Tidal Friction in Triple Systems: A Means of Producing Close Stellar and Planetary Orbits

In hierarchical triple systems the combination of tidal friction (TF) with fluctuations of eccentricity due to the third body can lead to potentially large but slow changes in the inner orbit, especially if the two orbits have high (40° or more) relative inclination. We model the dynamical evolution of triple systems using a force law which includes a combination of point-mass gravity, quadrupolar distortion (QD) of each body by the other two, and a dissipative TF term. In hypothetical cases of triple systems with relative orbital inclination i = 100° (as in the well-known triple stellar system β Per), the effect of the third star is periodically to increase the inner eccentricity up to nearly unity, provided we neglect the effects of QD and TF. The combined effect of QD and TF may reduce the fluctuations of the inner eccentricity, and in some cases the binary orbit may shrink quite drastically after a suitably long interval of time. These results can be applied to systems where all three components are of stellar mass, and also to triple systems with one binary component, or even two components including the distant one, being Jupiter-like planets. This is potentially important for the long-term evolution of such systems and can probably explain the origin of very short-period orbits for some recently discovered extra-Solar planets, such as τ Boo — which has both a Jupiter-like companion in a 3d orbit and an M2V companion in a ~2000 yr orbit.