modelling aimed at probing its unusual thermal and chemical structure. The nebula exhibits a nested bipolar morphology, including inner and outer shells, faint outer lobes, and polar knots. Remarkably, all these components share a dynamical age of order ${10^4}\, {\rm yr}$. Thermal broadening of the $\mathrm{H}\,\alpha$ line relative to heavier ions implies a hydrogen temperature 3000–6000 K, in contrast to the $\sim {10^4}\, {\rm K}$ derived from collisionally excited line diagnostics. This provides independent support for the presence of at least two distinct temperature/metallicity phases, as previously proposed to explain extreme abundance discrepancies. Photoionization models with sinusoidally varying metallicity successfully reproduce the observed nebular spectrum and morphology. A mixture of fluctuations with both extreme and moderate metallicity contrasts is required to simultaneously fit the ${\rm O\, {\small II}}$ and the [${\rm O\, {\small III}}$] observations. The nebular ${\rm He\, {\small II}}$ emission demands a hotter and more luminous central star than previously inferred, consistent with an $\approx {0.58}\, \mathrm{M}_{\odot }$ post-asymptotic giant branch progenitor evolving toward a CO white dwarf. Ou 5 thus reinforces the link between close-binary nuclei and extreme abundance discrepancies, and provides a valuable benchmark for understanding how common-envelope ejections give rise to the thermal and abundance inhomogeneities observed in planetary nebulae.