Kei Kotake

Fukuoka University, Hukuoka, Fukuoka, Japan

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Publications (129)377.3 Total impact

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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 three-dimensional radiation hydrodynamics core-collapse 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 state-of-the-art three-dimensional core-collapse 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.
    08/2014;
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    ABSTRACT: By performing neutrino-radiation 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 so-called 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.
    06/2014;
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    ABSTRACT: We present an overview of axisymmetric core-collapse supernova simulations employing neutrino transport scheme by the isotropic diffusion source approximation. Studying 101 solar-metallicity progenitors covering zero-age 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 long-term evolution over 1.0 s after bounce, most of the computed models exhibit neutrino-driven 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 one-dimensional 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 neutrino-driven convection and the standing accretion-shock 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.
    06/2014;
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    ABSTRACT: We investigate the effects of rotation on the evolution of core-collapse 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 non-rotating 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 non-rotating 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).
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    ABSTRACT: We examine the synthesis of 44Ti in a neutrino-driven 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 (> 1O-4M☉ and 1-2 respectively) are not realized in all the models.
    04/2014; 1594(1).
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    ABSTRACT: Based on multi-dimensional neutrino-radiation hydrodynamic simulations, we report several cutting-edge issues about the long-veiled explosion mechanism of core-collapse supernovae (CCSNe). In this contribution, we pay particular attention to whether three-dimensional (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 neutrino-driven explosions. Based on our recent parametric studies using a light-bulb scheme, we discuss impacts of nuclear energy deposition behind the supernova shock and stellar rotation on the neutrino-driven mechanism, both of which have yet to be included in the self-consistent 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 multi-messenger CCSN observables.
    04/2014; 1594(1).
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    ABSTRACT: We perform a series of simplified numerical experiments to explore how rotation impacts on the three-dimensional (3D) hydrodynamics of core-collapse supernovae. For the sake of our systematic study, we employ a light-bulb scheme to trigger explosions and a three-flavor neutrino leakage scheme to treat deleptonization effects and neutrino losses from proto-neutron star interior. Using a 15 solar mass progenitor, we compute thirty models in 3D with a wide variety of initial angular momentum and light-bulb neutrino luminosity. We find that the rotation can help onset of neutrino-driven explosions for the models in which the initial angular momentum is matched to that obtained in recent stellar evolutionary calculations (0.3-3 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). · 6.73 Impact Factor
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    ABSTRACT: We revisit potential impacts of nuclear burning on the onset of the neutrino-driven explosions of core-collapse supernovae. By changing the neutrino luminosity and its decay time to obtain parametric explosions in one- and two-dimensional (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 silicon-rich layer and/or later as it touches the oxygen-rich 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 Limongi-Chieffi progenitor in both 1D and 2D because the progenitor model possesses a massive oxygen layer, with an inner-edge 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 (~0.1-0.2 × 1051 erg) in 2D than in 1D (at most ~0.6 × 1051 erg). This is because neutrino-driven 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 neutrino-driven explosions.
    01/2014; 782(2).
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    ABSTRACT: Bearing in mind the application to high-magnetic-field (high-B) radio pulsars, we investigate two-dimensional (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 self-consistently 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 finite-difference 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 X-ray spectrum could be well reproduced by a single temperature blackbody fitting.
    Publications- Astronomical Society of Japan 01/2014; 66(2):50. · 2.44 Impact Factor
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    ABSTRACT: We present numerical results on two- (2D) and three-dimensional (3D) hydrodynamic core-collapse 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 energy-dependent 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 neutrino-heating mechanism, which is fostered by neutrino-driven convection and the standing-accretion-shock instability (SASI). Reflecting the stochastic nature of multi-dimensional (multi-D) neutrino-driven explosions, the blast morphology changes from models to models. However, we find that the final fate of the multi-D 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 self-consistent 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). · 6.73 Impact Factor
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    ABSTRACT: We study properties of gravitational waves (GWs) from rotating core-collapse of a 15 solar mass star by performing three-dimensional general-relativistic 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 three-flavor neutrino transport, we solve the energy-averaged 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 non-axisymmetric 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 narrow-band and highly quasi-periodic signals. We point out that such feature reflects the growth of the one-armed spiral modes that develop under the influence of the standing-accretion-shock instability and the low-$T/|W|$ instability. The typical frequency of the quasi-periodic 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.
    04/2013;
