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Astron. Astrophys. 341, 567-573 (1999)

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1. Introduction

In recent discussions a strong difference of views has emerged concerning the correct fluid-dynamical treatment of the flow of gas over the surface of a contact binary. This difference of views concerns the question of whether there are any hydrodynamical arguments for assuming constancy of Jacobi energy (sum of potential and kinetic energy) over the system surface. The answer to this question could be of relevance for the theory of systems where very high velocities (in addition to the normal velocities of orbital and synchronised motions) seem to have been observed (Frasca et al., 1996).

My view (Hazlehurst 1997; hereafter H 97) that there are no hydrodynamical arguments favouring constant Jacobi energy derived largely from a consideration of the symmetry-breaking properties of the Coriolis forces. After referring to the rôle of these forces in this respect I went on to state that they might even lead to topological changes in the flow.

This view has however been sharply challenged (Kähler 1997; hereafter K 97) in a paper where the author sees no reason whatever for expecting a change in fluid topology from the above cause.

I shall therefore in this paper try to give a detailed picture of how the topological changes can actually occur. Since the differences of view described above do not involve the `viscosity question' I shall retain the inviscid assumption in the interests of simplicity and, I hope, of transparency. I shall also assume that there is a steady circulation of gas through the system involving both supply and removal of material from the surface layers. This means that not all of the surface streamlines will close up in the surface itself. I shall use the simplest possible model, to be described in the next section, to represent this situation.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998