Rapid Orbital Decay in Detached Binaries: Evidence for Circumbinary Disks

Some short-period, detached binary systems have recently been reported as experiencing very rapid orbital decay, much faster than is expected from the angular-momentum loss caused by gravitational radiation alone. As these systems contain fully convective stars, magnetic braking is not believed to be operative, making the large orbital-period derivative puzzling. Here, we explore whether a resonant interaction between the binary and a surrounding circumbinary (CB) disk could account for the observed orbital decay. Our calculations indicate that the observed orbital-period derivatives in seven detached binaries can be produced by the resonant interaction between the binary and a CB disk if the latter has a mass in the range of 10(-4)-10(-2)M(circle dot), which is of the same order as the inferred disk mass (similar to 2.4 x 10(-4) M-circle dot) in the post-common-envelope binary NN Ser.