A new look at the evolution of Wolf-Rayet central stars of planetary nebulae

On the basis of recent observational evidence and new theoretical results, we construct a speculative scenario for the evolution of Wolf-Rayet central stars of planetary nebulae. Although single-star evolutionary calculations have succeeded recently in reproducing the composition of these objects, it is clear from the latest infrared observations that a new perspective has to be adopted; the simultaneous presence of carbon- and oxygen-rich dust (double-dust chemistry), while being a rare phenomenon for H-rich central stars, is found around the vast majority of cool Wolf-Rayet central stars. This correlation between Wolf-Rayet characteristics and double-dust chemistry points to a common mechanism. Within the binary evolution framework established by Soker, two scenarios are proposed, responsible for the majority (80%-85%) and minority (15%-20%) of Wolf-Rayet central stars. In the first scenario, proposed here for the first time, a low-mass main-sequence star, brown dwarf, or planet spirals into the asymptotic giant branch star, inducing extra mixing, hence a chemistry change, and terminating the asymptotic giant branch evolution. In the second scenario, previously proposed, a close binary companion is responsible for the formation of a disk around either the binary or the companion. This long-lived disk harbors the O-rich dust. Both models are speculative, although they are supported by several observations and recent theoretical results.