Helium-like X-ray line complexes show that the hottest plasma on the zeta supergiant Puppis is in its wind

We present an analysis of Chandra grating spectra of key helium-like line complexes to put constraints on the location with respect to the photosphere of the hottest (T greater than or similar to 6 x 10(6) K) plasma in the wind of the O supergiant zeta Pup and to explore changes in the 18 yr between two sets of observations of this star. We fit two models - one empirical and one wind-shock-based - to the S XV, Si XIII, and Mg XI line complexes and show that an origin in the wind flow, above r approximate to 1.5 R*, is strongly favoured over an origin less than 0.3 R* above the photosphere (r less than or similar to 1.3 R*), especially in the more recent, very long-exposure data set. There is a modest increase in the line and continuum fluxes, line widths, wind absorption signatures, and of the hot plasma's distance from the photosphere in the 18 yr since the first Chandra grating observation of zeta Pup. Both modes of modelling include the effects of dielectronic recombination satellite emission line blending on the helium-like complexes - the first time this has been accounted for in the analysis of He-like line ratios in O stars.