  Asymmetric shapes and evidence for binary central stars suggest a
common-envelope origin for many bipolar planetary nebulae. The bipolar
components of the nebulae are observed to expand faster than the rest and the
more slowly expanding material has been associated with the bulk of the
envelope ejected during the common-envelope phase of a stellar binary system.
Common-envelope evolution in general remains one of the biggest uncertainties
in binary star evolution and the origin of the fast outflow has not been
explained satisfactorily. We perform three-dimensional magnetohydrodynamic
simulations of common-envelope interaction with the moving-mesh code AREPO.
Starting from the plunge-in of the companion into the envelope of an asymptotic
giant branch star and covering hundreds of orbits of the binary star system, we
are able to follow the evolution to complete envelope ejection. We find that
magnetic fields are strongly amplified in two consecutive episodes. First, when
the companion spirals in the envelope and, second, when it forms a contact
binary with the core of the former giant star. In the second episode, a
magnetically-driven, high-velocity outflow of gas is launched self-consistently
in our simulations. The outflow is bipolar and the gas is additionally
collimated by the ejected common envelope. The resulting structure reproduces
typical morphologies and velocities observed in young planetary nebulae. We
propose that the magnetic driving mechanism is a universal consequence of
common envelope interaction responsible for a substantial fraction of observed
planetary nebulae. Such a mechanism likely also exists in the common-envelope
phase of other binary stars that lead to the formation of Type Ia supernovae,
X-ray binaries and gravitational-wave merger events.
