SIMBAD references

We examine mechanisms that may explain the luminosities and relatively low temperatures of extended X-ray emission in planetary nebulae. By building a simple flow structure for the wind from the central star during the proto- and early planetary nebula phase, we estimate the temperature of the X-ray-emitting gas and its total X-ray luminosity. We conclude that in order to account for the X-ray temperature and luminosity, both the evolution of the wind from the central star and the adiabatic cooling of the post-shocked wind's material must be considered. The X-ray-emitting gas results mainly from shocked wind segments that were expelled during the early planetary nebula phase when the wind speed was moderate, ∼500 km/s. Alternatively, the X-ray-emitting gas may result from a collimated fast wind blown by a companion to the central star. Heat conduction and mixing between hot and cool regions are likely to occur in some cases and may determine the detailed X-ray morphology of a nebula but are not required to explain the basic properties of the X-ray-emitting gas.