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B.D. Moore, R.J. Dufour (Rice Univ.), J.P. Aufdenberg (CfA), R. Sankrit (Johns Hopkins Univ.)
The Wolf-Rayet star WR 136, classified as a WN6, is the central star powering the ring nebula NGC 6888. The material in the nebula was shed by the central star during its preceeding post-main sequence phase, presumably a red supergiant (RSG). Evolutionary models of massive stars suggest an initial mass for the progenitor star of ~30 M\sun; models for the WR star suggest a mass of 10 M\sun. Of the 20 M\sun shed, however, only one-quarter can be accounted for in the nebular shell. It has been suggested previously that this material is present as a rarified shell outside the visible nebula, with all of the material ionized by the considerable UV flux from the central star. Here we present FUSE observations of WR 136 (HD 192163). In addition to the expected stellar and wind features of the WR star (e.g., P Cygni profiles) there are narrow absorption lines from intervening material. We identify and separate the various components, focussing on those lines we associate with the proposed rarified circumstellar matter. Particular attention is paid to the C~{\sc iii} \lambda 977 line; in conjunction with a grid of photoionization models, we can infer a range of C/H for the rarified material, and by extension the ring nebula.
This research was supported in part by NASA grant NAG5-12266 to Rice University.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.