arXiv subjects

Ko Nakamura

Publications and source records attributed to Ko Nakamura.

At least 19 recordsLinked to original sources

arXiv2026

Rotation-induced Relaxation of Supernova Constraints on Axionlike Particles

We study how rotation modifies the constraints on MeV-scale axion-like particles (ALPs) coupled to photons derived from SN 1987A. We constrain the ALP parameter space based on both the energy-loss argument and the gamma-ray limits, and examine how these constraints are affected by stellar rotation. Adopting initial angular velocities of ${\Omega}_{0} = 0.0 and 1.0 rad s^{-1}$ in the iron core, we carry out two-dimensional core-collapse supernova simulations for three progenitor models - a $14 + 9M_{\odot}$ binary and $13M_{\odot}$ and $18M_{\odot}$ single stars with solar metallicity - and estimate ALP emission rates through post-processing. We find that rotation suppresses ALP emission by reducing the core temperature via centrifugal support. Rotation also reduces the neutrino luminosity, but the suppression of ALP emission is more effective, leading to relaxed constraints within a simplified criterion based on the energy-loss argument. This relaxation is particularly pronounced in the rotating $18M_{\odot}$ model, where a substantial decrease in the central temperature occurs at $t_{pb} = 0.8 - 1 s$. In this simplified criterion, such rapid temporal variations in temperature indicate that the resulting constraints depend sensitively on both the evaluation time and the underlying supernova model. For a gamma-ray limit from the SN 1987A observation, rotation has a negligible impact on the constraint. This is because the ALP-induced gamma-ray fluence observed at Earth is proportional to the fourth power of the ALP-photon coupling constant, making the constraint relatively insensitive to the rotational suppression of ALP emission.

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arXiv2025

Inhomogeneous stellar mixing in the final hours before the Cassiopeia A supernova

Understanding stars and their evolution is a key goal of astronomical research and has long been a focus of human interest. In recent years, theorists have paid much attention to the final interior processes within massive stars, as they can be essential for revealing neutrino-driven supernova mechanisms and other potential transients of massive star collapse. However, it is challenging to observe directly the last hours of a massive star before explosion, since it is the supernova event that triggers the start of intense observational study. Here we report evidence for a final phase of stellar activity known as a ``shell merger'', an intense shell burning in which the O-burning shell swallows its outer C-/Ne-burning shell, deep within the progenitor's interior moments before the supernova explosion. In the violent convective layer created by the shell merger, Ne, which is abundant in the stellar O-rich layer, is burned as it is pulled inward, and Si, which is synthesized inside, is transported outward. The remnant still preserves some traces of such Ne-rich downflows and Si-rich upflows in the O-rich layer, suggesting that inhomogeneous shell-merger mixing began just hours ($\lesssim 10^4$ s) before its gravitational collapse. Our results provide the first observational evidence that the final stellar burning process rapidly alters the internal structure, leaving a pre-supernova asymmetry. This breaking of spherical symmetry facilitates the explosion of massive stars and influences various supernova and remnant characteristics, including explosion asymmetries and the neutron star's kick and spin.

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arXiv2025

Progenitor Dependence of Neutrino-driven Supernova Explosions with the Aid of Heavy Axion-like Particles

We perform spherically symmetric simulations of core-collapse supernovae with the aid of heavy axion-like particles (ALPs) which interact with photons and redistribute energy within supernova matter. We explore a wide ALP parameter space that includes MeV-scale ALP mass $m_{\,a}$ and the ALP-photon coupling constant $g_{\,a \gamma} \sim 10^{\,-10} \, \rm{GeV}^{\,-1}$ , employing three progenitor models with zero-age main-sequence mass of $11.2\,M_\odot$, $20.0\,M_\odot$, and $25.0\,M_\odot$. We find a general trend that, given $m_{\,a}\lesssim 300\,$MeV, heavier ALPs are favorable for the shock wave to be successfully revived, aiding the onset of the neutrino-driven explosion. However, if ALPs are heavier than $\sim 400\,$MeV, the explosion is failed or weaker than that for the models with smaller $m_{\,a}$, because of an insufficient temperature inside the supernova core to produce heavy ALPs. The maximum temperature in the core depends on the initial progenitor structure. Our simulations indicate that the high-temperature environment in the collapsing core of massive progenitors leads to a significant impact of ALPs on the explodability.

