Spectral diagnostics of luminous blue supergiants

The spectral analysis of the most luminous stars as individuals provides a unique tool for the determination of the properties of young populations in galaxies. Generally this requires hydrodynamic model atmospheres including the effects of spherical extension and stellar winds. Standard codes for this problem have now been developed by many groups. It is shown how the properties of the most luminous stars in different galaxies (temperature, gravity, radii, luminosity, masses, mass-loss rates, wind velocities, abundances) can be determined using spectral information from the UV to the IR. Recent results for O, B, A-supergiants in the Galaxy, LMC, SMC, M31, M33 and beyond the Local Group are presented. Special emphasis is given to the wind momentum - luminosity relationship as a tool for the determination of extragalactic distances out to the Virgo cluster of galaxies. (orig.)57 refs.Available from TIB Hannover: RR 4697(993) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Stellar wind momentum in galaxies and a new parametrization of the radiative line force

We demonstrate that the classical concept of ''line force multipliers'' k, #alpha#, #delta# to calculate the radiative line acceleration is insufficient. General tables with these force multiplier parameters as function of effective temperature only are bound to fail in predicting accurate wind properties across the HRD in all stages of stellar evolution. As a consequence, we develop a new parametrization method which is more precise and will allow a much better description of radiation driven winds. (orig.)18 refs.Available from TIB Hannover: RR 4697(1053) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

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

Fundamental parameters of galactic luminous OB stars. Pt. 2 A spectroscopic analysis of HDE 226 868 and the mass of Cygnus X-1

We present a spectroscopic analysis of the O9.7 Iab star HDE 226868, which is the optical counterpart of Cygnus X-1. We use this extreme supergiant to test the differences in the stellar parameters derived using plane parallel hydrostatic model atmospheres and spherical non-hydrostatic models ('Unified Model'). We find that the difference is significant, but smaller than it is needed to explain the mass and helium discrepancy between the theories of stellar atmospheres and evolution. We also find that the dilution effect of He I 4471 is not due only to the presence of a strong wind. We use the derived atmospheric parameters to show that Cygnus X-1 has to be a black hole (provided it is a single compact object) in a way that is independent of the distance to the system and assumptions about the mass-luminosity relation of the visible star. We also derive a probable mass for Cygnus X-1. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Stellar winds and the EUV continuum excess of early B-giants

The strong excess of radiation in the Lyman- and He I-continuum of the B-giant element of CMa, detected very recently with EUVE (Cassinelli et al. 1994), is partially explained as a consequence of transonic velocity fields of weak stellar winds. Velocity induced density changes or Doppler-shifts reduce the opacity and increase the escape probability in the resonance transitions of hydrogen and neutral helium at the depths of formation of the ground state continua. As a result the ground state occupation numbers and the corresponding absorption edges are reduced significantly, and the emergent Lyman- and He I-fluxes are enhanced. A small sequence of simple model atmospheres for B-giants including spherical extension and stellar winds is calculated to demonstrate the importance of this effect and to investigate its dependence on the mass-loss rate. It is shown that the number of hydrogen and helium ionizing photons of early B-giants is a strong function of the mass-loss rate. This might have consequences for the Diffuse ionizing radiation field in galaxies. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Non-LTE line formation for neutral oxygen Model atom and first results on A-type stars

Available from: http://www.mpa-garching.mpg.de / 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