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R.J. Dufour, B.D. Moore (Rice Univ.), J.J. Hester, P.A. Scowen (Arizona State Univ.), B.A. Buckalew (Rice Univ.)
HST/WFPC2 imagery and STIS spectra of the circumstellar shell NGC~6888 are presented. The nebula was formed by the collision of the stellar wind of the WN6 star HD~192163 with material shed during its preceding red supergiant phase. The resulting nebula is an 8 pc \times 5 pc photoionized shell enveloped in a shock-heated skin of emission powered by the internal pressure of the wind-blown bubble. We present color-coded pictures of our new WFPC2 imagery along the shell's northern perimeter. These included three fields imaged using the F502N, F656N, and F658N filters, and a previously studied region using F487N and F658N. STIS spectra covering the ~1650-9500 Å~ wavelength range of four knots and surrounding medium were obtained for comprehensive studies of spatial variations in various emission lines. Collectively the new data permit a global view of the shell material at different times relative to the breakout of the hot bubble interior. The NE pointing was observed previously with WFPC2, and analyzed by comparison with photoionization models. The earlier work, presented in Moore, Hester and Scowen 2000 (MHS00) relied on ground-based observations. Our new observations show that previous estimates of [N~II]/H\alpha were low by 20-50%. The lower gas density in the new models is in better agreement with the evolutionary scenario discussed in MHS00. The new ionizing flux is closer to that of the stellar model of Crowther & Smith 1996. The absence of the UV C~II] & C~III] lines in our STIS spectra allows only an upper limit on the carbon abundance in the nebula. Support for the observations came from AURA/STScI to Rice & ASU as General Observer Program GO-08568.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.