Hot Gas in the Circumstellar Bubble S308

S308 is a circumstellar bubble blown by the WN4 star HD50896. It is one ofthe only two single-star bubbles that show detectable diffuse X-ray emission.We have obtained XMM-Newton EPIC observations of the northwest quadrant ofS308. The diffuse X-ray emission shows a limb-brightened morphology, with aclear gap extending from the outer edge of the diffuse X-ray emission to theouter rim of the nebular shell. The X-ray spectrum of the diffuse emission isvery soft, and is well fitted by an optically thin plasma model for aN-enriched plasma at temperatures of ~1.1x10^6 K. A hotter gas component mayexist but its temperature is not well constrained as it contributes less than6% of the observed X-ray flux. The total X-ray luminosity of S308, extrapolatedfrom the bright northwest quadrant, is <=(1.2+-0.5)x10^{34} ergs/s. We haveused the observed bubble dynamics and the physical parameters of the hotinterior gas of S308 in conjunction with the circumstellar bubble model ofGarcia-Segura & Mac Low (1995) to demonstrate that the X-ray-emitting gas mustbe dominated by mixed-in nebular material.Comment: 18 pages, 3 figures, to appear in the Astrophysical Journal, Dec 20 issu