The etched hourglass nebula MyCn 18. II. A spatio-kinematic model

We have observed MyCn18, a young planetary nebula, with Hubble Space Telesc ope/Wide Field Planetary Camera 2 and have presented the observational resu lts in an earlier paper. Here we present a detailed spatio-kinematic model of H alpha emission from the nebula. We model the hourglass-shaped nebular lobes as thin-walled structures in the shape of modified paraboloids. Varia tions in the H alpha. surface brightness with radius imply abrupt changes i n the exponent of the power-law gas density in the hourglass walls. These d ensity variations might reflect temporal variations in the mass-loss rate o f the progenitor asympotic giant branch (AGE) star. Alternatively, they may be a result of the complex interaction between the fast and slow winds. Fr om the H alpha surface brightness variations we infer that the nebula is de nsity-bounded everywhere, except in the vicinity of the dense waist. We der ive a total mass of 0.013 M . for the walls of the ionized nebula and an up per limit of 0.006 M . for the mass in the interior. This appears to be a s mall fraction of the total mass ejected in the AGE phase. The nebular kinem atics are well modeled with a radial velocity field where velocity increase s with latitude. The density and velocity structures derived from our model are consistent with general predictions of the interacting winds hypothesi s for planetary nebula formation.