The Importance of XUV Radiation as a Solution to the P V Mass Loss Rate Discrepancy in O-Stars

TL;DR

This paper investigates the role of XUV radiation in resolving the discrepancy in O-star mass loss rates derived from P V line observations, suggesting that XUV emission lines can explain the observed ionization without requiring large reductions in mass loss rates.

Contribution

The study demonstrates that strong XUV emission lines can significantly affect P V ionization balance, negating the need for large mass loss rate reductions in O-stars.

Findings

XUV lines reduce P V fractional abundance significantly.

Large mass loss reductions are unnecessary for consistency.

The X-ray optical depth relation remains valid.

Abstract

A controversy has developed regarding the stellar wind mass loss rates in O-stars. The current consensus is that these winds may be clumped which implies that all previously derived mass loss rates using density-squared diagnostics are overestimated by a factor of ~ 2. However, arguments based on FUSE observations of the P V resonance line doublet suggest that these rates should be smaller by another order of magnitude, provided that P V is the dominant phosphorous ion among these stars. Although a large mass loss rate reduction would have a range of undesirable consequences, it does provide a straightforward explanation of the unexpected symmetric and un-shifted X-ray emission line profiles observed in high energy resolution spectra. But acceptance of such a large reduction then leads to a contradiction with an important observed X-ray property: the correlation between He-like ion source radii and their equivalent X-ray continuum optical depth unity radii. Here we examine the phosphorous ionization balance since the P V fractional abundance, q(P V), is fundamental to understanding the magnitude of this mass loss reduction. We find that strong "XUV" emission lines in the He II Lyman continuum can significantly reduce q(P V). Furthermore, owing to the unique energy distribution of these XUV lines, there is a negligible impact on the S V fractional abundance (a key component in the FUSE mass loss argument). We conclude that large reductions in O-star mass loss rates are not required, and the X-ray optical depth unity relation remains valid.