arXiv subjects

Kei Kotake

Publications and source records attributed to Kei Kotake.

At least 91 records · Page 5Linked to original sources

arXiv2012

On the Importance of the Equation of State for the Neutrino-Driven Supernova Explosion Mechanism

By implementing widely-used equations of state (EOS) from Lattimer & Swesty (LS) and H. Shen et al. (SHEN) in core-collapse supernova simulations, we explore possible impacts of these EOS on the post-bounce dynamics prior to the onset of neutrino-driven explosions. Our spherically symmetric (1D) and axially symmetric (2D) models are based on neutrino radiation hydrodynamics including spectral transport, which is solved by the isotropic diffusion source approximation. We confirm that in 1D simulations neutrino-driven explosions cannot be obtained for any of the employed EOS. Impacts of the EOS on the post-bounce hydrodynamics are more clearly visible in 2D simulations. In 2D models of a 15 M_sun progenitor using the LS EOS, the stalled bounce shock expands to increasingly larger radii, which is not the case using the SHEN EOS. Keeping in mind that the omission of the energy drain by heavy-lepton neutrinos in the present scheme could facilitate explosions, we find that 2D models of an 11.2 M_sun progenitor produce neutrino-driven explosions for all the EOS under investigation. Models using the LS EOS are slightly more energetic compared to those with the SHEN EOS. The more efficient neutrino heating in the LS models coincides with a higher electron antineutrino luminosity and a larger mass that is enclosed within the gain region. The models based on the LS EOS also show a more vigorous and aspherical downflow of accreting matter to the surface of the protoneutron star (PNS). The accretion pattern is essential for the production and strength of outgoing pressure waves, that can push in turn the shock to larger radii and provide more favorable conditions for the explosion. [abbreviated]

astro-ph.HE↗

arXiv2012

Explosive Nucleosynthesis in Magnetohydrodynamical Jets from Collapsars II. Heavy-Element Nucleosynthesis of s, r, p-Processes

We investigate the nucleosynthesis in a massive star of 70 M_solar with solar metallicity in the main sequence stage. The helium core mass after hydrogen burning corresponds to 32 M_solar. Nucleosynthesis calculations have been performed during the stellar evolution and the jetlike supernova explosion of a collapsar model, where the weak s-, p-, and r-processes are taken into account. We confirm that s-elements of 60 < A < 90 are highly overproduced relative to the solar abundances in the hydrostatic nucleosynthesis. During oxygen burning, p-elements of A > 90 are produced via photodisintegrations of seed s-elements. However, the produced p-elements are disintegrated in later stages except for ^{180}Ta. In the explosive nucleosynthesis, elements of 90 < A < 160 are significantly overproduced relative to the solar values owing to the r-process. Only heavy p-elements (N > 50) are overproduced via the p-process. Compared with the previous study of r-process nucleosynthesis calculations in the collapsar model of 40 M_solar by Fujimoto et al. 2007, 2008, our jet model cannot contribute to the third peak of the solar r-elements and intermediate p-elements. Averaging the overproduction factors over the progenitor masses with the use of Salpeter's IMF, we suggest that the 70 M_solar star could contribute to the solar weak s-elements of 60 < A < 90 and neutron-rich elements of 90 < A < 160. We confirm the primary synthesis of light p-elements in the ejected matter of high peak temperature. The ejected matter has [Sr/Eu] \sim -0.4, which is different from that of a typical r-process-enriched star CS22892-052 ([Sr/Eu] \sim -1). We find that Sr-Y-Zr isotopes are primarily synthesized in the explosive nucleosynthesis in a similar process of the primary production of light p-elements, which has been considered as one of the sites of a lighter element primary process (LEPP).

astro-ph.SR↗

arXiv2012

Core-Collapse Supernovae as Supercomputing Science: a status report toward 6D simulations with exact Boltzmann neutrino transport in full general relativity

