We have obtained a very deep exposure ( 813 \mathrm { ks } ) of \zeta Puppis ( O4 supergiant ) with the Chandra HETG Spectrometer .
Here we report on analysis of the 1 – 9 \mathrm { \AA } region , especially well suited for Chandra , which has a significant contribution from continuum emission between well separated emission lines from high-ionization species .
These data allow us to study the hottest plasma present through the continuum shape and emission line strengths .
Assuming a powerlaw emission measure distribution which has a high-temperature cut-off , we find that the emission is consistent with a thermal spectrum having a maximum temperature of 12 \mathrm { MK } as determined from the corresponding spectral cut-off .
This implies an effective wind shock velocity of 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 strictly self-excited line-deshadowing-instability models , suggesting a need for a fraction 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 changing boundary conditions .