SPIN AND ORBITAL ANGULAR-MOMENTUM EXCHANGE IN BINARY STAR SYSTEMS

We present a comprehensive model for studying the angular momentum (AM ) evolution in binary star systems, taking into account: (i) evolution ary 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; a nd (iii) AM loss from one or both component stars due to stellar winds . This allows us to assess whether, when and how the synchronization o f spin and orbital rotation rates, and the circularization of eccentri c orbits, is achieved within a composite system of two evolving stars. We develop the formalism for spin and orbital AM exchange in binary s ystems such that 'standard' (and sometimes rivaling) theories of tidal interactions and stellar winds can easily be incorporated and compare d, 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 sta rs, we use a 2-component model for each star (as in MacGregor & Brenne r 1991), with possibly differentially rotating core and envelope zones . These two zones are coupled through viscomagnetic mechanisms. The mo del calculations presented illustrate how the combined effects of stru ctural evolution, tidal interactions, stellar winds, and the visco-mag netic coupling mechanisms lead to rich scenarios for the AM evolution. We concentrate in this paper on the model and its potential for gaini ng 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 interpr etation of many observational results, but this is postponed to a fort hcoming paper (Keppens et al. 1996).