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The presence of O VI ions can be indicative of plasma temperatures of a fewtimes 10^5 K that is expected in heat conduction layers between the hot shockedstellar wind gas at several 10^6 K and the cooler (~10,000 K) nebular gas ofplanetary nebulae (PNe). We have used FUSE observations of PNe to search fornebular O VI emission or absorption as a diagnostic of conduction layer toensure the presence of hot interior gas. Three PNe showing nebular O VI, namelyIC 418, NGC 2392, and NGC 6826, have been selected for Chandra observations anddiffuse X-ray emission is indeed detected in each of these PNe. Among thethree, NGC 2392 has peculiarly high diffuse X-ray luminosity and plasmatemperature compared with those expected from its stellar wind's mechanicalluminosity and terminal velocity. The limited effects of heat conduction on theplasma temperature of a hot bubble at the low terminal velocity of the stellarwind of NGC 2392 may partially account for its high plasma temperature, but thehigh X-ray luminosity needs to be powered by processes other than the observedstellar wind, probably caused by the presence of an unseen binary companion ofthe CSPN of NGC 2392. We have compiled relevant information on the X-ray,stellar, and nebular properties of PNe with a bubble morphology and found thatthe expectations of bubble models including heat conduction compare favorablywith the present X-ray observations of hot bubbles around H-rich CSPNe, buthave notable discrepancies for those around H-poor [WR] CSPNe. We note that PNewith more massive central stars can produce hotter plasma and higher X-raysurface brightness inside central hot bubbles.

Diffuse X-ray Emission from Planetary Nebulae with Nebular O VI