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 backgrou nd 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 temper atures, these authors achieved good fits to the data only by postulati ng two intermixed shock families of independent temperature and filing 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 sa me quality with only two parameters. This supports the idea that the t wo- or multicomponent X-ray spectra are indeed due to stratified cooli ng layers.