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 10,000 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.

Figures (24)

• Figure 1: The 7 positions of the long-slits are indicated and labeled over an [OIII] image of Ou 5
• Figure 2: a. The central d slit is projected over the [OIII] image of Ou 5. Some regions of interest are indicated. The blue squares indicate the positions of the polar knots. The pink regions indicate the approximate length of the outer lobes and the central yellow region indicates the extent of the inner lobes. b. The P-V array in the right corresponds to the bi-dimensional [OIII] spectrum from slit d.
• Figure 2: Correspondence between model components (Fig. \ref{['fig:geometrical-model']}) and morphological features (Fig. \ref{['fig:rgb-image']}b)
• Figure 3: (a) Three-color RGB image of Ou 5 from ALFOSC-NOT in filters of HeII $\lambda 4686$ (red), $\mathrm{H}\alpha$$\lambda 6563$ (green), and [OIII] $\lambda 5007$ (blue), superimposed on a deep SPM + NOT co-added [OIII] image (inverse grayscale). (b) Schematic diagram of nebular components.
• Figure 4: Profile cuts along the equator and polar axis of Ou 5 from the ALFOSC-NOT images shown in Fig. \ref{['fig:rgb-image']}. Colored lines show the median profile in each emission line filter for a strip of width [-NoValue-]4arcsec, while gray shading shows the interquartile range (first to third quartile). Positions with a broad interquartile range correspond to stars that fall in the strip. (a) Cuts along equator. The profile is affected by stars at offsets of $-34$, $-16$, $0$, $+27$, and $+33$. (b) Cuts along the nebular axis. The profile is affected by stars at offsets of $-10$, $-4$, $0$, and $+6$.