This is a status report on our endeavor to reveal the mechanism of core-collapse supernovae (CCSNe) by large-scale numerical simulations. Multi-dimensionality of the supernova engine, general relativistic magnetohydrodynamics, energy and lepton number transport by neutrinos emitted from the forming neutron star as well as nuclear interactions there, are all believed to play crucial roles in repelling infalling matter and producing energetic explosions. These ingredients are nonlinearly coupled with one another in the dynamics of core-collapse, bounce, and shock expansion. Serious quantitative studies of CCSNe hence make extensive numerical computations mandatory. Since neutrinos are neither in thermal nor in chemical equilibrium in general, their distributions in the phase space should be computed. This is a six dimensional (6D) neutrino transport problem and quite a challenge even for those with an access to the most advanced numerical resources such as the "K computer". To tackle this problem, we have embarked on multi-front efforts. In particular we report in this paper our recent progresses in the treatments of multi-dimensional (multi-D) radiation-hydrodynamics. We are currently proceeding on two different paths to the ultimate goal; in one approach we employ an approximate but highly efficient scheme for neutrino transport and treat 3D hydrodynamics and/or general relativity rigorously; some neutrino-driven explosions will be presented and comparisons will be made between 2D and 3D models quantitatively; in the second approach, on the other hand, exact but so far Newtonian Boltzmann equations are solved in two and three spatial dimensions; we will show some demonstrative test simulations. We will also address the perspectives of exa-scale computations on the next generation supercomputers.

astro-ph.HE↗

arXiv2012

Fully General Relativistic Simulations of Core-Collapse Supernovae with An Approximate Neutrino Transport

We present results from the first generation of multi-dimensional hydrodynamic core-collapse simulations in full general relativity (GR) that include an approximate treatment of neutrino transport. Using a M1 closure scheme with an analytic variable Eddington factor, we solve the energy-independent set of radiation energy and momentum based on the Thorne's momentum formalism. To simplify the source terms of the transport equations, a methodology of multiflavour neutrino leakage scheme is partly employed. Our newly developed code is designed to evolve the Einstein field equation together with the GR radiation hydrodynamic equations. We follow the dynamics starting from the onset of gravitational core-collapse of a 15 $M_{\odot}$ star, through bounce, up to about 100 ms postbounce in this study to study how the spacial multi-dimensionality and GR would affect the dynamics in the early postbounce phase. Our 3D results support the anticipation in previous 1D results that the neutrino luminosity and average neutrino energy of any neutrino flavor in the postbounce phase increase when switching from SR to GR hydrodynamics. This is because the deeper gravitational well of GR produces more compact core structures, and thus hotter neutrino spheres at smaller radii. By analyzing the residency timescale to the neutrino-heating timescale in the gain region, we show that the criterion to initiate neutrino-driven explosions can be most easily satisfied in 3D models, irrespective of SR or GR hydrodynamics. Our results suggest that the combination of GR and 3D hydrodynamics provides the most favorable condition to drive a robust neutrino-driven explosion.

astro-ph.HE↗

arXiv2012

Gravitational Wave Signatures of Hyperaccreting Collapsar Disks

By performing two-dimensional special relativistic (SR) magnetohydrodynamic simulations, we study possible signatures of gravitational waves (GWs) in the context of the collapsar model for long-duration gamma-ray bursts. In our SR simulations, the central black hole is treated as an absorbing boundary. By doing so, we focus on the GWs generated by asphericities in neutrino emission and matter motions in the vicinity of the hyperaccreting disks. We compute nine models by adding initial angular momenta and magnetic fields parametrically to a precollapse core of a $35 M_{\odot}$ progenitor star. As for the microphysics, a realistic equation of state is employed and the neutrino cooling is taken into account via a multiflavor neutrino leakage scheme. To accurately estimate GWs produced by anisotropic neutrino emission, we perform a ray-tracing analysis in general relativity by a post-processing procedure. By employing a stress formula that includes contributions both from magnetic fields and special relativistic corrections, we study also the effects of magnetic fields on the gravitational waveforms. We find that the GW amplitudes from anisotropic neutrino emission show a monotonic increase with time, whose amplitudes are much larger than those from matter motions of the accreting material. We show that the increasing trend of the neutrino GWs stems from the excess of neutrino emission in the direction near parallel to the spin axis illuminated from the hyperaccreting disks. We point out that a recently proposed future space-based interferometer like Fabry-Perot type DECIGO would permit the detection of these GW signals within $\approx$ 100 Mpc.