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arXiv2024

Gravitational-Wave Signatures of Nonstandard Neutrino Properties in Collapsing Stellar Cores

We present a novel multi-messenger approach for probing nonstandard neutrino properties through the detection of gravitational waves (GWs) from collapsing stellar cores and associated supernova explosions. We show that neutrino flavor conversion inside the proto-neutron star (PNS), motivated by physics Beyond the Standard Model (BSM), can significantly boost PNS convection. This effect leads to large-amplitude GW emission over a wide frequency range during an otherwise relatively quiescent GW phase shortly after core bounce. Such a signal provides a promising new avenue for exploring nonstandard neutrino phenomena and other BSM physics impacting PNS convection.

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arXiv2024

Three-dimensional Magneto-hydrodynamic Simulations of Core-collapse Supernovae: I. Hydrodynamic evolution and protoneutron star properties

We present results from three-dimensional, magnetohydrodynamic, core-collapse simulations of sixteen progenitors following until 0.5 s after bounce. We use non-rotating solar-metallicity progenitor models with zero-age main-sequence mass between 9 and 24 $M_{\odot}$. The examined progenitors cover a wide range of the compactness parameter including a peak around $23 M_{\odot}$. We find that neutrino-driven explosions occur for all models within 0.3 s after bounce. We also find that the properties of the explosions and the central remnants are well correlated with the compactness. Early shock evolution is sensitive to the mass accretion rate onto the central core, reflecting the density profile of the progenitor stars. The most powerful explosions with diagnostic explosion energy $E_{\rm exp} \sim 0.75 \times 10^{51}$ erg are obtained by 23 and 24 $M_{\odot}$ models, which have the highest compactness among the examined models. These two models exhibit spiral SASI motions during 150-230 ms after bounce preceding a runaway shock expansion and leave a rapidly rotating neutron star with spin periods $\sim 50$ ms. Our models predict the gravitational masses of the neutron star ranging between $1.22 M_{\odot}$ and $1.67 M_{\odot}$ and their spin periods 0.04-4 s. The number distribution of these values roughly matches observation. On the other hand, our models predict small hydrodynamic kick velocity (15-260 km/s), although they are still growing at the end of our simulations. Further systematic studies, including rotation and binary effects, as well as long-term simulations up to several seconds, will enable us to explore the origin of various core-collapse supernova explosions.

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arXiv2022

Three-dimensional simulation of a core-collapse supernova for a binary star progenitor of SN 1987A

We present results from a self-consistent, non-rotating core-collapse supernova simulation in three spatial dimensions using a binary evolution progenitor model of SN 1987A by Urushibata et al. (2018). This 18.3 solar-mass progenitor model is evolved from a slow-merger of 14 and 8 solar-mass stars, and it satisfies most of the observational constraints such as red-to-blue evolution, lifetime, total mass and position in the Hertzsprung-Russell diagram at collapse, and chemical anomalies. Our simulation is initiated from a spherically symmetric collapse and mapped to the three-dimensional coordinates at 10 ms after bounce to follow the non-spherical hydrodynamics evolution. We obtain the neutrino-driven shock revival for this progenitor at 350 ms after bounce, leading to the formation of a newly-born neutron star with average gravitational mass of 1.35 solar mass and spin period of 0.1 s. We also discuss the detectability of gravitational wave and neutrino signals for a Galactic event with the same characteristics as SN 1987A. At our final simulation time (660 ms postbounce), the diagnostic explosion energy, though still growing, is smaller (0.15 foe) compared to the observed value (1.5 foe). The 56Ni mass obtained from the simulation (0.01 solar mass) is also smaller than the reported mass from SN 1987A (0.07 solar mass). Long-term simulation including several missing physical ingredients in our 3D models such as rotation, magnetic fields, or more elaborate neutrino opacity should be done to bridge the gap between the theoretical predictions and the observed values.

