Do post-common envelope objects form a distinct subset of PNe?

Abstract

A new H-alpha surface brightness - radius (SB-r) relation has proved to be a useful statistical distance indicator for planetary nebulae. Known close binary PNe with reliable primary distances are shown to inhabit a distinct locus in SBâ-r space. Comparing the ionized masses of this sample with a volume-limited ensemble of PNe with the same range of surface brightness, leads to the conclusion that post common-envelope (CE) PNe have systematically lower ionized masses than "normal" PNe. Post-CE PNe are also morphologically distinct from the majority of elliptical PNe. A comparison of optical and near-infrared colours of the central stars of PNe in a volume limited sample leads to an estimated binary fraction of 52 - 58 per cent, similar to the known binary fraction of G-type main-sequence stars. Close binaries form a subset of these so we conclude that only a minority (12 - 33 per cent) of PNe have passed through a CE phase.