The X-ray Emission from Planetary Nebulae

Planetary nebulae, the progeny of low- and intermediate-mass stars, are formed by the interaction of the fast stellar wind from its central star, which is currently evolving towards the white dwarf phase, and the slow wind ejected in the late stages of the asymptotic giant branch. The strong interaction between these two stellar winds causes a reverse shock to progress throughout the fast stellar wind, shock-heating this tenuous wind up to X-ray-emitting temperatures and producing a hot bubble. Processes of mixing, mass evaporation, and heat conduction at the interphase between the hot bubble and the optical nebula produce optimal conditions for diffuse X-ray emission. Chandra and XMM-Newton have allowed us for the first time to assess the effects of the X-ray-emitting plasma in the interior of planetary nebulae on their formation and evolution. Moreover, sources of hard X-ray emission have been discovered at their central stars, which provide evidence for their evolution in binary systems, processes of accretion, and for the unstable nature of their stellar winds.