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Astron. Astrophys. 318, 275-288 (1997)


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Spin and orbital angular momentum exchange in binary star systems

R. Keppens

Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg, Germany

Received 2 December 1995 / Accepted 22 June 1996

Abstract

We present a comprehensive model for studying the angular momentum (AM) evolution in binary star systems, taking into account: (i) evolutionary effects of both component stars on the Pre-Main Sequence (PMS), on the Main Sequence (MS) and during the (initial) ascent onto the giant branch; (ii) spin-orbital AM exchange through 'tidal' interactions; and (iii) AM loss from one or both component stars due to stellar winds. This allows us to assess whether, when and how the synchronization of spin and orbital rotation rates, and the circularization of eccentric orbits, is achieved within a composite system of two evolving stars.

We develop the formalism for spin and orbital AM exchange in binary systems such that 'standard' (and sometimes rivaling) theories of tidal interactions and stellar winds can easily be incorporated and compared, in so far as they lead to qualitative differences in the overall AM evolution.

When using our model for a binary system of solar-type stars, we use a 2-component model for each star (as in MacGregor & Brenner 1991), with possibly differentially rotating core and envelope zones. These two zones are coupled through visco-magnetic mechanisms. The model calculations presented illustrate how the combined effects of structural evolution, tidal interactions, stellar winds, and the visco-magnetic coupling mechanisms lead to rich scenarios for the AM evolution. We concentrate in this paper on the model and its potential for gaining new insights in the physical effects that play a role in the binary AM balance. It is pointed out how it can be used for a direct interpretation of many observational results, but this is postponed to a forthcoming paper (Keppens et al. 1996).

Key words: stars: binaries: close – stars: evolution – stars: rotation

Send offprint requests to: R. Keppens, FOM institute for Plasma-Physics Rijnhuizen, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998

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