QUANTITATIVE UV SPECTROSCOPY OF EARLY O-STAR IN THE MAGELLANIC CLOUDS- THE DETERMINATION OF THE STELLAR METALLICITIES

UV spectra of 4 O-stars in the Magellanic Clouds obtained with the Fai nt Object Spectrograph of the Hubble Space Telescope are analyzed with respect to metallicity. With the stellar parameters T-eff, log g, R, and the mass loss rates (M) over dot known from optical analyses, the metal abundances including iron group elements are derived in two ste ps. First, hydrodynamic radiation driven wind NLTE models with metalli city as a free parameter are constructed to fit the observed wind mome ntum rate and thus, yield a dynamical metallicity. Then, synthetic spe ctra are computed for different metal abundances and compared to the o bserved spectra to obtain a spectroscopic metallicity. In general, the results obtained from both methods agree. For the two stars in the Sm all Magellanic Cloud (NGC 346#3, O3 III(f) and AV 243, O6 V) metallic ities of log z/z(circle dot) = -0.7 and -0.8, respectively, are found. The O3 star shows evidence for CNO-cycled matter in its atmosphere. T he metallicity of the two stars in the Large Cloud (Sk-68 degrees 137, O3 III(f) and Sk-67 degrees 166, O4 If(+)) are constrained to log z/ z(circle dot) = -0.3 and -0.1. Because of saturation effects in the co res of the pseudophotospheric metal lines the determination of the LMC metallicity is less reliable.