One key feature of the interacting stellar winds model of the formation of planetary nebulae ( PNe ) is the presence of shock-heated stellar wind confined in the central cavities of PNe .
This so-called hot bubble should be detectable in X-rays .
Here we present XMM-Newton observations of NGC 3242 , a multiple-shell PN whose shell morphology is consistent with the interacting stellar winds model .
Diffuse X-ray emission is detected within its inner shell with a plasma temperature \sim 2.35 \times 10 ^ { 6 } K and an intrinsic X-ray luminosity \sim 2 \times 10 ^ { 30 } ergs s ^ { -1 } at the adopted distance of 0.55 kpc .
The observed X-ray temperature and luminosity are in agreement with “ ad-hoc ” predictions of models including heat conduction .
However , the chemical abundances of the X-ray-emitting plasma seem to imply little evaporation of cold material into the hot bubble , whereas the thermal pressure of the hot gas is unlikely to drive the nebular expansion as it is lower than that of the inner shell rim .
These inconsistencies are compounded by the apparent large filling factor of the hot gas within the central cavity of NGC 3242 .