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T. Lanz (U. Maryland, GSFC), J.-C. Bouret, S. R. Heap (NASA's GSFC), I. Hubeny (AURA, GSFC), D. J. Hillier (U. Pittsburgh), D. J. Lennon (IAC), L. J. Smith, C. J. Evans (UCL), S. P. Owocki (U. Delaware)
We have analyzed the UV and optical spectrum of the O9.5V Star, NGC 346-12, using sophisticated, NLTE line-blanketed model atmospheres calculated with our code TLUSTY. The following stellar parameters were derived: T\rm eff=30,000\,K, \log g=3.5. An abundance analysis yields a metallicity, [Fe/H]=-1.0. The N/C abundance ratio is 25 times the solar ratio, indicating that material processed through the CNO-cycle has been brought up to the surface. Assuming a distance modulus, (m-M)=18.9, for the SMC, we have derived the luminosity, radius, and mass of the star. We found, similarly to higher luminosity galactic stars, a discrepancy between the mass derived from the spectroscopic analysis and from stellar evolution theory. We conclude that it is very likely that NGC 346-12 is a fast rotator, whose evolution has been affected by rotation. Furthermore, the wind of NGC 346-12 appears abnormal: while the C IV resonance lines do not reveal any indication of a wind, a weak P-Cygni profile is observed in \ion{N}{5} 1240. Various possible explanations for the low inferred ion density in the stellar wind, including an enhanced degree of wind ionization associated with ion frictional heating, or ion runaway due to frictional decoupling from the hydrogen-helium bulk plasma, are examined.
This work was supported through a NASA/NRC RA award and STScI grants (GO 7437, AR 7985).
The author(s) of this abstract have provided an email address for comments about the abstract: lanz@nova.gsfc.nasa.gov