S308 is a circumstellar bubble blown by the WN4 star HD 50896. 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 an N-enriched plasma at temperatures of ～1.1 x 10~6 K. A hotter gas component may exist but its temperature is not well constrained since 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) x 10~(34) ergs s~(-1). 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, to demonstrate that the X-ray-emitting gas must be dominated by mixed-in nebular material.
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