astro-ph.HE↗

arXiv2012

Multimessengers from core-collapse supernovae: multidimensionality as a key to bridge theory and observation

Core-collapse supernovae are dramatic explosions marking the catastrophic end of massive stars. The only means to get direct information about the supernova engine is from observations of neutrinos emitted by the forming neutron star, and through gravitational waves which are produced when the hydrodynamic flow or the neutrino flux is not perfectly spherically symmetric. The multidimensionality of the supernova engine, which breaks the sphericity of the central core such as convection, rotation, magnetic fields, and hydrodynamic instabilities of the supernova shock, is attracting great attention as the most important ingredient to understand the long-veiled explosion mechanism. Based on our recent work, we summarize properties of gravitational waves, neutrinos, and explosive nucleosynthesis obtained in a series of our multidimensional hydrodynamic simulations and discuss how the mystery of the central engines can be unraveled by deciphering these multimessengers produced under the thick veils of massive stars.

astro-ph.HE↗

arXiv2012

Multiple physical elements to determine the gravitational-wave signatures of core-collapse supernovae

We review recent progress in the theoretical predictions of gravitational waves (GWs) of core-collapse supernovae. Following a brief summary of the methods in the numerical modeling, we summarize multiple physical elements that determine the GW signatures which have been considered to be important in extracting the information of the long-veiled explosion mechanism from the observation of the GWs. We conclude with a summary of the most urgent tasks to make the dream come true.

astro-ph.HE↗

arXiv2012

R-process Nucleosynthesis during the Magnetohydrodynamics Explosions of a Massive Star

We investigate the possibility of the r-process during the magnetohydrohynamical explosion of supernova in a massive star of 13 solar mass with the effects of neutrinos induced. We adopt five kinds of initial models which include properties of rotation and the toroidal component of the magnetic field . The simulations which succeed the explosions are limitted to a concentrated magnetic field and strong differential rotation. Low $Y_{e}$ ejecta produce heavy elements and the third peak can be reprocuced. However, the second peak is low because $Y_{e}$ distribution as a function of radius is steep and ejecta corresponding to middle $Y_{e}$ is very few.

astro-ph.HE↗

arXiv2012

Three-dimensional Hydrodynamic Core-Collapse Supernova Simulations for an $11.2 M_{\odot}$ Star with Spectral Neutrino Transport

We present numerical results on three-dimensional (3D) hydrodynamic core-collapse simulations of an $11.2 M_{\odot}$ star. By comparing one-(1D) and two-dimensional(2D) results with those of 3D, we study how the increasing spacial multi-dimensionality affects the postbounce supernova dynamics. The calculations were performed with an energy-dependent treatment of the neutrino transport that is solved by the isotropic diffusion source approximation scheme. By performing a tracer-particle analysis, we show that the maximum residency time of material in the gain region is shown to be longer for 3D due to non-axisymmetric flow motions than 2D, which is one of advantageous aspects of 3D models to obtain neutrino-driven explosions. 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. Nevertheless the emitted neutrino energies are made smaller due to the enhanced cooling. Our results indicate whether these advantages for driving 3D explosions could or could not overwhelm the disadvantages is sensitive to the employed numerical resolutions. An encouraging finding is that the shock expansion tends to become more energetic for models with finer resolutions. To draw a robust conclusion, 3D simulations with much more higher numerical resolutions and also with more advanced treatment of neutrino transport as well as of gravity is needed, which could be hopefully practicable by utilizing forthcoming Petaflops-class supercomputers.

