Publications (135)393.81 Total impact
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ABSTRACT: We present a new multidimensional radiationhydrodynamics code for massive stellar corecollapse in full general relativity (GR). Employing an M1 analytical closure scheme, we solve spectral neutrino transport of the radiation energy and momentum based on a truncated moment formalism. Regarding neutrino opacities, we take into account the socalled standard set in stateoftheart simulations, in which inelastic neutrinoelectron scattering, thermal neutrino production via pair annihilation and nucleonnucleon bremsstrahlung are included. In addition to gravitational redshift and Doppler effects, these energycoupling reactions are incorporated in the moment equations in a covariant form. While the Einstein field equations and the spatial advection terms in the radiationhydrodynamics equations are evolved explicitly, the source terms due to neutrinomatter interactions and energy shift in the radiation moment equations are integrated implicitly by an iteration method. To verify our code, we conduct several test simulations of corecollapse, bounce, and shockstall of a 15 solar mass star in the Cartesian coordinates and make a detailed comparison with published results. We first investigate how accurate the adopted closure scheme reproduces results from sphericallysymmetric simulations with fullBoltzmann neutrino transport. A good agreement of the hydrodynamic features and the spectral neutrino properties supports the reliability of the GR transport scheme in the momentum space. These results demonstrate the robustness of our code that is intended to model corecollapse supernovae. For the actual application, we discuss that higher numerical resolutions in both space and momentumspace are needed, which could be possibly practicable by using nextgeneration Exaflopsclass supercomputers.  [Show abstract] [Hide abstract]
ABSTRACT: Using predictions from threedimensional (3D) hydrodynamics simulations of corecollapse supernovae (CCSNe), we present a coherent network analysis to detection, reconstruction, and the source localization of the gravitationalwave (GW) signals. By combining with the GW spectrogram analysis, we show that several important hydrodynamics features imprinted in the original waveforms persist in the waveforms of the reconstructed signals. The characteristic excess in the GW spectrograms originates not only from rotating corecollapse and bounce, the subsequent ring down of the protoneutron star (PNS) as previously identified, but also from the formation of magnetohydrodynamics jets and nonaxisymmetric instabilities in the vicinity of the PNS. Regarding the GW signals emitted near at the rotating core bounce, the horizon distance, which we set by a SNR exceeding 8, extends up to $\sim$ 18 kpc for the most rapidly rotating 3D model among the employed waveform libraries. Following the rotating core bounce, the dominant source of the GW emission transits to the nonaxisymmetric instabilities that develop in the region between the stalled shock and the PNS. We point out that the horizon distances from the nonaxisymmetric instabilities are generally longer when seen from the direction parallel to the rotational axis of the source than seen from the equator. Among the 3D generalrelativistic models in which the nonaxisymmetric instabilities set in, the horizon distances extend maximally up to $\sim$ 40 kpc seen from the pole and they are rather insensitive to the imposed initial rotation rates. Our results suggest that in addition to the best studied GW signals due to rotating corecollapse and bounce, the quasiperiodic signals due to the nonaxisymmetric instabilities and the detectability should deserve further investigation to elucidate the innerworking of the rapidly rotating CCSNe.  [Show abstract] [Hide abstract]
ABSTRACT: Magnetorotational instability (MRI) in a convectivelystable layer around the neutrinosphere is simulated by a threedimensional model of supernova core. To resolve MRIunstable modes, a thin layer approximation considering only the radial global stratification is adopted. Our intriguing finding is that the convectivelystable layer around the neutrinosphere becomes fullyturbulent due to the MRI and its nonlinear penetration into the stronglystratified MRIstable region. The intensity of the MRIdriven turbulence increases with magnetic flux threading the core, but is limited by a free energy stored in the differential rotation. The turbulent neutrinosphere is a natural consequence of rotating corecollapse and could exert a positive impact on the supernova mechanism.11/2014; 798(1). DOI:10.1088/20418205/798/1/L22 
Article: The red supergiant and supernova rate problems: implications for corecollapse supernova physics
