THE FORMATION OF BIPOLAR PLANETARY-NEBULAE AND CLOSE WHITE-DWARF BINARIES

We systematically investigate the effects of binary interactions on the formation and shaping of planetary nebulae (PNe) and the various evolutionary channels leading to the formation of close white dwarf binaries, in particular cataclysmic variables (CVs), double degenerate binaries (DDs) and potential progenitors of Type Ia supernovae (SNe). Using Monte Carlo simulations, we explore the consequences of various binary mass-ratio distributions, different initial-final mass relations, different population parameters (such as age and metallicity), and different theoretical assumptions concerning the modelling of the common-envelope phase. Our results agree well with observations and - where comparable - with previous studies. Our main conclusions are the following. (1) The morphology of 34 to 43 per cent of all planetary nebulae is affected by binary interactions, if we assume that 50 per cent of all stellar systems are binaries with orbital periods less than approximately 100 yr. (2) The main types of binary interactions considered (gravitational focusing, common-envelope ejection and binary merger) are all of comparable importance. (3) Massive binaries are slightly more likely to produce bipolar PNe, provided that the initial mass-ratio distribution is biased towards a mass ratio near unity. (4) The orbital periods of close white dwarf binaries that experienced a common-envelope (CE) phase extend to longer orbital periods than found in earlier studies, because the binding energy of the CE of initially wide systems is reduced. (5) Best agreement with observations, in particular the fraction of PNe with close binary nuclei and the birth rates of CVs and DDs, is obtained if the process that leads to the ejection of CEs is very efficient. (6) The rate of mergers of two CO white dwarfs with a total mass larger than the Chandrasekhar mass, possibly leading to a Type Ia supernova, is marginally consistent with the observational SN Ia rate, although not in our best model. (7) Our simulations support the initial-final mass relation proposed by Han, Podsiadlowski & Eggleton and the use of PNe as a standard distance candle.