Hot bubbles of planetary nebulae with hydrogen-deficient winds. III. Formation and evolution in comparison with hydrogen-rich bubbles

D. Schönberner, M. Steffen

Published 2024-01-29Version 1

We seek to understand the evolution of Wolf-Rayet central stars by comparing the diffuse X-ray emission from their wind-blown bubbles with that from their hydrogen-rich counterparts with predictions from hydrodynamical models. We simulate the dynamical evolution of heat-conducting wind-blown bubbles using a post-AGB-model of 0.595~Msun, allowing for variations of its evolutionary timescale and wind power. For Wolf-Rayet central stars, the wind is hydrogen-poor, more dense, and slower compared to O-type central stars. We use the CHIANTI software to compute the X-ray properties of bubble models along the evolutionary paths and 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. 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~Msun 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 eventually, evaporation begins, leading to chemically stratified bubbles. The bubbles of the youngest Wolf-Rayet objects appear chemically uniform, the chemically stratified bubbles of the evolved Wolf-rayet objects have excessively low characteristic temperatures that cannot be explained by our modelling. 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.