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arXiv2021

SNEWPY: A Data Pipeline from Supernova Simulations to Neutrino Signals

Current neutrino detectors will observe hundreds to thousands of neutrinos from a Galactic supernovae, and future detectors will increase this yield by an order of magnitude or more. With such a data set comes the potential for a huge increase in our understanding of the explosions of massive stars, nuclear physics under extreme conditions, and the properties of the neutrino. However, there is currently a large gap between supernova simulations and the corresponding signals in neutrino detectors, which will make any comparison between theory and observation very difficult. SNEWPY is an open-source software package which bridges this gap. The SNEWPY code can interface with supernova simulation data to generate from the model either a time series of neutrino spectral fluences at Earth, or the total time-integrated spectral fluence. Data from several hundred simulations of core-collapse, thermonuclear, and pair-instability supernovae is included in the package. This output may then be used by an event generator such as sntools or an event rate calculator such as SNOwGLoBES. Additional routines in the SNEWPY package automate the processing of the generated data through the SNOwGLoBES software and collate its output into the observable channels of each detector. In this paper we describe the contents of the package, the physics behind SNEWPY, the organization of the code, and provide examples of how to make use of its capabilities.

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arXiv2021

A three-dimensional hydrodynamics simulation of oxygen-shell burning in the final evolution of a fast-rotating massive star

We perform for the first time a 3D hydrodynamics simulation of the evolution of the last minutes pre-collapse of the oxygen shell of a fast-rotating massive star. This star has an initial mass of 38 M$_\odot$, a metallicity of $\sim$1/50 Z$_\odot$, an initial rotational velocity of 600 km s$^{-1}$, and experiences chemically homogeneous evolution. It has a silicon- and oxygen-rich (Si/O) convective layer at (4.7-17)$\times 10^{8}$ cm, where oxygen-shell burning takes place. The power spectrum analysis of the turbulent velocity indicates the dominance of the large-scale mode ($\ell \sim 3$), which has also been seen in non-rotating stars that have a wide Si/O layer. Spiral arm structures of density and silicon-enriched material produced by oxygen-shell burning appear in the equatorial plane of the Si/O shell. Non-axisymmetric, large-scale ($m \le 3$) modes are dominant in these structures. The spiral arm structures have not been identified in previous non-rotating 3D pre-supernova models. Governed by such a convection pattern, the angle-averaged specific angular momentum becomes constant in the Si/O convective layer, which is not considered in spherically symmetrical stellar evolution models. Such spiral arms and constant specific angular momentum might affect the ensuing explosion or implosion of the star.

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arXiv2021

Three-dimensional Hydrodynamics Simulations of Precollapse Shell Burning in the Si- and O-rich Layers

We present 3D hydrodynamics simulations of shell burning in two progenitors with zero-age main-sequence masses of 22 and 27 $M_{\odot}$ for $\sim$65 and 200 s up to the onset of gravitational collapse, respectively. The 22 and 27 $M_{\odot}$ stars are selected from a suite of 1D progenitors. The former and the latter have an extended Si- and O-rich layer with a width of $\sim$10$^9$ cm and $\sim$5$\times 10^9$ cm, respectively. Our 3D results show that turbulent mixing occurs in both of the progenitors with the angle-averaged turbulent Mach number exceeding $\sim$0.1 at the maximum. We observe that an episodic burning of O and Ne, which takes place underneath the convection bases, enhances the turbulent mixing in the 22 and 27 $M_\odot$ models, respectively. The distribution of nucleosynthetic yields is significantly different from that in 1D simulations, namely, in 3D more homogeneous and inhomogeneous in the radial and angular direction, respectively. By performing a spectrum analysis, we investigate the growth of turbulence and its role of material mixing in the convective layers. We also present a scalar spherical harmonics mode analysis of the turbulent Mach number. This analytical formula would be helpful for supernova modelers to implement the precollapse perturbations in core-collapse supernova simulations. Based on the results, we discuss implications for the possible onset of the perturbation-aided neutrino-driven supernova explosion.

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arXiv2019

Long-term Simulations of Multi-Dimensional Core-collapse Supernovae: Implications for Neutron Star Kicks

