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B.D. Moore, J.J. Hester, P.A. Scowen (Arizona State University), D.K. Walter (South Carolina State University)
Wind-blown nebulae are formed in the complex interaction of slow winds, fast winds, stellar ionizing radiation, and the surrounding ambient environment. The pressure of the confined wind drives a shock into the circumstellar medium, sweeping it into a shell of material whose ionization state is maintained both by shocks and by the UV flux of the central star. Unlike normal H II region emission or objects such as supernova remnants, the properties of wind-blown nebulae are the simultaneous result of both shock physics and photoionization.
We present WF/PC and WFPC2 observations of the wind-blown nebulae NGC 6888 and NGC 7635, together with analysis of these data based on photoionization and shock model calculations. NGC 6888 is the nebula formed by the Wolf-Rayet stage of the central star HD 192163. It is comprised of material lost during the previous red supergiant phase of the star. This material is now visible as a fragmented shell. Together the expanding shell and confined wind are driving shocks into the medium external to the shell. NGC 7635, by contrast, is formed within a blister H~II region by the wind from an O star. Of particular interest is a location where the wind shock impinges on the photoionized photoevaporative flow from the H~II region cavity. Also of interest are a number of dense knots located very close to the central star. The high spatial resolution of our HST images allows us to study the sharply stratified ionization structure present throughout these nebulae.
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