Eccentric Binary Model for Off‐Center Planetary Nebula Nuclei

Abstract

We examine the influence of eccentric binary progenitors on the morphologies of their descendant planetary nebulae. In particular, we consider how mass loss via a stellar wind by an asymptotic giant branch (AGB) star in an eccentric binary can lead to the displacement of the central star in the equatorial plane. We postulate that the mass-loss rate from the AGB star varies systematically with orbital phase. Such variations may be due to several effects, including a tidal enhancement of the stellar wind near periastron and a cessation of the stellar wind when the Roche lobe of the AGB star encroaches on its extended atmosphere. Our results may pertain to binary systems with semimajor axes in the range of a ≃ 7-80 AU, which corresponds to orbital periods in the range P ≃ 15-500 yr. We apply the results to planetary nebulae in general, and MyCn 18 (the Hourglass Nebula) in particular, where the central star was recently found by the Hubble Space Telescope to be displaced from the center of the nebula. The results of this paper may be applied to circumstellar matter around more massive stars, such as progenitors of supernovae, by rescaling the physical properties of the binary stars and the wind velocities.