X-Ray Emitting Gas in Circumstellar Nebulae

Recent high-resolution 2D radiation-hydrodynamic numerical simulations of the formation and evolution of hot bubbles around evolved stars are described. The simulations take into account the evolution of the stellar parameters such as ionizing photon rate, wind velocity and mass-loss rate for a range of initial stellar masses. For low-mass stars, a planetary nebula with a lifetime of a few thousand years forms around the central hot star, while for massive stars the result is a Wolf-Rayet nebula, which has a lifetime of tens of thousands of years. In both cases, instabilities in the fast wind-slow wind interaction zone produce clumps and filaments in the swept-up shell of nebular material. Turbulent mixing and thermal conduction at the corrugated interface can produce quantities of intermediate temperature and density gas between the hot, shocked wind bubble, and the swept-up photoionized nebular material, which can emit in soft, diffuse X-rays. Sampling of the resultant theoretical spectra helps to make meaningful comparisons with recent observations of planetary nebulae.