Hot bubbles of planetary nebulae with hydrogen-deficient winds

Abstract

Aims. We seek to understand the evolution of Wolf–Rayet central stars by comparing the diffuse X-ray emission from their windblown bubbles with that from their hydrogen-rich counterparts with predictions from hydrodynamical models. Methods. We simulate the dynamical evolution of heat-conducting wind-blown bubbles using our 1D radiation-hydrodynamics code NEBEL/CORONA. We use a post-AGB-model of 0.595 M⊙ but allow for variations of its evolutionary timescale and wind power. We follow the evolution of the circumstellar structures for different post-AGB wind prescriptions: for O-type central stars and for Wolf–Rayet central stars where the wind is hydrogen-poor, more dense, and slower. We use the CHIANTI software to compute the X-ray properties of bubble models along the evolutionary paths. We explicitly allow for non-equilibrium ionisation of key chemical elements. A sample of 12 planetary nebulae with diffuse X-ray emission – seven harbouring an O-type and five a Wolf–Rayet nucleus – is used to test the bubble models. Results. The properties of most hydrogen-rich bubbles (X-ray temperature, X-ray luminosity, size) and their central stars (photon and wind luminosity) are fairly well represented by bubble models of our 0.595 M⊙ AGB remnant. The bubble evolution of Wolf–Rayet objects is different, thanks to the high radiation cooling of their carbon- and oxygen-rich winds. The bubble formation is delayed, and the early evolution is dominated by condensation instead of evaporation. Eventually, evaporation begins and leads to chemically stratified bubbles. The bubbles of the youngest Wolf–Rayet objects appear chemically uniform, and their X-ray properties can be explained by faster-evolving nuclei. The bubbles of the evolved Wolf–Rayet objects have excessively low characteristic temperatures that cannot be explained by our modelling. Conclusions. The formation of nebulae with O-type nuclei follows mainly a single path, but the formation pathways leading to the Wolf–Rayet-type objects appear diverse. Bubbles with a pure Wolf–Rayet composition can exist for some time after their formation despite the presence of heat conduction.