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We have obtained a very deep exposure (813 ks) of $\zeta\,$Puppis (O4supergiant) with the Chandra/HETG Spectrometer. Here we report on analysis ofthe 1-9 \r{A} region, especially well suited for Chandra, which has asignificant contribution from continuum emission between well separatedemission lines from high-ionization species. These data allow us to study thehottest plasma present through the continuum shape and emission line strengths.Assuming a powerlaw emission measure distribution which has a high-temperaturecut-off, we find that the emission is consistent with a thermal spectrum havinga maximum temperature of 12 MK. This implies an effective wind shock velocityof $900\,\mathrm{km\,s^{-1}}$, well below the wind terminal speed of$2250\,\mathrm{km\,s^{-1}}$. For X-ray emission which forms close to the star,the speed and X-ray flux are larger than can be easily reconciled with strictlyself-excited line-deshadowing-instability models, suggesting a need for afraction of the wind to be accelerated extremely rapidly right from the base.This is not so much a dynamical instability as a nonlinear response to changingboundary conditions.

A Deep Exposure in High Resolution X-Rays Reveals the Hottest Plasma in the $\zeta\,$Puppis Wind