Current status of numerical-relativity simulations in Kyoto

Abstract

We describe the current status of our numerical simulations for the collapse of a massive stellar core to a black hole (BH) and the merger of binary neutron stars (BNS), performed in the framework of full general relativity incorporating finite-temperature equations of state (EOS) and neutrino cooling. For the stellar core collapse simulation, we present the latest numerical results. We employed a purely nucleonic EOS derived by Shen et al. [Nucl. Phys. A 637, 435 (1998)]. As an initial condition, we adopted a 100 M presupernova model calculated by Umeda and Nomoto [Astrophys. J. 637, 1014 (2008)], which has a massive core (M ≈ 3M) with a high value of entropy per baryon (s ≈ 4kB). Changing the degree of rotation for the initial condition, we clarify the strong dependence of the outcome of the collapse on this. When the rotation is rapid enough, the shock wave formed at the core bounce is deformed to a torus-like shape. Then, the infalling matter accumulates in the central region due to the oblique shock at the torus surface, hitting the proto-neutron star and dissipating the kinetic energy there. As a result, outflows can be launched. The proto-neutron eventually collapses to a BH and an accretion torus is formed around it.We also found that the evolution of the BH and torus depends strongly on the rotation initially given. In the BNS merger simulations, we employ an EOS incorporating a degree of freedom for hyperons derived by Shen et al. [Astrophys. J. Suppl. 197, 20 (2011)], in addition to the purely nucleonic EOS. The numerical simulations show that for the purely nucleonic EOS, a hypermassive neutron star (HMNS) with a long lifetime (10 ms) is the outcome for the total mass M ≤ 3.0M. In contrast, the formed HMNS collapses to a BH in a shorter time scale with the hyperonic EOS for M ≥ 2.7M. It is shown that the typical total neutrino luminosity of the HMNS is ∼(3–10)×1053 ergs/s and the effective amplitude of gravitational waves from the HMNS is (2–6)×10−22 at f ≈ 2–2.5 kHz for a source distance of 100 Mpc.