Other
Scientific paper
Sep 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005aspc..332..266w&link_type=abstract
The Fate of the Most Massive Stars, ASP Conference Series, Vol. 332, Proceedings of the conference held 23-28 May, 2004 in Grand
Other
Scientific paper
A dichotomy has been found in the relative line strengths of CNO in the spectra of five O2 giants in the Magellanic Clouds. Three of them have N lines from all ionization stages present stronger than those of C, O while the other two have the opposite; the former group also has enhanced He line strengths. The CNO differences are seen in both the optical lines and in the UV stellar-wind profiles. The N-strong spectra have been reclassified as ON2 III(f*). These effects strongly suggest mixing of CNO-cycled material into the atmospheres and winds of the ON2 stars. To further investigate this interpretation, model-atmosphere abundance analyses have been performed. The results for N/C relative to the respective interstellar media are 46, 46, and 52 for the ON2 spectra vs 3.8 and 2.9 for the O2. The corresponding N/O results are 14, 23, and 12 vs 2.9 and 1.4. In the context of current evolutionary models, the implication is that the ON2 objects are more highly evolved and/or had higher rotational velocities on the main sequence. We have compared the results with Geneva models for rotating massive stars. The surprisingly large CNO effects in such hot, near-main sequence objects suggest that the ON2 stars are subject to stronger early mixing than predicted by the standard models, or that they have evolved homogeneously, moving back toward the main sequence.
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