The Etched Hourglass Nebula MYCN 18. II. A Spatio-kinematic Model

We have observed MyCn18, a young planetary nebula, with Hubble Space Telescope/Wide Field Planetary Camera 2 and have presented the observational results in an earlier paper. Here we present a detailed spatio-kinematic model of Hα emission from the nebula. We model the hourglass-shaped nebular lobes as thin-walled structures in the shape of modified paraboloids. Variations in the Hα surface brightness with radius imply abrupt changes in the exponent of the power-law gas density in the hourglass walls. These density variations might reflect temporal variations in the mass-loss rate of the progenitor asympotic giant branch (AGB) star. Alternatively, they may be a result of the complex interaction between the fast and slow winds. From the Hα surface brightness variations we infer that the nebula is density-bounded everywhere, except in the vicinity of the dense waist. We derive a total mass of 0.013 M_solar for the walls of the ionized nebula and an upper limit of 0.006 M_solar for the mass in the interior. This appears to be a small fraction of the total mass ejected in the AGB phase. The nebular kinematics are well modeled with a radial velocity field where velocity increases with latitude. The density and velocity structures derived from our model are consistent with general predictions of the interacting winds hypothesis for planetary nebula formation.