A New Look at the Evolution of Wolf‐Rayet Central Stars of Planetary Nebulae

Abstract

ABSTRACTOn 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.