Gravitational waves from the dynamical bar instability in a rapidly rotating star - art. no. 084024

A rapidly rotating, axisymmetric star can be dynamically unstable to an m = 2 "bar" mode that transforms the star from a disk shape to an elongated ba r. The fate of such a bar-shaped star is uncertain. Some previous numerical studies indicate that the bar is short lived, lasting for only a few bar-r otation periods, while other studies suggest that the bar is relatively lon g lived. This paper contains the results of a numerical simulation of a rap idly rotating gamma = 5/3 fluid star. The simulation shows that the bar sha pe is long lived: once the bar is established, the star retains this shape for more than 10-bar-rotation periods, through the end of the simulation. T he results are consistent with the conjecture that a star will retain its b ar shape indefinitely on a dynamical timescale, as long as its rotation rat e exceeds the threshold for secular bar instability. The results are descri bed in terms of a low-density neutron star, but can be scaled to represent, for example, a burned-out stellar core that is prevented from complete col lapse by centrifugal forces. Estimates for the gravitational-wave signal in dicate that a dynamically unstable neutron star in our galaxy can be detect ed easily by the first generation of ground based gravitational-wave detect ors. The signal for an unstable neutron star in the Virgo cluster might be Been by the planned advanced detectors. The Newtonian-quadrupole approximat ion is used throughout this work.