THE 0.1-2.5 KEV X-RAY-SPECTRUM OF THE O4F STAR ZETA-PUPPIS

We have obtained a high quality ROSAT PSPC spectrum of the bright O4 f star zeta Pup. Allowing for the wind X-ray opacity, as computed from detailed non-LTE stellar wind models of zeta Pup, and under the assump tion that the X-rays arise from shocks distributed throughout the wind , we have been able to match the observed X-ray spectrum (0.1 to 2.5ke V). The best model fit is obtained when He++ recombines to He+ in the outer regions of the stellar wind, as predicted by recent detailed coo l wind model calculations. With a single temperature plasma, the best model fit indicates a temperature of log T(s)(K) = 6.5 to 6.6 correspo nding to shock velocities of around 500 km s-1. A 2 temperature plasma yields a significantly improved fit, and indicates temperatures of lo g T(s)(K) = 6.2 and 6.7 for the 2 components. The hotter component acc ounts for 55% of the intrinsic (75% of the observed) X-ray flux. Due t o absorption by the stellar wind, and to a minor extent stellar occult ation, less than 5% of the total emitted X-ray flux escapes the star. The models require significant X-ray emission (particularly at energie s less than 0.5 keV) from large radii (r > 100R). In models without r ecombination, the fits, even with a 2 temperature plasma, are unaccept able. A significant K shell absorption is predicted by these models, b ut is definitely not present in the observational data. The analysis s uggests that the X-ray flux provides an invaluable diagnostic of the i onization of helium in the stellar wind of stars with low reddening.