Spin and orbital angular momentum exchange in binary star systems II. Ascending the giant branch: a new path to FK Comae stars

Using the model by Keppens (1997), we investigate the angular momentum (AM) evolution in asymmetric binary star systems from Zero-Age Main Sequence times until at least one component has ascended the giant branch. We concentrate on stars ranging in mass from 0.9 M. - 1.7 M. in almost synchronous, short period systems (P-orb < 9 days). We address synchronization and circularization by tidal interaction, allowing for structural evolution and stellar winds. A Weber-Davis prescription is used to quantify the wind influence, thereby accounting for changes in its acceleration mechanism from the interplay of the evolving thermal-magneto-centrifugal effects. We identify a scenario for fast in-spiraling components with d In P-orb/dt similar or equal to -O(10(-8)) which is primarily driven by fast structural evolution as the heaviest component ascends the giant branch. This leads to the formation of contact systems, which ultimately coalesce and form FK Comae-like objects on relatively short timescales due to the continuing expansion of the primary. The obtained mass loss rates and orbital period variations d ln P-orb/dt are confronted with their observed ranges. The predicted mass loss rates agree with the solar value on the main sequence and with the Reimers relation in the giant phase. Observations of period evolution in close, active binaries suggest, however. that other influences than those considered here must play an important role. Finally, we point out how the mass asymmetry of the binary system can be a crucial ingredient in the angular momentum evolution: while the primary dictates the spin-orbital AM exchange in the system, the slowly evolving lighter component can develop an efficient magnetocentrifugally driven wind and thereby drain the AM from the system.