  S308 is a circumstellar bubble blown by the WN4 star HD50896. It is one of
the only two single-star bubbles that show detectable diffuse X-ray emission.
We have obtained XMM-Newton EPIC observations of the northwest quadrant of
S308. The diffuse X-ray emission shows a limb-brightened morphology, with a
clear gap extending from the outer edge of the diffuse X-ray emission to the
outer rim of the nebular shell. The X-ray spectrum of the diffuse emission is
very soft, and is well fitted by an optically thin plasma model for a
N-enriched plasma at temperatures of ~1.1x10^6 K. A hotter gas component may
exist but its temperature is not well constrained as it contributes less than
6% of the observed X-ray flux. The total X-ray luminosity of S308, extrapolated
from the bright northwest quadrant, is <=(1.2+-0.5)x10^{34} ergs/s. We have
used the observed bubble dynamics and the physical parameters of the hot
interior gas of S308 in conjunction with the circumstellar bubble model of
Garcia-Segura & Mac Low (1995) to demonstrate that the X-ray-emitting gas must
be dominated by mixed-in nebular material.
