Penetration and circulation currents in irradiated spherically symmetric stellar atmospheres

We study the motions driven in a stellar atmosphere by heating from a companion in a close binary system. Earlier work is extended by considering a spherical atmosphere, so as to be able to investigate the penetration of the motions into the unilluminated part of the star. The incident radiation is assumed to be a monodirectional beam. Although this beam does not penetrate directly to the dark side of the star, except in a very narrow region at the limb, radiative equilibrium is restored only gradually and the circulation currents are found to persist up to about 160 into the shadow region. The flow speeds are found to be exceptionally large, but it is argued that the inclusion of inertial terms, or of turbulence, would reduce them to reasonable values. The amount of energy carried by circulation into the dark side is estimated and the result supports the view that enough energy can be carried to explain the light curve of HZ Herculis. (author)