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ABSTRACT: Mapping supernovae to their progenitors is fundamental to understanding the collapse of massive stars. We investigate the red supergiant problem, which concerns why red supergiants with masses $\sim16$ to $30 M_\odot$ have not been identified as progenitors of Type IIP supernovae, and the supernova rate problem, which concerns why the observed cosmic supernovae rate is smaller than the observed cosmic star formation rate. We find key physics to solving these in the compactness parameter, which characterizes the density structure of the progenitor. If massive stars with compactness above $\xi_{2.5} \sim 0.2$ fail to produce canonical supernovae, (i) stars in the mass range $16$ to $30 M_\odot$ populate an island of stars that have high $\xi_{2.5}$ and do not produce canonical supernovae, and (ii) the fraction of such stars is consistent with the missing fraction of supernovae relative to the star formation rate. We support this scenario with a series of two and threedimensional radiation hydrodynamics corecollapse simulations. Using more than 300 progenitors covering initial masses 10.8 to $75 M_\odot$ and three initial metallicities, we show that high compactness is conducive to failed explosions. We then argue that a critical compactness of $\sim 0.2$ as the divide between successful and failed explosions is consistent with stateoftheart threedimensional corecollapse simulations. Our study implies that numerical simulations of core collapse need not produce robust explosions in a significant fraction of compact massive star initial conditions.Monthly Notices of the Royal Astronomical Society Letters 08/2014; 445(1). DOI:10.1093/mnrasl/slu146 · 5.52 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: By performing neutrinoradiation hydrodynamic simulations in spherical symmetry (1D) and axial symmetry (2D) with different progenitor models by Woosley & Heger (2007) from 12 $M_\odot$ to 100 $M_\odot$, we find that all 1D runs fail to produce explosion and several 2D runs succeed. The difference of shock evolution can be interpreted by the difference of mass accretion history, which is determined by the density structure of the progenitor. The exploding models exhibit high neutrino luminosity with low mass accretion rate. This is consistent with the discussion about the socalled critical curve in the mass accretion rate and neutrino luminosity plane, above which there is no steady solution of the accretion flow so that a dynamical expanding shock wave is expected. In addition, we developed a phenomenological model to evaluate the trajectories in this plane. This model reasonably reproduces the numerical results by using the initial density structure of the progenitors alone. By this model, we can predict the possibility of explosion by using the initial density structure of the progenitors alone.  [Show abstract] [Hide abstract]
ABSTRACT: We present an overview of axisymmetric corecollapse supernova simulations employing neutrino transport scheme by the isotropic diffusion source approximation. Studying 101 solarmetallicity progenitors covering zeroage main sequence mass from 10.8 to 75.0 solar masses, we systematically investigate how the differences in the structures of these multiple progenitors impact the hydrodynamics evolution. By following a longterm evolution over 1.0 s after bounce, most of the computed models exhibit neutrinodriven revival of the stalled bounce shock at about 200  800 ms postbounce, leading to the possibility of explosion. Pushing the boundaries of expectations in previous onedimensional studies, our results show that the time of shock revival, evolution of shock radii, and diagnostic explosion energies are tightly correlated with the compactness parameter xi which characterizes the structure of the progenitors. Compared to models with low xi, models with high xi undergo high ram pressure from the accreting matter onto the stalled shock and it takes longer time before the shock expansion is initiated under the influence of neutrinodriven convection and the standing accretionshock instability. We find that the accretion luminosity becomes higher for models with high xi, which makes the diagnostic energy higher and the synthesized nickel mass bigger. We point out that these explosion characteristics tend to show a monotonic increase as a function of the compactness parameter xi.  [Show abstract] [Hide abstract]
ABSTRACT: We examine the synthesis of 44Ti in a neutrinodriven aspherical supernova (SN), focusing on reaction rates related to 44Ti and rotation of a progenitor. We have performed 2D hydrodynamic simulations of SN of a 15M☉ progenitor, whose angular velocity is manually set to be a cylindrical distribution and have followed explosive nucleosynthesis in the ejecta. We find that the faster rates of 40Ca(α,γ)44Ti and the slower rate of 44Ti(α,p)47V lead to more massive ejection of 44Ti and 56Ni and larger ratios <44Ti/56Ni>. Faster rotation also results in more massive ejection of 44Ti and 56Ni. Ratios <44Ti/56Ni> are however independent from rotation. Large masses of 44Ti and large ratios SUP>Ni> observed in SN 1987A and Cas A (> 1O4M☉ and 12 respectively) are not realized in all the models.04/2014; 1594(1). DOI:10.1063/1.4874085  [Show abstract] [Hide abstract]
