The evolutionary sequence of planetary nebulae

Abstract

We develop a simple model for the evolution of planetary nebulae (PNs) using the large and homogeneous data set accumulated in recent times on the Magellanic Cloud PN population. In this model, a high-velocity radiation pressure driven wind interacts with the dense un-ionized shell of material ejected during the asymptotic giant branch evolution. This wind serves to confine by ram pressure the ionized gas and to accelerate the nebula. During the optically thin period, the nebulae fade at almost constant expansion velocity. Higher mass planetary nebulae nuclei (PNN) accelerate their associated nebulae to higher velocities. This model reproduces the observed density-radius-ionized mass-dynamical age relationships and serves to define the geometrical relationship between the ionized nebula and the central star. Using a grid of model atmospheres, we have generated theoretical evolutionary tracks on the excitation class versus Hβ flux plane, which we show to be a form of the log Teff versus log (L/L⊙)plane (Hertzsprung-Russell diagram) transformed to explicit coordinates of the nebular parameters. A comparison of these tracks with the observed points demonstrates that the PNN in the Magellanic Clouds are confined in the mass range 0.55-0.7 M⊙, with a peak at ∼0.64 M⊙. Finally, we have used photoionization models computed along the evolutionary tracks to show that secondary line ratios used to define the excitation class are also correctly predicted, and we present evidence that the third dredge-up episode appears to have enhanced both He and N in the more massive PNs in the LMC. Show more