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

TL;DR

This study investigates Ou 5, a bipolar planetary nebula with extreme abundance discrepancies, revealing complex thermal and chemical structures, a binary core, and implications for nebular evolution and abundance inhomogeneities.

Contribution

It provides detailed imaging, spectroscopy, and photoionization models that uncover the nebula's morphology, temperature phases, and binary nature, advancing understanding of abundance discrepancies in planetary nebulae.

Findings

Presence of nested bipolar morphology with multiple shells and lobes

Detection of at least two distinct temperature/metallicity phases

Identification of a binary core consistent with a post-AGB star

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 10,000 yr. Thermal broadening of the H alpha line relative to heavier ions implies a hydrogen temperature 3000 K to 6000 K, in contrast to the ~ 10,000 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 O II and the [O III] observations. The nebular He II emission demands a hotter and more luminous central star than previously inferred, consistent with a ~ 0.58 solar mass post-AGB 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.