Quantitative UV spectroscopy of early O stars on the Magellanic Clouds The determination of the stellar metallicities

UV spectra of 4 O-stars in the Magellanic Clouds obtained with the Faint Object Spectrograph of the Hubble Space Telescope are analyzed with respect to metallicity. With the stellar parameters T_e_f_f, log g, R_*, and the mass loss rates M known from optical analyses the metal abundances including iron group elements are derived in two steps. First, hydrodynamic radiation driven wind NLTE models with metallicity as a free parameter are constructed to fit the observed wind momentum rate and, thus, yield a dynamical metallicity. Then, synthetic spectra are computed for different metal abundances and compared to the observed spectra to obtain a spectroscopic metallicity. In general, the results obtained from both methods agree. For the two stars in the Small Magellanic Cloud (NGC 346 3, O3III(f*) and AV 243, O6V) metallicities of log z/z_sun=-0.7 and -0.8, respectively, are found. The O3 star shows evidence for CNO-cycled matter in its atmosphere. The metallicity of the two stars in the Large Cloud (Sk-68 137, O3III(f*) and Sk-67 166, O4If"+) are constrained to log z/z_sun=-0.3 and -0.1. Because of saturation effects in the cores of the pseudophotospheric metal lines the determination of the LMC metallicity is less reliable. (orig.)Available from TIB Hannover: RR 4697(1052) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman