From bipolar to elliptical: Simulating the morphological evolution of planetary nebulae

In this paper we model the evolution of pre-planetary nebula (PPN) and planetary nebula (PN) morphologies as a function of nebular age. The aim of this work is to understand if shape transitions from one evolutionary phase to the other can be driven by changes in the parameters of the mass-loss from the central star. We carry out 2.5D hydrodynamical simulations of mass-loss at the end stages of stellar evolution for intermediate mass stars. Changes in wind velocity, mass-loss rate and mass-loss geometry are tracked. We focus on the transition from mass-loss dominated by a short-duration jet flow (driven during the PPN phase) to mass-loss driven by a spherical fast wind (produced by the central star of the PN). Our results show that while jet-driven nebulae can be expected to be dominated by bipolar morphologies, systems that begin with a jet but are followed by a spherical fast wind will evolve into elliptical objects. Systems that begin with an aspherical asymptotic giant branch wind evolve into butterfly-shaped nebula with, or without, a jet phase. In addition, our models show that spherical nebulae are highly unlikely to derive from either bipolar PPN or elliptical PN over relevant time-scales. The morphological transitions seen in our simulations may however provide insight into the driving mechanisms of both PPN and PN as point to evolutionary changes in the central engine.