Kei Kotake

Magnetorotational Effects on Anisotropic Neutrino Emission and Convection in Core-Collapse Supernovae

We perform a series of two-dimensional hydrodynamic simulations of the magnetorotational collapse of a supernova core. We employ a realistic equation of state and take into account electron capture and neutrino transport by the so-called leakage scheme. Recent stellar evolution calculations imply that the magnetic fields of the toroidal components are much stronger than the poloidal ones at the presupernova stage. In this study we systematically investigate the effects of the toroidal magnetic fields on the anisotropic neutrino radiation and convection. Our results show that the shapes of the shock wave and the neutrino spheres generally become more oblate for the models whose profiles of rotation and the magnetic field are shell type and become, in contrast, more prolate for the models whose profiles of rotation and the magnetic field are cylindrical than for the corresponding models without the magnetic fields. Furthermore, we find that magnetorotational instability induced by nonaxisymmetric perturbations is expected to develop within the prompt-shock timescale. Combined with the anisotropic neutrino radiation, which heats matter near the rotational axis preferentially, the growth of the instability may enhance the heating near the axis. This might suggest that magnetar formation is accompanied by a jetlike explosion.

Anisotropic Neutrino Radiation in Rotational Core Collapse

We have done a series of two-dimensional hydrodynamic simulations of the rotational collapse of a supernova core and estimated the anisotropy of neutrino radiation from nonspherical neutrino spheres. We have employed a realistic equation of state and approximated electron captures and neutrino transport by the so-called leakage scheme. We have calculated heating rates outside the neutrino sphere, assuming that neutrinos are emitted isotropically from each point on the neutrino sphere. We have found that neutrinos heat matter near the rotational axis more strongly than those near the equatorial plane. This might induce a globally anisotropic explosion.

Magneto-driven Shock Waves in Core-Collapse Supernovae

We perform a series of two-dimensional magnetohydrodynamic simulations of the rotational core collapse of a magnetized massive star. We employ a realistic equation of state and take into account the neutrino cooling by the so-called leakage scheme. In this study we systematically investigate how the strong magnetic field and the rapid rotation affect the propagation of the shock waves. Our results show that in the case of the strong initial poloidal magnetic field, the toroidal magnetic field amplified by the differential rotation becomes strong enough to generate a tightly collimated shock wave along the rotational axis. On the other hand, in the case of the weak initial magnetic field, although the differential rotation amplifies the toroidal magnetic field over the long rotational period, the launched shock wave is weak and the shape of it becomes wider. The former case is expected to be accompanied by the formation of the so-called magnetar. Our models with rapid rotation and strong magnetic field can create a nozzle formed by the collimated shock wave. This might be the analogous situation to the collapsar that is plausible as the central engine of the gamma-ray bursts.

Gravitational radiation from rotational core collapse: Effects of magnetic fields and realistic equations of state

We perform a series of two-dimensional, axisymmetric, magnetohydrodynamic simulations of the rotational collapse of a supernova core. In order to calculate the waveforms of the gravitational wave, we derive the quadrupole formula including the contributions from the electromagnetic fields. Recent stellar evolution calculations imply that the magnetic fields of the toroidal components are much stronger than those of the poloidal ones at the presupernova stage. Thus, we systematically investigate the effects of the toroidal magnetic fields on the amplitudes and waveforms. Furthermore, we employ the two kinds of the realistic equation of states, which are often used in the supernova simulations. Then, we investigate the effects of the equation of states on the gravitational wave signals. With these computations, we find that the peak amplitudes are lowered by an order of 10% for the models with the strongest toroidal magnetic fields. However, the peak amplitudes are mostly within sensitivity range of laser interferometers such as TAMA and the first LIGO for a source at a distance of 10 kpc. Furthermore, we point out that the amplitudes of second peaks are still within the detection limit of the first LIGO for the source, although the characteristics of second peaks are reduced by the magnetic fields. We stress the importance of the detection, since it will give us information about the angular momentum distribution of massive evolved stars. When we compare the gravitational waves from the two realistic equation of states, significant differences are not found, except that the typical frequencies of the gravitational wave become slightly higher for the softer equation of state.

Gravitational radiation from axisymmetric rotational core collapse

We perform a series of two-dimensional hydrodynamic simulations of the rotational collapse of a supernova core in axisymmetry. We employ a realistic equation of state (EOS) and take into account electron capture and neutrino transport by the so-called leakage scheme. It is an important step to apply the realistic EOS coupled with microphysics to 2D simulations for computing gravitational radiation in rotational core collapse. We use the quadrupole formula to calculate the amplitudes and the waveforms of the gravitational wave assuming Newtonian gravity. With these computations, we extend the conventional category of the gravitational waveforms. Our results show that the peak amplitudes of the gravitational wave are mostly within the sensitivity range of laser interferometers such as TAMA and the first LIGO for a source at a distance of 10 kpc. Furthermore, we find that the amplitudes of the second peaks are within the detection limit of the first LIGO for the source, and first point out the importance of the detection, since it will give us information as to the angular momentum distribution of evolved massive stars.

Decaying cold dark matter and the evolution of the cluster abundance

The cluster abundance and its redshift evolution are known to be powerful tools for constraining the amplitude of mass fluctuations σ8 and the mass density parameter Ωm0. We study the impact of the finite decay rate of cold dark matter particles on the cluster abundances. On the basis of a spherical model in a decaying cold dark matter universe, we calculate the mass function of clusters and compare it with observed cluster abundance. We find the decay of cold dark matter particles significantly changes the evolution of the cluster abundance. In particular, we point out that the lifetime of dark matter particles comparable to the age of the universe lowers the ratio of the local cluster abundance to the high-redshift cluster abundance and can account for the observed evolution of the cluster abundance quite well. The strong dependence of the cluster abundance on the decay rate of dark matter suggests that distant cluster surveys may offer clues to the nature of dark matter.

NORTH-SOUTH NEUTRINO HEATING ASYMMETRY IN STRONGLY MAGNETIZED AND ROTATING STELLAR CORES

We perform a series of two-dimensional magnetohydrodynamic simulations of supernova cores. Since the distributions of the angular momentum and the magnetic fields of strongly magnetized stars are quite uncertain, we systematically change the combinations of the strength of the angular momentum, the rotations law, the degree of differential rotation, and the profiles of the magnetic fields to construct the initial conditions. By so doing, we estimate how the rotation-induced anisotropic neutrino heating are affected by the strong magnetic fields through parity-violating effects and first investigate how the north-south asymmetry of the neutrino heating in a strongly magnetized supernova core could be. As for the microphysics, we employ a realistic equation of state based on the relativistic mean field theory and take into account electron captures and the neutrino transport via the neutrino leakage scheme. With these computations, we find that the parity-violating corrections reduce

The role of neutrinos, rotations and magnetic fields in collapse-driven supernovae

lesssim 0.5 %

Gravitational collapse of rotating massive stars

of the neutrino heating rate than that without the magnetic fields in the vicinity of the north pole of a star, on the other hand, enhance about

Quasi-Periodic Gravitational-Wave Emission due to the SASI Motion

lesssim 0.5 %