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

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.

💡 Research Summary

The paper investigates how X‑ray emission, presumed to arise from wind‑embedded shocks, influences the structure of line‑driven winds in O‑type stars. Using a non‑local thermodynamic equilibrium (NLTE) wind code, the authors introduce an artificial X‑ray source characterized by a luminosity ratio Lₓ/L ≈ 10⁻⁷ and explore its impact on key wind parameters such as mass‑loss rate (Ṁ) and terminal velocity (v∞). The study covers a representative sample of O‑stars spanning effective temperatures from about 30 000 K to 45 000 K, with model parameters calibrated to observed Ṁ and v∞ values.

The main findings can be summarized as follows:

1. Mass‑Loss Rate: Inclusion of X‑rays modifies the ionisation balance, enhancing the population of high‑ionisation species (e.g., O VI, N V). However, the overall line‑driving force is only weakly affected, leading to changes in Ṁ of less than 10 %. This confirms that the wind’s momentum deposition remains dominated by UV line absorption rather than by X‑ray‑induced ionisation.

2. Terminal Velocity: For cooler O‑stars (Teff < 35 000 K) the extra electrons supplied by X‑ray ionisation increase the line opacity slightly, raising v∞ by up to ~5 %. In hotter stars the effect is negligible because the line driving is already saturated.

3. Lₓ/L ≈ 10⁻⁷ Relation: The authors propose a physical explanation for the observed near‑constant X‑ray to bolometric luminosity ratio. In dense winds, the post‑shock cooling time scales with wind density, producing a roughly constant fraction of the wind kinetic energy that is radiated as X‑rays, thus naturally yielding Lₓ/L ≈ 10⁻⁷. In contrast, stars with optically thin “weak winds” have long cooling lengths; the shocked plasma does not cool efficiently, leading to an excess X‑ray output. This mechanism offers a potential solution to the weak‑wind problem, linking the anomalously low mass‑loss rates to enhanced X‑ray emission.

4. Pv Problem: While X‑rays improve the agreement between modeled and observed ionisation fractions for many species, they do not significantly alter the Pv ionisation fraction. Consequently, the long‑standing discrepancy between predicted and observed Pv line strengths cannot be resolved by invoking X‑ray ionisation alone.

5. X‑ray Line Profiles: The paper extends the analysis to the transfer of X‑ray photons in the wind, focusing on helium‑like ions (e.g., Si XIII, Mg XI). It demonstrates that line absorption in the cool, dense part of the wind can reshape the emergent X‑ray line profiles, producing asymmetries that must be accounted for when interpreting high‑resolution spectra from Chandra or XMM‑Newton.

Overall, the work provides a comprehensive quantitative assessment of X‑ray feedback on O‑star winds. It confirms that X‑rays have a modest dynamical impact but play a crucial role in shaping the ionisation structure, especially in low‑density winds. By linking the X‑ray luminosity to wind density‑dependent cooling, the authors offer a unified explanation for both the empirical Lₓ/L ≈ 10⁻⁷ scaling and the weak‑wind phenomenon. The study also highlights the necessity of including wind absorption effects in the modelling of X‑ray line profiles, thereby setting a clear direction for future observational and theoretical investigations.