astro-ph.HE↗

arXiv2011

Gravitational Wave Signatures of Magnetohydrodynamically-Driven Core-Collapse Supernova Explosions

By performing a series of two-dimensional, special relativistic magnetohydrodynamic (MHD) simulations, we study signatures of gravitational waves (GWs) in the magnetohydrodynamically-driven core-collapse supernovae. In order to extract the gravitational waveforms, we present a stress formula including contributions both from magnetic fields and special relativistic corrections. By changing the precollapse magnetic fields and initial angular momentum distributions parametrically, we compute twelve models. As for the microphysics, a realistic equation of state is employed and the neutrino cooling is taken into account via a multiflavor neutrino leakage scheme. With these computations, we find that the total GW amplitudes show a monotonic increase after bounce for models with a strong precollapse magnetic field ($10^{12}$G) also with a rapid rotation imposed. We show that this trend stems both from the kinetic contribution of MHD outflows with large radial velocities and also from the magnetic contribution dominated by the toroidal magnetic fields that predominantly trigger MHD explosions. For models with weaker initial magnetic fields, the total GW amplitudes after bounce stay almost zero, because the contribution from the magnetic fields cancels with the one from the hydrodynamic counterpart. These features can be clearly understood with a careful analysis on the explosion dynamics. We point out that the GW signals with the increasing trend, possibly visible to the next-generation detectors for a Galactic supernova, would be associated with MHD explosions with the explosion energies exceeding $10^{51}$ erg.

astro-ph.HE↗

arXiv2011

Impacts of Collective Neutrino Oscillations on Supernova Explosions

By performing a series of one- and two-dimensional (1-, 2D) hydrodynamic simulations with spectral neutrino transport, we study possible impacts of collective neutrino oscillations on the dynamics of core-collapse supernovae. To model the spectral swapping which is one of the possible outcome of the collective neutrino oscillations, we parametrize the onset time when the spectral swap begins, the radius where the spectral swap occurs, and the threshold energy above which the spectral interchange between heavy-lepton neutrinos and electron/anti-electron neutrinos takes place, respectively. By doing so, we systematically study how the neutrino heating enhanced by the spectral swapping could affect the shock evolution as well as the matter ejection. We also investigate the progenitor dependence using a suite of progenitor models (13, 15, 20, and 25 $M_\odot$). We find that there is a critical heating rate induced by the spectral swapping to trigger explosions, which significantly differs between the progenitors. The critical heating rate is generally smaller for 2D than 1D due to the multidimensionality that enhances the neutrino heating efficiency. For the progenitors employed in this paper, the final remnant masses are estimated to range in 1.1-1.5$M_\odot$. For our 2D model of the $15M_\odot$ progenitor, we find a set of the oscillation parameters that could account for strong supernova explosions ($\sim 10^{51}$ erg), simultaneously leaving behind the remnant mass close to $\sim 1.4 M_\odot$.

astro-ph.HE↗

arXiv2011

Explosive nucleosynthesis in the neutrino-driven aspherical supernova explosion of a non-rotating 15$M_{\odot}$ star with solar metallicity

We investigate explosive nucleosynthesis in a non-rotating 15$M_\odot$ star with solar metallicity that explodes by a neutrino-heating supernova (SN) mechanism aided by both standing accretion shock instability (SASI) and convection. To trigger explosions in our two-dimensional hydrodynamic simulations, we approximate the neutrino transport with a simple light-bulb scheme and systematically change the neutrino fluxes emitted from the protoneutron star. By a post-processing calculation, we evaluate abundances and masses of the SN ejecta for nuclei with the mass number $\le 70$ employing a large nuclear reaction network. Aspherical abundance distributions, which are observed in nearby core-collapse SN remnants, are obtained for the non-rotating spherically-symmetric progenitor, due to the growth of low-mode SASI. Abundance pattern of the supernova ejecta is similar to that of the solar system for models whose masses ranges $(0.4-0.5) \Ms$ of the ejecta from the inner region ($\le 10,000\km$) of the precollapse core. For the models, the explosion energies and the \nuc{Ni}{56} masses are $ \simeq 10^{51} \rm erg$ and $(0.05-0.06) \Ms$, respectively; their estimated baryonic masses of the neutron star are comparable to the ones observed in neutron-star binaries. These findings may have little uncertainty because most of the ejecta is composed by matter that is heated via the shock wave and has relatively definite abundances. The abundance ratios for Ne, Mg, Si and Fe observed in Cygnus loop are well reproduced with the SN ejecta from an inner region of the $15\Ms$ progenitor.

