The outer evolution of instability-generated structure in radiatively driven stellar winds

Abstract

Abstract. We investigate stochastic structure in hot-star winds. The structure (i.e. inhomogeneities such as clumps and shocks) is generated by the instability of the line driving mechanism in the inner wind. It is self-excited in the sense that it persists even in the absence of explicit perturbations. The evolution of structure as it moves out with the flow is quantified by the radial dependence of statistical properties such as the clumping factor and the velocity dispersion. We find that structure evolves under the influence of two competing mechanisms. Dense clumps pressure-expand into the rarefied gas that separates them, but this expansion is counteracted by supersonic collisions among the clumps, which tend to compress them further. Because of such ongoing collisions, clumps can survive over an extended region out of pressure equilibrium with the rarefied surrounding gas. Moreover, the linedriving force has little rôle in maintaining the structure beyond about 20-30 R * , implying that the outer evolution can be simplified as a pure gasdynamical problem. In modelling the distant wind structure we find it is necessary to maintain a relatively fine constant grid spacing to resolve the often quite narrow dense clumps. We also find that variations in the heating and cooling, particularly the "floor" temperature to which shock-compressed gas is allowed to cool, can affect both the density and temperature variation. Finally, we find that increasing the value of the line-driving cut-off parameter κmax can significantly enhance the level of flow structure. Overall, the results of our work suggest that structure initiated in the inner wind acceleration region can survive to substantial distances (∼100 R * ), and thus can have an important influence on observational diagnostics (e.g. infrared and radio emission) formed in the outer wind.

NLTE models of line-driven stellar winds

“…On the other hand, a detailed numerical study of wind stability (cf. Feldmeier et al 1997;Runacres & Owocki 2002), and of effects of stellar rotation on the stellar wind (cf. Petrenz & Puls 2000) showed that basic wind parameters (i.e.…”

“…neglect of wind non-stationarity (cf. Runacres & Owocki 2002), wind 3D structure (Dessart & Owocki 2003), or wind magnetic fields (ud-Doula & Owocki 2002). Among these unsolved problems is the long-standing momentum problem of WR stars (for the latest progress see Gräfener et al 2002) or problems with fitting of X-ray line profiles (Kramer et al 2003).…”

Predictions of the effect of clumping on the wind properties of O-type stars

“…It is the result of a positive feedback in which a small increase in the velocity of a fluid parcel exposes the parcel to more intense, unattenuated radiation from the star and causes it to be further accelerated (Owocki et al 1988;Feldmeier 1995;Owocki & Puls 1996, 1999. Simulations of this self-excited wind instability show that the gas compression in clumps typically starts at about 0.3-0.4 ∞ and can extend to very large radii (Runacres & Owocki 2002). These simulations, however, do not account for (transonic) velocity curvature terms.…”

XMM‐Newtonand Very Large Array Observations of the Variable Wolf‐Rayet Star EZ Canis Majoris: Evidence for a Close Companion?

“…Similar conclusions have been reached for some Otype stars (e.g., Hillier et al 1993). Detailed hydrodynamic simulations are now needed to determine whether instability-generated wind shocks can persist to hundreds of radii in W-R stars, analogous to those recently undertaken for OB stars by Runacres & Owocki (2002).…”