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

Vela X-1: How to produce asymmetric eclipses

Light curves of the X-ray pulsar Vela X-1 obtained with the ASCA satellite show a strong asymmetry in the hard energy band during the eclipse of the X-ray source: a steep drop at ingress is followed by a gradual decline, whereas only a steep increase is observed at egress. X-ray scattering off the dense accretion wake trailing the neutron star cannot explain the gradual decline because of the long persistence of the latter (#DELTA##phi#=0.11). Instead we propose that scattering in an extended photoionization wake is responsible. This wake is caused by the switch-off of the radiative force that drives the B supergiant wind during the passage through the highly ionized Stroemgren region surrounding the X-ray source. The piled-up gas then trails the neutron star, which moves relative to the B star surface (no corotation). A model for the Vela X-1 system which assumes that the B star does not rotate gives too large a phase extent for the dense wake and can be ruled out. Including the B star rotation, the relative azimuthal motion of the neutron star is slower and the wake covers a smaller phase interval. Finally, we assume that the Stroemgren sphere does not reach too deep into the wind accelerating region. The wind can then reach a certain fraction of the terminal velocity before the radiative force is switched off. This elongates the photoionization wake further and both its phase extent and its scattering efficiency match the observations. (orig.)Available from TIB Hannover: RR 4697(894) / 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