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

Abstract

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_eff,log g, R_{*}, and the mass loss rates dot {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 (NGC346#3, O3III(f(*) ) and AV243, 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-68mbox {^\circ}137, O3III(f(*) ) and Sk-67mbox {^\circ}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. Based on observations with the NASA/ESA {\em Hubble Space Telescope