[Paper Review] Binary planetary nebulae nuclei towards the Galactic bulge. II. A penchant for bipolarity and low-ionisation structures

B. Miszalski, A. Acker+2 more|arXiv (Cornell University)|Jul 14, 2009

Stellar, planetary, and galactic studies92 references99 citations

TL;DR

This study analyzes 30 post-common-envelope planetary nebulae (PNe) with binary central stars, using Gemini South narrow-band imaging to assess morphology. It finds that 60% are intrinsically bipolar when inclination effects are accounted for, providing the strongest observational evidence linking common-envelope evolution to bipolar nebulae, and reveals a high incidence of low-ionisation structures consistent with binary-driven formation mechanisms.

ABSTRACT

Considerable effort has been applied towards understanding the precise shaping mechanisms responsible for the diverse range of morphologies exhibited by planetary nebulae (PNe). A binary companion is increasingly gaining support as a dominant shaping mechanism, however morphological studies of the few PNe that we know for certain were shaped by binary evolution are scarce or biased. Newly discovered binary central stars (CSPN) from the OGLE-III photometric variability survey have significantly increased the sample of post common-envelope (CE) nebulae available for morphological analysis. We present Gemini South narrow-band images for most of the new sample to complement existing data in a qualitative morphological study of 30 post-CE nebulae. Nearly 30% of nebulae have canonical bipolar morphologies, however this rises to 60% once inclination effects are incorporated with the aid of geometric models. This is the strongest observational evidence yet linking CE evolution to bipolar morphologies. A higher than average proportion of the sample shows low-ionisation knots, filaments or jets suggestive of a binary origin. These features are also common around emission-line nuclei which may be explained by speculative binary formation scenarios for H-deficient CSPN.

Motivation & Objective

• To overcome sampling bias in previous morphological studies of post-common-envelope (CE) planetary nebulae (PNe) by analyzing a larger, more representative sample of binary central stars.
• To test the hypothesis that CE evolution produces highly aspherical nebulae, particularly bipolar morphologies, by accounting for inclination effects using geometric models.
• To investigate the prevalence and origin of low-ionisation structures (LIS) in post-CE nebulae, assessing their link to binary systems.
• To compare morphological trends in post-CE nebulae with theoretical expectations of high density contrast during the CE phase.
• To explore the connection between LIS and emission-line nuclei, particularly [WO] stars, to assess binary formation scenarios for H-deficient central stars.

Proposed method

• Acquired narrow-band imaging data for 30 post-CE nebulae using the Gemini South telescope under programs GS-2008B-Q-65 and GS-2009A-Q-35.
• Combined new Gemini data with existing archival imaging to perform a qualitative morphological analysis of nebulae, focusing on bipolarity and low-ionisation structures.
• Applied geometric bipolar models and inclination corrections to estimate intrinsic 3D morphologies, using established frameworks such as those by Pollacco & Bell (1997) and De Marco (2009).
• Evaluated nebular morphology using Gatley’s rule to assess the likelihood of bipolarity based on projected 2D appearance.
• Identified low-ionisation structures (LIS) such as knots, filaments, and jets via [NII] and Hα imaging, and correlated their presence with binary central star systems.
• Cross-referenced nebulae with emission-line nuclei, particularly [WO] stars, to assess morphological and chemical similarities suggesting shared binary formation pathways.

Experimental results

Research questions

• RQ1 What is the true fraction of post-common-envelope planetary nebulae that are intrinsically bipolar when inclination effects are properly accounted for?
• RQ2 To what extent do low-ionisation structures (LIS) in post-CE nebulae correlate with binary central stars, and what does this imply about their formation mechanism?
• RQ3 How does the morphological distribution of post-CE nebulae compare to theoretical predictions of high density contrast during the common-envelope phase?
• RQ4 Are emission-line nuclei, especially [WO] stars, preferentially associated with nebulae showing low-ionisation structures and bipolar morphologies?
• RQ5 Can the presence of LIS in nebulae with similar morphologies (e.g. NGC 6337 and Sab 41) be used as indirect evidence for a binary origin?

Key findings

• Approximately 30% of the 30 post-CE nebulae exhibit canonical bipolar morphology in 2D projection, but this fraction rises to at least 60% when inclination effects are corrected using geometric models.
• The intrinsic bipolar fraction is estimated at 60–70%, providing the strongest observational evidence to date that common-envelope evolution produces highly aspherical nebulae due to high density contrast.
• 40% of the sample show low-ionisation structures (LIS), a significantly higher fraction than the ~17% observed in the general planetary nebula population, suggesting a strong link to binary-driven mass loss.
• LIS are preferentially located in the orbital plane (as knots or filaments) or along the poles (as low-surface-brightness jets), consistent with models involving dust and H₂ clumps distributed during the CE phase.
• Two-thirds of a broader sample of nebulae with LIS were found to host emission-line nuclei, with extreme LIS features common in nebulae with [WO] central stars, supporting speculative binary formation scenarios.
• No round, single-shell nebulae were found in the sample; the roundest multiple-shell nebula (PHR 1804−2913) may still be pole-on bipolar, suggesting CE evolution may not easily produce spherical PNe.