Introduction

The main body of the book has been concerned with the effects of axial rotation upon the structure and evolution of single stars. As was pointed out in Section 1.4, further challenging problems arise from the study of double stars whose components are close enough to raise tides on the surface of each other. Indeed, tidal interaction in a detached close binary will continually change the spin and orbital parameters of the system (such as the orbital eccentricity e, mean orbital angular velocity Ω0, inclination ω, and rotational angular velocity Ω of each component). Unless there are sizeable stellar winds emanating from the binary components, the total angular momentum will be conserved during these exchange processes. However, as a result of tidal dissipation of energy in the outer layers of the components, the total kinetic energy of a close binary system will decrease monotonically. Ultimately, this will lead to either a collision or an asymptotic approach toward a state of minimum kinetic energy. Such an equilibrium state is characterized by circularity (e = 0), coplanarity (ω = 0), and corotation (Ω = Ω0); that is to say, the orbital motion is circular, the rotation axes are perpendicular to the orbital plane, and the rotations are perfectly synchronized with the orbital revolution.