The Relative Stability against Merger of Close, Compact Binaries

The orbital separation of compact binary stars will shrink with time owing to the emission of gravitational radiation. This inspiral phase of a binary system's evolution generally will be very long compared to the system's orbital period, but the final coalescence may be dynamical and driven to a large degree by hydrodynamic effects, particularly if there is a critical separation at which the system becomes dynamically unstable toward merger. Indeed, if weakly relativistic systems (such as white dwarf-white dwarf binaries) encounter a point of dynamical instability at some critically close separation, coalescence may be entirely a classical, hydrodynamic process. Therefore, a proper investigation of this stage of binary evolution must include three-dimensional hydrodynamic simulations. We have constructed equilibrium sequences of synchronously rotating, equal-mass binaries in circular orbit with a single parameter—the binary separation—varying along each sequence. Sequences have been constructed with various polytropic as well as realistic white dwarf and neutron star equations of state. Using a Newtonian, finite-difference hydrodynamics code, we have examined the dynamical stability of individual models along these equilibrium sequences. Our simulations indicate that no points of instability exist on the sequences we analyzed that had relatively soft equations of state (polytropic sequences with polytropic index n = 1.0 and 1.5 and two white dwarf sequences). However, we did identify dynamically unstable binary models on sequences with stiffer equations of state (n = 0.5 polytropic sequence and two neutron star sequences). We thus infer that binary systems with soft equations of state are not driven to merger by a dynamical instability. For the n = 0.5 polytropic sequence, the separation at which a dynamical instability sets in appears to be associated with the minimum energy and angular momentum configuration along the sequence. Our simulations suggest but do not conclusively demonstrate that, in the absence of relativistic effects, this same association may also hold for binary neutron star systems.