The influence of dynamic tides on the apsidal-motion rate in close binaries with an evolved main-sequence star

Abstract

The validity of the classical formula for the rate of secular apsidal motion in close binaries is investigated for a sequence of models of a 5M⊙ star ranging from the last stages of the C 12 → N reaction to the phase where hydrogen is exhausted in the core. For binaries with short orbital periods, the apsidal-motion rates predicted by the classical formula deviate from the rates determined within the framework of the theory of dynamic tides due to the effects of the compressibility of the stellar fluid and due to resonances of dynamic tides with free oscillation modes of the component stars (Smeyers & Willems 2001). As the star evolves on the main sequence, the deviations caused by the compressibility of the stellar fluid increase with increasing radius of the star. The additional deviations caused by the resonances are largest near the end of the core-hydrogen burning phase. Both of these deviations increase with increasing values of the orbital eccentricity.