The Nonlinear Evolution of Massive Stellar Core Collapses That “Fizzle”

Core collapse in a massive rotating star may pause before nuclear density is reached, if the core contains total angular momentum J greater than or similar to 10(49) g cm(2) s(-1). In such aborted or "fizzled" collapses, temporary equilibrium objects form that, although rapidly rotating, are secularly and dynamically stable because of the high electron fraction per baryon Y-e > 0.3 and the high entropy per baryon S-b/k approximate to 1-2 of the core material at neutrino trapping. These fizzled collapses are called "fizzlers." In the absence of prolonged infall from the surrounding star, the evolution of fizzlers is driven by deleptonization, which causes them to contract and spin up until they either become stable neutron stars or reach the dynamic instability point for barlike modes. The barlike instability case is of current interest because the bars would be sources of gravitational wave (GW) radiation. In this paper, we use linear and nonlinear techniques, including three-dimensional hydrodynamic simulations, to study the behavior of fizzlers that have deleptonized to the point of reaching dynamic bar instability. The simulations show that the GW emission produced by bar-unstable fizzlers has rms strain amplitude r(15)h = 10(-23) to 10(-22) for an observer on the rotation axis, with wave frequency of roughly 60-600 Hz. Here h is the strain and r(15) = (r/15 Mpc) is the distance to the fizzler in units of 15 Mpc. If the bars that form by dynamic instability can maintain GW emission at this level for 100 periods or more, they may be detectable by the Laser Interferometer Gravitational-Wave Observatory at the distance of the Virgo Cluster. They would be detectable as burst sources, defined as sources that persist for similar to10 cycles or less, if they occurred in the Local Group of galaxies. The long-term behavior of the bars is the crucial issue for the detection of fizzler events. The bars present at the end of our simulations are dynamically stable but will evolve on longer time-scales because of a variety of effects, such as shock heating, infall, deleptonization, and cooling, as well as gravitational radiation and Newtonian gravitational coupling to surrounding material. Long-term simulations including these effects will be necessary to determine the ultimate fate and GW production of fizzlers with certainty.