Departure from Axisymmetry in Planetary Nebulae

Many planetary nebulae (PNs) exhibit symmetries that range from unremarkable spherical and elliptical shapes to quite exotic bipolar and point-symmetric shapes. However, there are many that exhibit distinctly nonaxisymmetric structure in either (1) the shape of the nebula, or (2) the off-centered position of the illuminating star. By examining a large number of well-resolved images of PNs, we estimate that ~30%-50% of all PNs exhibit distinctly nonaxisymmetric structure. In this paper, we discuss how such departures from axisymmetry can arise from the binary nature of the progenitors of the PNs. The scenarios include (1) relatively close binaries with eccentric orbits, and (2) longer orbital period systems with either circular or eccentric orbits. In the first mechanism, the departure from axisymmetry is caused by the variation of mass loss and/or mass transfer with the changing distance between the companions in their eccentric orbit. In the second mechanism, the departure from axisymmetry is the result of the time-varying vector direction of the mass-losing star, or that of a possible pair of jets from the companion, as the stars move around their orbit. In order to assess the fraction of PNs whose nonaxisymmetric morphologies are expected to arise in binary systems, we have carried out a detailed population synthesis study. In this study, a large number of primordial binaries are evolved through the lifetimes of both stars, including wind mass loss. We then assess whether the primary or the secondary (or both) produces a PN. The expected deviations from axisymmetry are then classified for each binary and the results tabulated. We find that ~25% of elliptical and ~30%-50% of bipolar PNs are expected to acquire nonaxisymmetric structure from binary interactions.