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Y.-H. Chu, M.A. Guerrero, R.A. Gruendl (UIUC), G. Garcia-Segura (UNAM), H. Wendker (U. Hamburg)
S\,308 is a circumstellar bubble blown by the fast wind of the Wolf-Rayet star HD\,50896. Diffuse X-ray emission from its interior has been previously detected by ROSAT; however, a significant fraction of the diffuse emission was occulted by the PSPC window support structure, making it difficult to determine the distribution of X-ray emission. Spectral analysis is also difficult because of its low surface brigthness and contamination by point sources. We have obtained XMM-Newton observations of the northewest quadrant of S\,308 with 10'' resolution. The exposure time was 47 ks, but a high-background flare occurred during the observation and limited the useful exposure times to 22.7 ks and 16.9 ks for the EPIC/MOS and EPIC/pn observations, respectively. Even with reduced exposures, diffuse X-ray emission from S\,308 was clearly detected. The X-ray emission shows a limb-brightened morphology; however, a distinct gap with widths of 100--200'' (0.7--1.4 pc) exists between the optical shell rim and the outer edge of the diffuse X-ray emission. This is in sharp contrast to NGC\,6888, in which the X-ray emission extends all the way to the edge of the optical shell. The integrated X-ray spectrum of S\,308 is soft, showing prominent N\,{\sc vii} lines which reflect the enhanced N abundance in the nebula. Spectral fits with thin plasma model indicate that the hot gas in S\,308 has a temperature of T ~1.1\times106 K (kT ~ 0.094 keV). This temperature is lower than those of NGC\,6888 and planetary nebulae, 2--3\times106 K. The differences in morphology and plasma temperature between S\,308 and NGC\,6888 may be attributed to an evolutionary effect, as S\,308 is 70,000 yr old and NGC\,6888 is only 24,000 yr old.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.