In this paper we examine a variety of topics dealing with synchronization and orbital circularization in detached close binaries. First, we calculate the unsteady tidally induced currents in a nonsynchronous binary component that has a large density gradient in its interior. Boundary-layer theory is used to confirm the order of magnitude of the spin-down time t(sd) that was obtained in our previous investigations. It is thus found that the hydrodynamical spin-down is a very efficient mechanism because the time t(sd) is proportional to the factor (d/R) exp 4.125, where d/R is the distance ratio. Second, the basic physics that underlies the newly proposed mechanism is presented in simple, nontechnical terms. For these purposes, we explain the differences and similarities between a laboratory problem (with solid boundaries and an incompressible fluid) and the double-star problem (with a free boundary and a large density gradient). It is explained why, in both cases, the time t(sd) is always much shorter than the viscous decay time of residual motions. Third, we comment on some comparisons between theory and observation that have been made over the past few years.
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Tassoul et al. (1992) studied this question.