Influence of XUV radiation on Pv ionization fraction in hot star winds

TL;DR

This paper investigates how XUV radiation affects the ionization of phosphorus in hot star winds, revealing that it can alter mass-loss rate diagnostics and contribute to the weak wind problem in certain stars.

Contribution

It provides a detailed model showing XUV radiation's impact on ionization fractions and wind properties, highlighting limitations of current mass-loss rate estimates.

Findings

XUV radiation decreases Pv ionization fraction in hot star winds.

XUV radiation influences wind terminal velocity by ionizing heavier ions.

Theoretical mass-loss rates align with some observational diagnostics for giants and supergiants, but not for main-sequence stars.

Abstract

Different diagnostics of hot star wind mass-loss rates provide results that are difficult to reconcile with each other. The widely accepted presence of clumping in hot star winds implies a significant reduction of observational mass-loss rate estimates from diagnostics that depend on the square of the density. Moreover, the ultraviolet Pv resonance lines indicate a possible need for even stronger reduction of hot star mass-loss rates, provided that Pv is a dominant ionization stage of phosphorus at least in some hot stars. The latter assumption is challenged by a possible presence of the XUV radiation. Here we study the influence of the XUV radiation on the Pv ionization fraction in the hot star winds. By a detailed solution of the hydrodynamical, radiative transfer, and statistical equilibrium equations we confirm that sufficiently strong XUV radiation source may decrease the Pv ionization fraction, possibly depreciating the Pv lines as a reliable mass-loss rate indicator. On the other hand, the XUV radiation influences also the ionization fraction of heavier ions that drive the wind, leading to a decrease of the wind terminal velocity. Consequently, we conclude that the XUV radiation alone can not bring theory and observations in accord. We fit our predicted wind mass-loss rates by a suitable formula and compare the results with the observational mass-loss rate diagnostics. We show that for supergiants and giants the theoretical predictions do not contradict the mass-loss rate estimates based on X-ray line profiles or density squared diagnostics. On the other hand, for main-sequence stars the predicted mass-loss rates are still significantly higher than that inferred from Pv or X-ray lines. This indicates that the "weak wind problem" recently detected in low-luminosity main-sequence stars may occur to some extent also for the stars with higher luminosity.