EXPLOSION OF A ROTATING NEUTRON-STAR NEAR THE MINIMUM MASS

We analyze the explosion of a rotating neutron star just below the minimum mass. We follow the dynamical evolution of the star by solving the Newtonian equations of motion for a homogeneous, uniformly rotating spheroid with internal pressure and gravity. The matter consists of heavy nuclei, dripped neutrons, and free electrons, and the Harrison-Wheeler nuclear model is employed to construct the equation of state. At the outset, the oblate star is in hydrostatic equilibrium at the minimum mass along an equilibrium curve of fixed angular momentum. We perturb the matter away from beta-equilibrium, thereby driving the star unstable. We track its nonspherical motion up to the point at which the highly neutronized matter undergoes spontaneous fission. The explosion proceeds almost instantaneously, and an intense burst of antineutrinos signals the onset of abrupt acceleration. While expanding, the oblateness of the spheroid decreases as the centrifugal force weakens with increasing radius. We find antineutrino luminosities of approximately 10(50)-10(52) ergs s-1 and bulk kinetic energies of order 10(49) ergs. Gravitational radiation (GR) and gravitational wave amplitudes are computed as perturbations in the weak-field, slow-motion limit of general relativity. For a wide set of initial conditions, the emission in gravitational waves is found to be small, with GR energy efficiencies E(GR)/M(B) approximately 10(-14), and wave amplitudes h+ less-than-or-similar-to 5 x 10(-23) for a source distance of 10 kpc. We also estimate the photon luminosity from the explosion via a simple two-zone core-atmosphere model. The photon luminosity calculated in the diffusion approximation varies between 10(36) and 10(38) ergs s-1, with the photons emerging in the hard UV and soft X-ray bands. We discuss the influence of a companion neutron star on the rotational properties of the unstable star and on the dynamics of the explosion. Finally, we briefly discuss the astrophysical significance of this explosion scenario.