The evolutionary route to form planetary nebulae with central neutron star-white dwarf binary systems

We present a possible evolutionary pathway to form planetary nebulae (PNe) with close neutron star (NS)-white dwarf (WD) binary central stars. By employing the binary population synthesis technique, we find that the evolution involves two common envelope evolution (CEE) phases and a core collapse supernova explosion between them that forms the NS. Later the lower mass star engulfs the NS as it becomes a red giant, a process that leads to the second CEE phase and to the ejection of the envelope. This leaves a hot horizontal branch star that evolves to become a helium WD and an expanding nebula. Both the WD and the NS power the nebula. The NS in addition might power a pulsar wind nebula inside the expanding PN. From our simulations we find that the Galactic formation rate of NS-WD PNe is 1.8 x 10(-5) yr(-1) while the Galactic formation rate of all PNe is 0.42 yr(-1). There is a possibility that one of the observed Galactic PNe might be a NS-WD PN, and a few NS-WD PNe might exist in the Galaxy. The central binary systems might be sources for future gravitational wave detectors like LISA, and possibly of electromagnetic telescopes.