ABSTRACT: Based on multidimensional neutrinoradiation hydrodynamic simulations, we report several cuttingedge issues about the longveiled explosion mechanism of corecollapse supernovae (CCSNe). In this contribution, we pay particular attention to whether threedimensional (3D) hydrodynamics and/or general relativity (GR) would or would not help the onset of explosions. By performing 3D simulations with spectral neutrino transport, we show that it is more difficult to obtain an explosion in 3D than in 2D. In addition, our results from the first generation of full general relativistic 3D simulations including approximate neutrino transport indicate that GR can foster the onset of neutrinodriven explosions. Based on our recent parametric studies using a lightbulb scheme, we discuss impacts of nuclear energy deposition behind the supernova shock and stellar rotation on the neutrinodriven mechanism, both of which have yet to be included in the selfconsistent 3D supernova models. Finally we give an outlook with a summary of the most urgent tasks to extract the information about the explosion mechanisms from multimessenger CCSN observables.04/2014; 1594(1). DOI:10.1063/1.4874078  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the effects of rotation on the evolution of corecollapse supernova explosion using a 15 solar mass progenitor model with a variety of neutrino luminosity and rotational velocity. Stars should have some amount of angular momentum, which would affect stellar evolution and its final explosion. In this paper we focus on the effect of rotation on gravitational collapse of a core, on a core bounce of accreting matter, and on subsequent generation and evolution of a shock wave. We find that the rotation plays a positive role for supernova explosions. More rapidly rotating models present more rapid expansion of the shock front and more energetic explosions. When the rotational speed is moderate, the shock once stalls at about 200 km away from the center similarly to a nonrotating model. Then the rotating progenitor experiences effective neutrino heating especially around an equatorial plane and explodes even with somewhat low neutrino luminosity for which the nonrotating model cannot overcome accreting matter and finally collapses. When the rotational speed is fast, the shock expands to about 300 km immediately after the core bounce and then evolves to move outward without shock stalling. We conclude that this positive effect of rotation to explosions is dominant against some possible negative aspects, for example, lower neutrino luminosity caused by less contraction of the rotating core.04/2014; 1594(1). DOI:10.1063/1.4874084  [Show abstract] [Hide abstract]
ABSTRACT: We perform a series of simplified numerical experiments to explore how rotation impacts on the threedimensional (3D) hydrodynamics of corecollapse supernovae. For the sake of our systematic study, we employ a lightbulb scheme to trigger explosions and a threeflavor neutrino leakage scheme to treat deleptonization effects and neutrino losses from protoneutron star interior. Using a 15 solar mass progenitor, we compute thirty models in 3D with a wide variety of initial angular momentum and lightbulb neutrino luminosity. We find that the rotation can help onset of neutrinodriven explosions for the models in which the initial angular momentum is matched to that obtained in recent stellar evolutionary calculations (0.33 rad/s at the center). For models with larger initial angular momentum, a shock surface deforms to be more oblate due to larger centrifugal force. This makes not only a gain region more concentrated around the equatorial plane, but also the mass in the gain region bigger. As a result, buoyant bubbles tend to be coherently formed and rise in the equatorial region, which pushes the revived shock ever larger radii until a global explosion is triggered. We find that these are the main reasons that the preferred direction of explosion in 3D rotating models is often perpendicular to the spin axis, which is in sharp contrast to the polar explosions around the axis that was obtained in previous 2D simulations.The Astrophysical Journal 03/2014; 793(1). DOI:10.1088/0004637X/793/1/45 · 6.28 Impact Factor 
Article: Revisiting Impacts of Nuclear Burning for Reviving Weak Shocks in Neutrinodriven Supernovae