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    ABSTRACT: We studied roles of a turbulent resistivity in the core-collapse of a strongly magnetized massive star, carrying out 2D-resistive-MHD 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 non-rotating core. In each case, both an ideal-MHD model and resistive-MHD 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 post-bounce phase are an outward advection of magnetic field and a winding of poloidal magnetic field-lines 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 ideal-MHD 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). · 6.73 Impact Factor
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    ABSTRACT: We performed hydrodynamic simulations of core collapse and bounce for a progenitor model with 15.0 solar mass, using ZEUS-MP code in axi-symmetric 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 core-collapse 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.
    AIP Conference Proceedings. 11/2012; 1484(1).
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    ABSTRACT: We examine explosive nucleosynthesis during neutrino-driven, aspherical supernovae of Population III stars, based on two-dimensional (2D) hydrodynamic simulations of the explosion of 11-40M{sub Circled-Dot-Operator} stars with zero metallicity. The magnitude and asymmetry of the explosion energy are estimated with the simulations. By post-processing 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.
    AIP Conference Proceedings. 11/2012; 1484(1).
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    ABSTRACT: We investigate the nucleosynthesis during the stellar evolution and the jet-like supernova explosion of a massive star of 70 M{sub Circled-Dot-Operator} 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 r-processes are taken into account. As a result s-elements of 60 > A > 90 and r-elements of 90 > A > 160 are highly overproduced relative to the solar system abundances. We find that the Sr-Y-Zr isotopes are primarily synthesized in the explosive nucleosynthesis which could be one of the sites of the lighter element primary process (LEPP).
    AIP Conference Proceedings. 11/2012; 1484(1).
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    ABSTRACT: Bearing in mind the application of core-collapse supernovae, we study the nonlinear properties of the magnetorotational instability (MRI) by means of three-dimensional 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 proto-neutron stars (PNSs), we derive a scaling relation between the turbulent stress sustained by the MRI and the shear-vorticity ratio. Our parametric survey shows a power-law scaling between the turbulent stress (((w {sub tot}))) and the shear-vorticity ratio (g{sub q} ) as ((w {sub tot})){proportional_to}g {sup {delta}} {sub q} with an index of {delta} {approx} 0.5. The MRI-amplified magnetic energy has a similar scaling relative to the turbulent stress, while the Maxwell stress has a slightly smaller power-law 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 core-collapse supernovae is likely to be affected by MRI-driven turbulence, which we speculate, on the one hand, could harm the MHD-driven 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 neutrino-heating mechanism.
    The Astrophysical Journal 11/2012; 759(2). · 6.73 Impact Factor
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    ABSTRACT: We have investigated the revival of a shock wave by nuclear burning reactions at the central region of core-collapse supernovae. For this purpose, we performed hydrodynamic simulations of core collapse and bounce for 15 M ⊙ progenitor model, using ZEUS-MP code in axi-symmetric coordinates. Our numerical code is equipped with a simple nuclear reaction network including 13 α nuclei form 4He to 56Ni, and accounting for energy feedback from nuclear reactions as well as neutrino heating and cooling. 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 the input neutrino luminosity is low.
    Proceedings of the International Astronomical Union 09/2012; 7(S279):365-366.
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    ABSTRACT: We examine explosive nucleosynthesis during neutrino-driven, aspherical supernovae (SNe) of Population (Pop) III stars, based on two-dimensional (2D) hydrodynamic simulations of the explosion of 11-40Msolar Pop III stars. The magnitude and asymmetry of the explosion energy are estimated with the simulations. By post-processing 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. Faint SNe of 11 and 15Msolar progenitors show large [C/Fe] > 0.7, as observed in C-enhanced metalpoor stars. Moreover, observed increasing trend of [C/O] towards for lower [O/H] could rely on a fact that [C/O] increases and [O/H] decreases for a progenitor with a lower mass.
    09/2012;
  • Kei Kotake
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    ABSTRACT: Based on our multi-dimensional neutrino-radiation hydrodynamic simulations, we report several cutting-edge issues about the long-veiled explosion mechanism of core-collapse supernovae (CCSNe). In this contribution, we pay particular attention to whether three-dimensional (3D) hydrodynamics and/or general relativity (GR) would or would not help the onset of explosions. Our results from the first generation of full GR 3D simulations including approximate neutrino transport are quite optimistic, indicating that both of the two ingredients can foster neutrino-driven explosions. We give an outlook with a summary of the most urgent tasks to draw a robust conclusion to our findings.
    Proceedings of the International Astronomical Union 09/2012; 7(S279):126-133.
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    ABSTRACT: We have performed three-dimensional (3D) hydrodynamical simulations of core-collapse supernovae (SNe) with multigroup neutrino transport to study non-axisymmetric 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 multi-dimensionality affects the postbounce SN dynamics. The calculations were performed with an energy-dependent 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 neutrino-driven 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):409-410.

Publication Stats

1k Citations
377.30 Total Impact Points

Institutions

  • 2014
    • Fukuoka University
      Hukuoka, Fukuoka, Japan
  • 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
  • 2005–2008
    • Waseda University
      • Department of Computer Science and Engineering
      Tokyo, Tokyo-to, Japan
  • 2003–2008
    • The University of Tokyo
      • Department of Physics
      Tokyo, Tokyo-to, Japan