[Paper Review] Hydrodynamic model atmospheres for WR stars: Self-consistent modeling of a WC star wind

G. Gräfener, W.‐R. Hamann|CERN Bulletin|Oct 28, 2004

Stellar, planetary, and galactic studiesPhysics and Astronomy50 references94 citations

TL;DR

This paper presents the first self-consistent, non-LTE hydrodynamic model for a WC5 Wolf-Rayet star wind, coupling radiative transfer in the co-moving frame with hydrodynamic equations. It demonstrates that the 'Hot Iron Bump' (Fe ix–xvi) opacity drives strong radiative acceleration in optically thick layers, initiating mass loss, while lower-excitation Fe, C, and O ions drive outer wind acceleration, reproducing observed O vi emissions and wind structure with clumping effects included.

ABSTRACT

We present the first non-LTE atmosphere models for WR stars that incorporate a self-consistent solution of the hydrodynamic equations. The models account for iron-group line-blanketing and clumping, and compute the hydrodynamic structure of a radiatively driven wind consistently with the non-LTE radiation transport in the co-moving frame. We construct a self-consistent wind model that reproduces all observed properties of an early-type WC star (WC5). We find that the WR-type mass-loss is initiated at high optical depth by the so-called `Hot Iron Bump' opacities (Fe IX-XVI). The acceleration of the outer wind regions is performed by iron-group ions of lower excitation in combination with C and O. Consequently, the wind structure shows two acceleration regions, one close to the hydrostatic wind base in the optically thick part of the atmosphere, and another farther out in the wind. In addition to the radiative acceleration, the `Iron Bump' opacities are responsible for an intense heating of deep atmospheric layers. We find that the observed narrow OVI-emissions in the optical spectra of WC stars originate from this region. By their dependence on the clumping factor we gain important information about the location where the density inhomogeneities in WR-winds start to develop.

Motivation & Objective

• To develop a self-consistent model of a WC5 Wolf-Rayet star wind that couples non-LTE radiation transfer with hydrodynamics.
• To investigate whether radiative acceleration driven by Fe M-shell ions (Fe ix–xvi) can initiate mass loss in optically thick layers.
• To determine the role of clumping in radiative acceleration and its impact on observed wind properties like O vi emissions.
• To reproduce the observed terminal velocity, mass-loss rate, and spectral features of early-type WC stars.
• To assess the consistency of the sonic point location and wind acceleration structure with stellar structure models.

Proposed method

• Coupled the non-LTE atmosphere code with the hydrodynamic equations to solve for wind structure and radiation transport simultaneously.
• Used a fine frequency grid to resolve millions of overlapping spectral lines, ensuring accurate radiative force calculation in the co-moving frame.
• Incorporated iron-group line-blanketing (Fe ix–xvi) to model the 'Hot Iron Bump' opacity enhancement at ~160 kK.
• Applied a clumping factor D to account for density inhomogeneities, reducing effective mass-loss rates by √D.
• Solved the statistical equilibrium equations for all ionization stages under non-LTE conditions.
• Calculated the emergent flux and line profiles to compare with observations of WR 111 (WC5).

Experimental results

Research questions

• RQ1 Can radiative acceleration by Fe M-shell ions (Fe ix–xvi) drive a wind from an optically thick layer in a WC star?
• RQ2 Where is the sonic point located in a self-consistent hydrodynamic wind model for a WC5 star?
• RQ3 How do clumping effects influence the radiative acceleration and the observed O vi emission lines?
• RQ4 Does the observed O vi emission originate from the deep, hot layers driven by the 'Hot Iron Bump'?
• RQ5 How does the wind structure, particularly the two acceleration regions, compare with the predictions of standard CAK theory?

Key findings

• The sonic point is located at a high optical depth (τ ≈ 100) and high temperature (T_s = 199 kK), consistent with the 'Hot Iron Bump' opacity enhancement.
• Two distinct acceleration regions are identified: a steep rise in deep layers due to Fe ix–xvi, and a shallower rise in the outer wind due to lower-excitation Fe, C, and O ions.
• The observed narrow O vi emissions originate from the deep atmospheric layers heated by the 'Hot Iron Bump' and high stellar temperature (T_⋆ ≈ 140 kK).
• The radiative force shows strong dependence on the clumping factor D, with O vi lines serving as a diagnostic for the onset of clumping in the wind.
• The emergent flux and wind terminal velocity from the model are consistent with observations of the WC5 star WR 111.
• The model shows significant deviations from standard CAK theory in the outer wind, particularly in the response of radiative force to velocity gradient changes, likely due to line overlaps and recombination cascades.