Slowly rotating close binary stars in Cassini states

Abstract

ABSTRACT Recent asteroseismic measurements have revealed a small population of stars in close binaries, containing primaries with extremely slow rotation rates. Such stars defy the standard expectation of tidal synchronization in such systems, but they can potentially be explained if they are trapped in a spin-orbit equilibrium known as Cassini state 2 (CS2). This state is maintained by orbital precession due to an outer tertiary star, and it typically results in a very sub-synchronous rotation rate and high degree of spin-orbit misalignment. We examine how CS2 is affected by magnetic braking and different types of tidal dissipation. Magnetic braking results in a slower equilibrium rotation rate, while tidal dissipation via gravity waves can result in a slightly higher rotation rate than predicted by equilibrium tidal theory, and dissipation via inertial waves can result in much slower rotation rates. For seven binary systems with slowly rotating primaries, we predict the location of the outer tertiary predicted by the CS2 theory. In five of these systems, a tertiary companion has already been detected, although it is closer than expected in three of these, potentially indicating tidal dissipation via inertial waves. We also identify a few new candidate systems among a population of eclipsing binaries with rotation measurements via spot modulation.