Nonlinear Development of the Secular Bar-Mode Instability in Rotating Neutron Stars

We have modeled the nonlinear development of the secular bar-mode instability that is driven by gravitational radiation reaction (GRR) forces in rotating neutron stars. In the absence of any competing viscous effects, an initially uniformly rotating axisymmetric n = 1/2 polytropic star with a ratio of rotational to gravitational potential energy T/ img1.gif = 0.181 is driven by GRR forces to a barlike structure, as predicted by linear theory. The pattern frequency of the bar slows to nearly zero, that is, the bar becomes almost stationary as viewed from an inertial frame of reference as GRR removes energy and angular momentum from the star. In this "Dedekind-like" state, rotational energy is stored as motion of the fluid in highly noncircular orbits inside the bar. However, in less than 10 dynamical times after its formation the bar loses its initially coherent structure as the ordered flow inside the bar is disrupted by what appears to be a purely hydrodynamic short-wavelength "shearing"-type instability. The gravitational waveforms generated by such an event are determined, and an estimate of the detectability of these waves is presented.