astro-ph.HE↗

arXiv2011

Effects of Rotation on Stochasticity of Gravitational Waves in Nonlinear Phase of Core-Collapse Supernovae

By performing three-dimensional (3D) simulations that demonstrate the neutrino-driven core-collapse supernovae aided by the standing accretion shock instability (SASI), we study how the spiral modes of the SASI can have impacts on the properties of the gravitational-wave (GW) emission. To see the effects of rotation in the non-linear postbounce phase, we give a uniform rotation on the flow advecting from the outer boundary of the iron core, whose specific angular momentum is assumed to agree with recent stellar evolution models. We compute fifteen 3D models in which the initial angular momentum as well as the input neutrino luminosities from the protoneutron star are changed in a systematic manner. By performing a ray-tracing analysis, we accurately estimate the GW amplitudes generated by anisotropic neutrino emission. Our results show that the gravitational waveforms from neutrinos in models that include rotation exhibit a common feature otherwise they vary much more stochastically in the absence of rotation. The breaking of the stochasticity stems from the excess of the neutrino emission parallel to the spin axis. This is because the compression of matter is more enhanced in the vicinity of the equatorial plane due to the growth of the spiral SASI modes, leading to the formation of spiral flows circulating around the spin axis with higher temperatures. We point out that a recently proposed future space interferometers like Fabry-Perot type DECIGO would permit detection of these signals for a Galactic supernova.

astro-ph.HE↗

arXiv2010

Magnetic Field Decay Due to the Wave-Particle Resonances in the Outer Crust of the Neutron Star

Bearing in mind the application to the outer crust of the neutron stars (NSs), we investigate the magnetic field decay by means of the fully relativistic Particle-In-Cell simulations. Numerical computations are carried out in 2-dimensions, in which the initial magnetic fields are set to be composed both of the uniform magnetic fields that model the global fields penetrating the NS and of the turbulent magnetic fields that would be originated from the Hall cascade of the large-scale turbulence. Our results show that the whistler cascade of the turbulence transports the magnetic energy preferentially in the direction perpendicular to the uniform magnetic fields. It is also found that the distribution function of electrons becomes anisotropic because electrons with lower energies are predominantly heated in the direction parallel to the uniform magnetic fields due to the Landau resonance, while electrons with higher energies are heated mainly by the cyclotron resonance that makes the distribution function isotropic for the high energy tails. Furthermore we point out that the degree of anisotropy takes maximum as a function of the initial turbulent magnetic energy. As an alternative to the conventional ohmic dissipation, we propose that the magnetic fields in the outer crust of NSs, cascading down to the electron inertial scale via the whistler turbulence, would decay predominantly by the dissipation processes through the Landau damping and the cyclotron resonance.

astro-ph.HE↗

arXiv2010

Explosion geometry of a rotating 13 $M_{\odot}$ star driven by the SASI-aided neutrino-heating supernova mechanism

By performing axisymmetric hydrodynamic simulations of core-collapse supernovae with spectral neutrino transport based on the isotropic diffusion source approximation scheme, we support the assumption that the neutrino-heating mechanism aided by the standing accretion shock instability and convection can initiate an explosion of a 13 $M_{\odot}$ star. Our results show that bipolar explosions are more likely to be associated with models which include rotation. We point out that models, which form a north-south symmetric bipolar explosion, can lead to larger explosion energies than for the corresponding unipolar explosions.

