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

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2. A simple model

We shall in this paper assume the principle possibility of two stars being able to form a stable, stationary contact binary in which strong motions occur across the surface. We do not specify the evolutionary state of the system.

We shall regard the material involved in these motions as flowing from one star (say, the primary) to the other via the outer layers and then back again through the inside. Then, as seen from the surface, there would appear to be a source on the primary side of the system and a sink on the secondary side.

We start by agreeing with the statement in K 97 that the surface of a contact binary is topologically equivalent to a sphere. This being so, we can go a step further and attempt to represent the system by means of a rotating, roughly spherical object - at least as a topological analogue. A rather similar approach was used by Tassoul (1992); in our case we would require that the rotating object be supplied with a source on one side and a sink on the other.

In order to make the model `realistic' (and not merely topologically similar) a rather large depth of contact for the system represented by the model would appear advantageous.

Finally, our simple model says nothing about the driving mechanism of the circulation and is merely intended to represent the situation as seen from the surface layers. Nevertheless we have in mind something like the thermally-driven circulation of Nariai (1976), Webbink (1977) and others.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998