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S. R. Heap (NASA's GSFC), T. M. Lanz (UMD & NASA's GSFC)
Recent evolutionary models of massive stars (Maeder & Meynet 2000, 2001; Heger & Langer 2000) predict important effects of rotation including: increasing the rate of mass-loss; lowering the effective gravity; altering the evolutionary track on the HRD; extending the main-sequence phase (both on the HR diagram and in time); and mixing of CNO-processed elements up to the stellar surface. Observations suggest that rotation is a more important factor at lower metallicities because of higher initial rotational velocities and weaker winds. This makes the SMC, a low-metallicity galaxy (Z=0.2Z\odot), an excellent environment for discerning the role of rotation in massive stars.
We report on a FUSE+STIS+optical spectral analysis of 17 O-type stars in the SMC, where we found an enormous range in N abundances. Three stars in the sample have the same (low) CN abundances as the nebular material out of which they formed, namely C=0.085 C\odot and N=0.034 N\odot. However, more than half show N~N\odot, an enrichment factor of 30X! Such unexpectedly high levels of N have ramifications for the evolution of massive stars including precursors to supernovae. They also raise questions about the sources of nitrogen in the early universe.
This study was supported in part by grants from NASA's ADP, HST GO-7437, and FUSE B134.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.