Binary planetary nebulae nuclei towards the Galactic bulge^*

II. A penchant for bipolarity and low-ionisation structures

¹ Observatoire Astronomique, Université de Strasbourg, 67000, Strasbourg, France e-mail: [brent;acker]@newb6.u-strasbg.fr
² Department of Physics, Macquarie University, Sydney, NSW 2109, Australia e-mail: [brent;qap]@ics.mq.edu.au
³ Anglo-Australian Observatory, Epping, NSW 1710, Australia
⁴ Dépt. de physique, Univ. de Montréal CP 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, and Centre de recherche en astrophysique du Québec, Canada e-mail: moffat@astro.umontreal.ca

Received: 25 March 2009
Accepted: 7 July 2009

Abstract

Considerable effort has been applied towards understanding the precise shaping mechanisms responsible for the diverse range of morphologies exhibited by planetary nebulae (PNe). At least 10–20% of PNe have central stars (CSPN) with a close binary companion thought responsible for heavily shaping the ejected PN during common-envelope (CE) evolution, however morphological studies of the few available examples found no clear distinction between PNe and post-CE PNe. The discovery of several new binary central stars (CSPN) from the OGLE-III photometric variability survey has significantly increased the number of post-CE PNe available for morphological analysis to 30 PNe. High quality Gemini South narrow-band images are presented for most of the OGLE sample, while some previously known post-CE PNe are reanalysed with images from the literature. Nearly 30% of nebulae have canonical bipolar morphologies, however this could be as high as 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 suggesting they have a binary origin. These features are also common in nebulae around emission-line nuclei which may be explained by speculative binary formation scenarios for H-deficient CSPN.