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

TL;DR

This study analyzes the morphologies of planetary nebulae with binary central stars, finding a strong link between binary evolution and bipolar shapes, with many showing low-ionisation features indicative of binary origins.

Contribution

It provides the first comprehensive morphological analysis of a significant sample of post-common-envelope planetary nebulae, highlighting the connection between binary evolution and nebula shapes.

Findings

60% of nebulae are bipolar when inclination is considered

High prevalence of low-ionisation knots suggests binary influence

Strong evidence linking common-envelope evolution to bipolar morphologies

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.