astro-ph.HE↗

arXiv2010

General Relativistic Ray-Tracing Method for Estimating the Energy and Momentum Deposition by Neutrino Pair Annihilation in Collapsars

Bearing in mind the application to the collapsar models of gamma-ray bursts (GRBs), we develop a numerical scheme and code for estimating the deposition of energy and momentum due to the neutrino pair annihilation ($ν+ {\bar ν} \rightarrow e^{-} + e^{+}$) in the vicinity of accretion tori around a Kerr black hole. Our code is designed to solve the general relativistic neutrino transfer by a ray-tracing method. To solve the collisional Boltzmann equation in curved spacetime, we numerically integrate the so-called rendering equation along the null geodesics. For the neutrino opacity, the charged-current $β$-processes are taken into account, which are dominant in the vicinity of the accretion tori. The numerical accuracy of the developed code is certificated by several tests, in which we show comparisons with the corresponding analytic solutions. Based on the hydrodynamical data in our collapsar simulation, we estimate the annihilation rates in a post-processing manner. Increasing the Kerr parameter from 0 to 1, it is found that the general relativistic effect can increase the local energy deposition rate by about one order of magnitude, and the net energy deposition rate by several tens of percents. After the accretion disk settles into a stationary state (typically later than $\sim 9$ s from the onset of gravitational collapse), we point out that the neutrino-heating timescale in the vicinity of the polar funnel region can be shorter than the dynamical timescale. Our results suggest the neutrino pair annihilation has a potential importance equal to the conventional magnetohydrodynamic mechanism for igniting the GRB fireballs.

astro-ph.HE↗

arXiv2010

Neutrino Pair Annihilation in Collapsars; Ray-Tracing Method in Special Relativity

We develop a numerical scheme and code for estimating the energy and momentum transfer via neutrino pair annihilation ($ν+ {\bar ν} \to e^{-}+ e^{+}$), bearing in mind the application to the collapsar models of gamma-ray bursts (GRBs). To calculate the neutrino flux illuminated from the accretion disk, we perform a ray-tracing calculation in the framework of special relativity. The numerical accuracy of the developed code is certificated by several tests, in which we show comparisons with the corresponding analytical solutions. Using hydrodynamical data in our collapsar simulation, we estimate the annihilation rates in a post-processing manner. We show that the neutrino energy deposition and momentum transfers are strongest near the inner edge of the accretion disk. The beaming effects of special relativity are found to change the annihilation rates by several factors in the polar funnel region. After the accretion disk settles into a stationary state (typically later than $\sim 9$ s from the onset of gravitational collapse), we find that the neutrino-heating timescale in the vicinity of the polar funnel ($\lesssim 80$ km) can become shorter than the hydrodynamical timescale, indicating that the neutrino-heated outflows can be launched there. We point out that the momentum transfer can play as important role as the energy deposition for the efficient acceleration of neutrino-driven outflows. Our results suggest that the neutrino pair annihilation has a potential importance equal to the conventional magnetohydrodynamic mechanism for igniting the GRB fireballs.

astro-ph.HE↗

arXiv2009

Long-Term Evolution of Collapsar: Mechanism of Outflow Production

We present our numerical results of two-dimensional hydrodynamic (HD) simulations and magnetohydrodynamic (MHD) simulations of the collapse of rotating massive stars in light of the collapsar model of gamma-ray bursts (GRBs). Pushed by recent evolution calculations of GRB progenitors, we focus on lower angular momentum of the central core than the ones taken mostly in previous studies. By performing special relativistic simulations including both realistic equation of state and neutrino cooling, we follow a long-term evolution of the slowly rotating collapsars up to 10 s, accompanied by the formation of jets and accretion disks. We find such outfows can be launched both by MHD process and neutrino process. We investigeate the properties of these jets whether it can become GRBs or remains primary weak outflow.

astro-ph.HE↗