On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of ζ Puppis

We have studied superionization and X-ray line formation in the spectra of ζ Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O VI λλ1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O VI profiles. However, enough O VI can be produced if the voids are filled by a low-density gas. The recombination of O VI is very efficient in the dense material, but in the tenuous interclump region an observable amount of O VI can be maintained. We also find that different UV resonance lines are sensitive to different density regimes in ζ Pup: C IV is almost exclusively formed within the densest regions, while the majority of O VI resides between clumps. N V is an intermediate case, with contributions from both the tenuous gas and clumps.