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Title:
Time-dependent structure and energy transfer in hot star winds.
Authors:
Feldmeier, A.
Publication:
Astronomy and Astrophysics, v.299, p.523 (A&A Homepage)
Publication Date:
07/1995
Origin:
A&A via CDS; KNUDSEN
A&A Keywords:
STARS: EARLY TYPE, STARS: MASS-LOSS, HYDRO, DYNAMICS, INSTABILITIES
Bibliographic Code:
1995A&A...299..523F

Abstract

We present time-dependent hydrodynamical models of radiation driven hot star winds, which are subject to a strong instability intrinsic to the radiative line force. The calculations are done using a newly developed radiation hydrodynamics code applying the Smooth Source Function method (Owocki 1991) to calculate the radiative acceleration. Assuming spherical symmetry, the wind consists of a sequence of narrow, dense shells, where each shell is bounded by a pair of reverse and forward shocks, in good agreement with comparable models by Owocki (1992). We find frequent encounters of two shells with subsequent merging of the shells into one. For small periodic base perturbations, the wind structure is also periodic, without a stochastic component. For large base perturbations, on the other hand, a continuous spectrum of wave frequencies is excited in the wind. Furthermore, our models show the shock decay to set in from about 5 stellar radii on. The major theme of this paper is the energy transfer in the wind. Time-dependent supergiant wind models up to now simply assume radiative cooling to be efficient, and hence the shocks to be isothermal. To test this assumption and to calculate the X-ray emission, the energy equation is included in the simulations. A severe numerical shortcoming is then encountered, whereby all radiative cooling zones collapse and the shocks become isothermal again. We propose a new method to hinder this defect. Simulations of dense winds then prove radiative cooling to indeed be efficient up to 5 to 7 R_*_. Shock temperatures are between 10^6^ to 10^7^K, depending on the base perturbation. Beyond these radii, however, the cooling zones of strong shocks become broad and thereby alter the wind structure drastically: all reverse shocks disappear, leaving regions of previously heated gas. This gas cools as it advects to larger radii. Since, moreover, shell-shell collisions only occur up to 6 to 7 R_*_, the wind can be divided into two regions: an inner, active one with frequent shocks and shell-shell collisions; and an outer, quiescent region with "old" hot material, and with no further shell collisions.

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