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R.A. Gruendl, M.A. Guerrero, Y.-H. Chu (UIUC)
NGC\,6888, the Crescent Nebula, is a circumstellar bubble blown by the fast wind of the Wolf-Rayet star HD\,192163. Diffuse X-ray emission from its interior has been previously detected by Einstein, ROSAT, and ASCA with 30''--150'' angular resolution. We have obtained a Chandra ACIS-S observation of the northeast quadrant of NGC\,6888 with 1'' resolution. The observation clearly resolved point sources from the diffuse X-ray emission. A total of 13,000 counts from the diffuse emission were detected. The diffuse X-ray emission shows a limb-brightened morphology with spatial extent similar to that of the cool, dense nebular shell, but with the X-ray emission peaking interior to the optical emission. The integrated X-ray spectrum is soft, showing prominent N\,{\sc vii} lines consistent with the N-enriched nebular abundance. Spectral fits with a thin plasma model indicate that the hot gas in NGC\,6888 has a temperature of T ~2.8\times106 K (kT ~ 0.24 keV). This temperature is higher than that of S\,308, but similar to those found in planetary nebulae. The differences in morphology and plasma temperature between NGC\,6888 and S\,308 suggest an evolutionary effect consistent with their dynamic ages, 24,000 yr for NGC\,6888 and 70,000 yr for S\,308.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.