The 0.1-2.5 keV X-ray spectrum of the O4f star ζ Puppis

We have obtained a high quality ROSAT PSPC spectrum of the bright O4f 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 assumption that the X-rays arise from shocks distributed throughout the wind, we have been able to match the observed X-ray spectrum (0.1-2.5keV). The best model fit is obtained when He(++) recombines to He(+) in the outer regions of the stellar wind, as predicted by recent detailed cool wind model calculations. With a single temperature plasma, the best model fit indicates a temperature of log Ts(K) = 6.5-6.6, corresponding to shock velocities of around 500 km/s. A 2-temperature plasma yields a significantly improved fit and indicates temperatures of log Ts(K) = 6.2 and 6.7 for the 2 components. The hotter component accounts for 55 percent of the intrinsic (75 percent of the observed) X-ray flux.