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ABSTRACT: We revisit potential impacts of nuclear burning on the onset of the neutrinodriven explosions of corecollapse supernovae. By changing the neutrino luminosity and its decay time to obtain parametric explosions in one and twodimensional (1D and 2D, respectively) models with or without a 13 isotope α network, we study how the inclusion of nuclear burning could affect the postbounce dynamics for 4 progenitor models; 3 for 15.0 M ☉ stars and 1 for an 11.2 M ☉ star. We find that the energy supply due to the nuclear burning of infalling material behind the shock can energize the shock expansion, especially for models that produce only marginal explosions in the absence of nuclear burning. These models are energized by nuclear energy deposition when the shock front passes through the siliconrich layer and/or later as it touches the oxygenrich layer. Depending on the neutrino luminosity and its decay time, the diagnostic energy of the explosion increases up to a few times 1050 erg for models with nuclear burning compared to the corresponding models without. We point out that these features are most remarkable for the LimongiChieffi progenitor in both 1D and 2D because the progenitor model possesses a massive oxygen layer, with an inneredge radius that is smallest among the employed progenitors, which means that the shock can touch the rich fuel on a shorter timescale after bounce. The energy difference is generally smaller (~0.10.2 × 1051 erg) in 2D than in 1D (at most ~0.6 × 1051 erg). This is because neutrinodriven convection and the shock instability in 2D models enhance the neutrino heating efficiency, which makes the contribution of nuclear burning relatively smaller compared to 1D models. Considering uncertainties in progenitor models, our results indicate that nuclear burning should remain one of the important ingredients to foster the onset of neutrinodriven explosions.The Astrophysical Journal 01/2014; 782(2). DOI:10.1088/0004637X/782/2/91 · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Bearing in mind the application to highmagneticfield (highB) radio pulsars, we investigate twodimensional (2D) thermal evolutions of neutron stars (NSs). We pay particular attention to the influence of different equilibrium configurations on the surface temperature distributions. The equilibrium configurations are constructed in a systematic manner, in which both toroidal and poloidal magnetic fields are determined selfconsistently with the inclusion of general relativistic effects. To solve the 2D heat transfer inside the NS interior out to the crust, we have developed an implicit code based on a finitedifference scheme that deals with anisotropic thermal conductivity and relevant cooling processes in the context of a standard cooling scenario. In agreement with previous studies, the surface temperatures near the pole become higher than those in the vicinity of the equator as a result of anisotropic heat transfer. Our results show that the ratio of the highest to the lowest surface temperatures changes maximally by one order of magnitude, depending on the equilibrium configurations. Despite such difference, we find that the area of such hot and cold spots is so small that the simulated Xray spectrum could be well reproduced by a single temperature blackbody fitting.Publications Astronomical Society of Japan 01/2014; 66(2):50. DOI:10.1093/pasj/psu009 · 2.01 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present numerical results on two (2D) and threedimensional (3D) hydrodynamic corecollapse simulations of an 11.2$M_\odot$ star. By changing numerical resolutions and seed perturbations systematically, we study how the postbounce dynamics is different in 2D and 3D. The calculations were performed with an energydependent treatment of the neutrino transport based on the isotropic diffusion source approximation scheme, which we have updated to achieve a very high computational efficiency. All the computed models in this work including nine 3D models and fifteen 2D models exhibit the revival of the stalled bounce shock, leading to the possibility of explosion. All of them are driven by the neutrinoheating mechanism, which is fostered by neutrinodriven convection and the standingaccretionshock instability (SASI). Reflecting the stochastic nature of multidimensional (multiD) neutrinodriven explosions, the blast morphology changes from models to models. However, we find that the final fate of the multiD models whether an explosion is obtained or not, is little affected by the explosion stochasticity. In agreement with some previous studies, higher numerical resolutions lead to slower onset of the shock revival in both 3D and 2D. In the context of selfconsistent supernova models, our results systematically show, for the first time, that the shock expansion occurs more energetically in 2D than in 3D.The Astrophysical Journal 08/2013; 786(2). DOI:10.1088/0004637X/786/2/83 · 6.28 Impact Factor 
Article: Gravitational Wave Signatures from Lowmode Spiral Instabilities in Rapidly Rotating Supernova Cores
