Do most planetary nebulae derive from binary interactions? The binary fraction of central stars of planetary nebula

During the past 20 years, the idea that non-spherical planetary nebulae, comprising 80% of the total population, might need a binary or planetary interaction to be shaped, was discussed by various authors. It is now generally agreed that the varied morphologies of planetary nebulae cannot currently be explained by rotation and magnetic fields in single giant stars. Observationally, more binary central stars of planetary nebula have been discovered, opening new possibilities to understand the connections between binarity and morphology. So far, 45 binary central stars of planetary nebulae have been detected, most being close systems detected via flux variability. In order to determine the actual PN binary fraction, one needs a method that can detect wider binaries. We present here early results from a survey of high precision I-band and near infrared photometry of planetary nebula central stars aimed at detecting binaries with any separation. Eventually our survey will sample most of the 2-kpc volume limited sample of Frew (2008). At that time we expect that the binary fraction will reveal whether PN derive primarily from binaries or whether the current scenario, whereby single stars somehow, can generate non spherical planetary nebulae, is more in line with observation.