On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of Zeta Pup

TL;DR

This study investigates how the interclump medium influences superionization, specifically O VI formation, in Zeta Pup's stellar wind, revealing the importance of low-density gas in reproducing observed spectral features.

Contribution

The paper introduces a new stellar atmosphere code, XCMFGEN, and demonstrates that including a low-density interclump medium is crucial for accurately modeling O VI line formation.

Findings

Void interclump regions cannot reproduce O VI profiles.

Low-density interclump gas allows sufficient O VI production.

Different UV lines probe different density regimes.

Abstract

We have studied superionization and X-ray line formation in the spectra of Zeta 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 ll1032, 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 Zeta 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.