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

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