Collapse of a differentially rotating supermassive star

We investigate the gravitational collapse of rapidly rotating relativistic supermassive stars by means of a 3+1 hydrodynamical simulations in conformally flat spacetime of general relativity. We study the evolution of differentially rotating supermassive stars of q = J/M² ~ 1 (J is the angular momentum and M is the gravitational mass of the star) from R/M ~ 65 (R is the circumferential radius of the star) to the point where the conformally flat approximation breaks down. We find that the collapse of the star of q ≿ 1, a radially unstable differentially rotating star form a black hole of q ≿ 1. The main reason to prevent the formation of a black hole of q ≿ 1 is that quite a large amount of the angular momentum stays at the surface. We also find that the collapse is coherent and that it likely leads to the formation of a supermassive black hole with no appreciable disk nor bar. In the absence of nonaxisymmetric deformation, the collapse of differentially rotating supermassive stars are the promising sources of burst and quasinormal ringing waves in the Laser Interferometer Space Antenna.