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ABSTRACT: We study properties of gravitational waves (GWs) from rotating corecollapse of a 15 solar mass star by performing threedimensional generalrelativistic hydrodynamic simulations with an approximate neutrino transport. By parametrically changing the precollapse angular momentum, we focus on the effects of rotation on the GW signatures in the early postbounce evolution. Regarding threeflavor neutrino transport, we solve the energyaveraged set of radiation energy and momentum. In addition to the gravitational quadrupole radiation from matter motions, we take into account GWs from anisotropic neutrino emission. With these computations, our results present evidence that nonaxisymmetric instabilities play an essential role in determining the GW signatures in the rotating postbounce evolution. For our rapidly rotating models, we show that precollapse density inhomogeneities give rise to millisecond variations in the waveforms. During prompt convection, we find that the waveforms show narrowband and highly quasiperiodic signals. We point out that such feature reflects the growth of the onearmed spiral modes that develop under the influence of the standingaccretionshock instability and the low$T/W$ instability. The typical frequency of the quasiperiodic waveforms can be well explained by the propagating acoustic waves. Although the GW signals exhibit strong variability between the two polarizations and different observer directions, they are within the realm of next generation detectors such as by KAGRA and Advanced LIGO, if the source with sufficient angular momentum is located in our Galaxy.Physical Review D 04/2013; 89(4). DOI:10.1103/PhysRevD.89.044011 · 4.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We studied roles of a turbulent resistivity in the corecollapse of a strongly magnetized massive star, carrying out 2DresistiveMHD simulations. The three cases with different initial strengths of magnetic field and rotation are investigated; 1. strongly magnetized rotating core; 2.moderately magnetized rotating core; 3. very strongly magnetized nonrotating core. In each case, both an idealMHD model and resistiveMHD models are computed. As a result of computations, each model shows a matter eruption helped by a magnetic acceleration (and also by a centrifugal acceleration in the rotating cases). We found that a resistivity attenuates the explosion in case~1 and 2, while it enhances the explosion in case~3. We also found that in the rotating cases, main mechanisms for the amplification of a magnetic field in the postbounce phase are an outward advection of magnetic field and a winding of poloidal magnetic fieldlines by differential rotation, which are somewhat dampened down with the presence of a resistivity. Although the magnetorotational instability seems to occur in the rotating models, it will play only a minor role in a magnetic field amplification. Another impact of resistivity is that on the aspect ratio. In the rotating cases, a large aspect ratio of the ejected matters, $> 2.5$, attained in a idealMHD model is reduced to some extent in a resistive model. These results indicate that a resistivity possibly plays an important role in the dynamics of strongly magnetized supernovae.The Astrophysical Journal 11/2012; 764(1). DOI:10.1088/0004637X/764/1/10 · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We performed hydrodynamic simulations of core collapse and bounce for a progenitor model with 15.0 solar mass, using ZEUSMP code in axisymmetric coordinate. Our numerical code is equipped with a nuclear reaction network including 13 alpha nuclei form {sup 4}He to {sup 56}Ni to investigate the potential role played by nuclear reactions in reviving a stalled shock wave at the central region of corecollapse supernovae. We found that the energy released by nuclear reactions is significantly helpful in accelerating shock waves and is able to produce energetic explosion even if inputted neutrino luminosity is low.11/2012; 1484(1). DOI:10.1063/1.4763446  [Show abstract] [Hide abstract]
ABSTRACT: We examine explosive nucleosynthesis during neutrinodriven, aspherical supernovae of Population III stars, based on twodimensional (2D) hydrodynamic simulations of the explosion of 1140M{sub CircledDotOperator} stars with zero metallicity. The magnitude and asymmetry of the explosion energy are estimated with the simulations. By postprocessing calculations with a large nuclear reaction network, we have evaluated abundances and masses of ejecta from the aspherical SNe. We find that the evaluated abundance patterns are similar to those observed in extremely metal poor stars, as shown in spherical and 2D models, in which the explosion is manually and spherically initiated. Matter mixing induced via standing accretion shock instability is important for the abundances and masses of the SN ejecta.11/2012; 1484(1). DOI:10.1063/1.4763457 
Article: Nucleosynthesis in a Massive Star Associated with Magnetohydrodynamical Jets from Collapsars
