The X-ray emission from shock cooling zones in O star winds

A semi-empirical model is developed for the X-ray emission from O star winds, and used to analyze recent ROSAT PSPC spectra. The X-rays are assumed to originate from cooling zones behind shock fronts, where the cooling is primarily radiative at small radii in the wind, and due to expansion at large radii. The shocks are dispersed in a cold background wind whose X-ray opacity is provided by detailed NLTE calculations. This model is a natural extension of the Hillier et al. (1993) model of isothermal wind shocks. By assuming spatially constant shock temperatures, these authors achieved good fits to the data only by postulating two intermixed shock families of independent temperature and filling factor - i.e., by adjusting in parallel four parameters. By applying the present model to the analysis of high S/N PSPC spectra of three O-stars (#zeta# Pup, #iota# Ori, #zeta# Ori), we achieve fits of almost the same quality with only two parameters. This supports the idea that the two- or multi-component X-ray spectra are indeed due to stratified cooling layers. (orig.)36 refs.Available from TIB Hannover: RR 4697(985) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

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

Hot luminous stars in nearby galaxies

We discuss a new method for the determination of stellar distances using the dependence of the stellar mass-loss rate upon luminosity, the so-called wind momentum - luminosity relationship. We review some recent results on wind momentum rates and metallicities of O-stars and B-supergiants in the Magellanic Clouds and include some new results derived using HST data. Clear evidence for a metallicity effect in the Small Magellanic Cloud is found, the effect for the Large Magellanic Cloud being only moderate. We discuss the need to calibrate the relationship using supergiants in these galaxies so that the method may be applied to supergiants in more distant galaxies. The potential of this method with regard to both distance and metallicity determinations is illustrated by results for two supergiants in each of the galaxies M31 and M33. We also show new HST/WFPC2 images of massive O-stars in the Magellanic Clouds and, in combination with previous optical and HST/FOS spectra, suggest that these data constitute evidence that one of these stars, Sk-67 211, may have a mass of close to 200 M_sun. (orig.)31 refs.Available from TIB Hannover: RR 4697(925) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

O-star mass-loss and wind momentum rates in the Galaxy and the Magellanic Clouds Observations and theoretical predictions

A new, very fast approximate method is presented to determine mass-loss rates of O-stars from H_#alpha# line profiles. The method uses H and HeII departure coefficients from unified model atmospheres parametrized in a simple way as function of wind velocity together with photospheric NLTE line profiles as the inner boundary condition for a numerically exact radiative transfer solution to derive a wind contaminated H_#alpha#-profile. The method is also applied to H_#gamma# to determine stellar gravities corrected for wind emission. A detailed analytical discussion of H_#alpha# line formation in O-star winds is given and it is demonstrated that former very simple approaches considering only optically thin wind emission lead to significant systematic errors. Scaling relations and generalized curves of growth are presented that connect mass-loss rate, terminal velocity, stellar parameters and H_#alpha# equivalent width. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman