Hydrodynamic Instabilities in Close Binary Systems Frederic

Equilibrium uid con gurations for close binary systems can become globally unstable. Instabilities arise from the strong tidal interaction between the two components, which tends to make the e ective two-body potential governing the orbital motion steeper than 1=r. As a result, a circular orbit can become unstable to small radial perturbations. The instability can be either secular or dynamical. In both cases it leads to the coalescence and merging of the two components on a timescale generally much shorter than the lifetime of a stable system. In a secularly unstable system, orbital decay is driven by viscous dissipation and proceeds on a timescale comparable to the tidal circularization time of a stable binary. In a dynamically unstable system, the two stars suddenly plunge toward each other and merge hydrodynamically in just a few orbital periods. These instabilities are relevant to a wide variety of astrophysical problems involving coalescing binaries, such as the formation of blue stragglers, Type Ia supernovae, the orbital decay of massive X-ray binaries, and the calculation of gravitational radiation signals from close neutron star binaries.