  The presence of O VI ions can be indicative of plasma temperatures of a few
times 10^5 K that is expected in heat conduction layers between the hot shocked
stellar wind gas at several 10^6 K and the cooler (~10,000 K) nebular gas of
planetary nebulae (PNe). We have used FUSE observations of PNe to search for
nebular O VI emission or absorption as a diagnostic of conduction layer to
ensure the presence of hot interior gas. Three PNe showing nebular O VI, namely
IC 418, NGC 2392, and NGC 6826, have been selected for Chandra observations and
diffuse X-ray emission is indeed detected in each of these PNe. Among the
three, NGC 2392 has peculiarly high diffuse X-ray luminosity and plasma
temperature compared with those expected from its stellar wind's mechanical
luminosity and terminal velocity. The limited effects of heat conduction on the
plasma temperature of a hot bubble at the low terminal velocity of the stellar
wind of NGC 2392 may partially account for its high plasma temperature, but the
high X-ray luminosity needs to be powered by processes other than the observed
stellar wind, probably caused by the presence of an unseen binary companion of
the 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 that
the expectations of bubble models including heat conduction compare favorably
with the present X-ray observations of hot bubbles around H-rich CSPNe, but
have notable discrepancies for those around H-poor [WR] CSPNe. We note that PNe
with more massive central stars can produce hotter plasma and higher X-ray
surface brightness inside central hot bubbles.