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ABSTRACT: We investigate the nucleosynthesis during the stellar evolution and the jetlike supernova explosion of a massive star of 70 M{sub CircledDotOperator} having the solar metallicity in the main sequence stage. The nucleosynthesis calculations have been performed with large nuclear reaction networks, where the weak s, p, and rprocesses are taken into account. As a result selements of 60 > A > 90 and relements of 90 > A > 160 are highly overproduced relative to the solar system abundances. We find that the SrYZr isotopes are primarily synthesized in the explosive nucleosynthesis which could be one of the sites of the lighter element primary process (LEPP).11/2012; 1484(1). DOI:10.1063/1.4763437  [Show abstract] [Hide abstract]
ABSTRACT: Bearing in mind the application of corecollapse supernovae, we study the nonlinear properties of the magnetorotational instability (MRI) by means of threedimensional simulations in the framework of a local shearing box approximation. By systematically changing the shear rates that symbolize the degree of differential rotation in nascent protoneutron stars (PNSs), we derive a scaling relation between the turbulent stress sustained by the MRI and the shearvorticity ratio. Our parametric survey shows a powerlaw scaling between the turbulent stress (((w {sub tot}))) and the shearvorticity ratio (g{sub q} ) as ((w {sub tot})){proportional_to}g {sup {delta}} {sub q} with an index of {delta} {approx} 0.5. The MRIamplified magnetic energy has a similar scaling relative to the turbulent stress, while the Maxwell stress has a slightly smaller powerlaw index ({approx}0.36). By modeling the effect of viscous heating rates from MRI turbulence, we show that the stronger magnetic fields, or the larger shear rates initially imposed, lead to higher dissipation rates. For a rapidly rotating PNS with a spin period in milliseconds and with strong magnetic fields of 10{sup 15} G, the energy dissipation rate is estimated to exceed 10{sup 51} erg s{sup 1}. Our results suggest that the conventional magnetohydrodynamic (MHD) mechanism of corecollapse supernovae is likely to be affected by MRIdriven turbulence, which we speculate, on the one hand, could harm the MHDdriven explosions due to the dissipation of the shear rotational energy at the PNS surface; or, on the other hand, its energy deposition might be potentially favorable for the working of the neutrinoheating mechanism.The Astrophysical Journal 11/2012; 759(2). DOI:10.1088/0004637X/759/2/110 · 6.28 Impact Factor 
Article: 3D hydrodynamic corecollapse SN simulations for an 11.2 M ⊙ star with spectral neutrino transport
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ABSTRACT: We have performed threedimensional (3D) hydrodynamical simulations of corecollapse supernovae (SNe) with multigroup neutrino transport to study nonaxisymmetric effects in the context of neutrino heating explosion mechanism. By comparing one (1D) and two dimensional (2D) results with those of 3D, we study how the increasing spatial multidimensionality affects the postbounce SN dynamics. The calculations were performed with an energydependent treatment of the neutrino transport that is solved by the isotropic diffusion source approximation scheme. In agreement with previous studies, our 1D model does not produce explosions for the 11.2 M ⊙ star, while the neutrinodriven revival of the stalled bounce shock is obtained both in the 2D and 3D models. Our results show that convective matter motions below the gain radius become much more violent in 3D than 2D, making the neutrino luminosity larger for 3D. Enhanced by the large neutrino luminosity, the shock of the 3D model expands faster than that of the 2D. Our results show that the evolution of the shock is sensitive to the employed numerical resolutions. To draw a robust conclusion, 3D simulations with much higher numerical resolution and more advanced treatment of neutrino transport and gravity is needed.Proceedings of the International Astronomical Union 09/2012; 7(S279):409410. DOI:10.1017/S1743921312013580
Publication Stats
2k  Citations  
393.81  Total Impact Points  
Top Journals
Institutions

2013–2015

Fukuoka University
 Department of Applied Physics
Hukuoka, Fukuoka, Japan


2014

Universität Basel
 Department of Physics
Bâle, BaselCity, Switzerland


2006–2014

National Astronomical Observatory of Japan
 • Division of Theoretical Astronomy
 • Center for Computational Astrophysics
Edo, Tōkyō, Japan


2012

Kumamoto National College of Technology
Kumamoto, Kumamoto Prefecture, Japan


2008–2012

National Institutes Of Natural Sciences
Edo, Tōkyō, Japan


2003–2008

The University of Tokyo
 Department of Physics
Tokyo, Tokyoto, Japan 
RIKEN
 Biometal Science Laboratory
Вако, Saitama, Japan


1970–2008

Waseda University
 Department of Computer Science and Engineering
Tokyo, Tokyoto, Japan
