NLTE models of line-driven stellar winds III. Influence of X-ray radiation on wind structure of O stars

TL;DR

This study investigates how X-ray emission from wind shocks affects the structure, ionization, and observable features of stellar winds in O stars, finding minor effects on mass-loss rates but potential explanations for the weak wind problem.

Contribution

It introduces a NLTE wind model incorporating shock-induced X-ray emission to analyze its impact on wind properties and ionization, offering new insights into the weak wind problem and X-ray line profiles.

Findings

X-ray emission has a small effect on mass-loss rates.

Enhanced X-ray emission may explain the weak wind problem.

X-ray line profiles can be affected by wind absorption.

Abstract

We study the influence of X-rays on the wind structure of selected O stars. For this purpose we use our non-local thermodynamic equilibrium (NLTE) wind code with inclusion of additional artificial source of X-rays, assumed to originate in the wind shocks. We show that the influence of shock X-ray emission on wind mass-loss rate is relatively small. Wind terminal velocity may be slightly influenced by the presence of strong X-ray sources, especially for stars cooler than Teff < 35 000 K. We discuss the origin of the Lx/L \sim 10^-7 relation. For stars with thick wind this relation can be explained assuming that the cooling time depends on wind density. Stars with optically thin winds exhibiting the "weak wind problem" display enhanced X-ray emission which may be connected with large shock cooling length. We propose that this effect can explain the "weak wind problem". Inclusion of X-rays leads to a better agreement of the model ionization structure with observations. However, we do not found any significant influence of X-rays on Pv ionization fraction implying that the presence of X-rays cannot explain the Pv problem. We study the implications of modified ionization equilibrium due to shock emission on the line transfer in the X-ray region. We conclude that the X-ray line profiles of helium-like ions may be affected by the line absorption within the cool wind.