Circulation Currents in Irradiated Stellar Atmospheres

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Abstract
In general, the mutual heating of the components in a close binary system gives rise to rapid motions in their atmospheres. A simple model of this circulation is constructed, assuming axial symmetry and treating the incident radiation as a perturbation. The flow speeds decrease exponentially with optical depth, but are unrealistically high near the surface if the flow is taken to be laminar. In fact, a shear flow instability gives rise to a turbulent surface layer, for which an order-of-magnitude model is constructed. It is pointed out that radiative cooling may have a major influence on the structure of the turbulent shear flow and that the turbulence may be much weaker than suggested in an earlier paper on rotationally driven circulation. This may explain why it has proved difficult to confirm the presence of certain weak lines in 57 Cygni which had earlier been tentatively explained as arising in a chromosphere induced by strong turbulence. Although hydrodynamic effects probably do not play a major role in most binary systems, they may be important in binaries with X-ray sources. However, circulation currents seem unlikely to explain the anomalous light curve of HZ Herculis.

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