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.Comment: 18 pages, 3 figures, to appear in the Astrophysical Journal, Dec 20 issu

Abstract.We have observed the stellar wind blown bubble NGC 6888 with the ROSAT HRI. A map and a catalogue of X-ray filaments is derived, the typical filament being a few 0.1 pc in extent with an HRI count rate of a few 10 −4 s −1 . We show that this filamentary structure can qualitatively be modelled as local count rate enhancements due to denser gas near evaporating cool clumps which in turn are seen in the optical and probably are produced by instabilities after the passage of the primary shock front of the expanding bubble.

Abstract. The apparent shapes and orientations of optical nebulae in the Cygnus X complex provide possible evidence for the existence of a symmetry in the local spiral arm which may be related to the structure of the local magnetic field within the spiral arm. We have made a preliminary determination of the distances to about 90 nebulae in the Cygnus X complex by use of the values of interstellar absorption as a function of galactic coordinates. These values of absorption were determined from a comparison of optical and radio data for the nebulae. The more prominent nebulae are clumped at a distance of about 1.5 kpc. The total range in distances is from 1 kpc to at least 4 kpc. We have attempted to fit model spiral arms to this three-dimensional distribution of nebulae by approximating the spiral arm with a truncated cylinder. It has been possible to narrow the range of permissible orientations and sizes etc. for this local section of the Orion arm.

We present ASCA SIS observations of the wind-blown bubble NGC 6888. Owing to the higher sensitivity of the SIS for higher energy photons compared to the ROSAT PSPC, we are able to detect a T $ 8 ; 10 6 K plasma component in addition to the T $ 1:3 ; 10 6 K component previously detected in PSPC observations. No significant temperature variations are detected within NGC 6888. García-Segura & Mac Low's analytical models of WR bubbles constrained by the observed size, expansion velocity, and mass of the nebular shell underpredict the stellar wind luminosity and cannot reproduce simultaneously the observed X-ray luminosity, spectrum, surface brightness profile, and SIS count rate of NGC 6888's bubble interior. The agreement between observations and expectations from models may be improved if one or more of the following ad hoc assumptions are made: (1) the stellar wind luminosity was weaker in the past, (2) the bubble is at a special evolutionary stage and the nebular shell has recently been decelerated to 1 2 of its previous expansion velocity, and (3) the heat conduction between the hot interior and the cool nebular shell is suppressed. Chandra and XMM-Newton observations with high spatial resolution and high sensitivity are needed to accurately determine the physical conditions of NGC 6888's interior hot gas for critical comparisons with bubble models.