Core-collapse supernovae (CCSNe) are the final stage of massive stars, marking the birth of neutron stars (NSs). The aspherical mass ejection drives a natal kick of the forming NS. In this work we study the properties of the NS kick based on our long-term hydrodynamics CCSN simulations. We perform two-dimensional (2D) simulations for ten progenitors from a 10.8 to 20 $\, M_{\odot}$ star covering a wide range of the progenitor's compactness parameter, and two three-dimensional (3D) simulations for an 11.2 $\, M_{\odot}$ star. Our 2D models present a variety of explosion energies between $\sim 1.3 \times 10^{50}$ erg and $\sim 1.2 \times 10^{51}$ erg, and NS kick velocities between $\sim 100$ km s$^{-1}$ and $\sim 1500$ km s$^{-1}$. For the 2D exploding models, we find that the kick velocities tend to become higher with the progenitor's compactness. This is because the high progenitor compactness results in high neutrino luminosity from the proto-neutron star (PNS), leading to more energetic explosions. Since high-compactness progenitors produce massive PNSs, we point out that the NS masses and the kick velocities can be correlated, which is moderately supported by observation. Comparing 2D and 3D models of the 11.2 $\, M_{\odot}$ star, the diagnostic explosion energy in 3D is, as previously identified, higher than that in 2D, whereas the 3D model results in a smaller asymmetry in the ejecta distribution and a smaller kick velocity than in 2D. Our results confirm the importance of self-consistent CCSN modeling covering a long-term postbounce evolution in 3D for a quantitative prediction of the NS kicks.

astro-ph.HE↗

arXiv2019

One-, Two-, and Three-dimensional Simulations of Oxygen Shell Burning Just Before the Core-Collapse of Massive Stars

We perform two- (2D) and three-dimensional (3D) hydrodynamics simulations of convective oxygen shell-burning that takes place deep inside a massive progenitor star of a core-collapse supernova. Using one dimensional (1D) stellar evolution code, we first calculate the evolution of massive stars with an initial mass of 9-40 $M_\odot$. Four different overshoot parameters are applied, and CO core mass trend similar to previous works is obtained in the 1D models. Selecting eleven 1D models that have a silicon and oxygen coexisting layer, we perform 2D hydrodynamics simulations of the evolution $\sim$100 s until the onset of core-collapse. We find that convection with large-scale eddies and the turbulent Mach number $\sim$0.1 is obtained in the models having a Si/O layer with a scale of 10$^8$ cm, whereas most models that have an extended O/Si layer up to a few $\times 10^9$ cm exhibit lower turbulent velocity. Our results indicate that the supernova progenitors that possess a thick Si/O layer could provide a preferable condition for perturbation-aided explosions. We perform 3D simulation of a 25 $M_\odot$ model, which exhibits large-scale convection in the 2D models. The 3D model develops large ($\ell = 2$) convection similar to the 2D model, however, the turbulent velocity is lower. By estimating the neutrino emission properties of the 3D model, we point out that a time modulation of the event rates, if observed in KamLAND and Hyper-Kamiokande, would provide an important information about structural changes in the presupernova convective layer.

astro-ph.SR↗

arXiv2019

Neutrino self-interaction and MSW effects on the supernova neutrino-process

We calculate the abundances of $^{7}$Li, $^{11}$B, $^{92}$Nb, $^{98}$Tc, $^{138}$La, and $^{180}$Ta produced by neutrino $(\nu)$ induced reactions in a core-collapse supernova explosion. We consider the modification by $\nu$ self-interaction ($\nu$-SI) near the neutrinosphere and the Mikheyev-Smirnov-Wolfenstein effect in outer layers for time-dependent neutrino energy spectra. Abundances of $^{7}$Li and heavy isotopes $^{92}$Nb, $^{98}$Tc and $^{138}$La are reduced by a factor of 1.5-2.0 by the $\nu$-SI. In contrast, $^{11}$B is relatively insensitive to the $\nu$-SI. We find that the abundance ratio of heavy to light nucleus, $^{138}$La/$^{11}$B, is sensitive to the neutrino mass hierarchy, and the normal mass hierarchy is more likely to be consistent with the solar abundances.

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arXiv2018

Progenitor Mass Distribution of Core-Collapse Supernova Remnants in Our Galaxy and Magellanic Clouds based on Elemental Abundances

We investigate a progenitor mass distribution of core-collapse supernova remnants (CCSNRs) in our Galaxy and the Large and Small Magellanic Clouds, for the first time. We count the number of CCSNRs in three mass ranges divided by the zero-age main-sequence mass, $M_{\rm ZAMS}$; A: $M_{\rm ZAMS} < 15\ {\rm M}_\odot$, B: $15\ {\rm M}_\odot < M_{\rm ZAMS} < 22.5\ {\rm M}_\odot$, C: $M_{\rm ZAMS} > 22.5\ {\rm M}_\odot$. Simple compilation of progenitor masses in the literature yields a progenitor mass distribution of $f_{\rm A}: f_{\rm B}: f_{\rm C} =0.24:0.28:0.48$, where $f$ is the number fraction of the progenitors. The distribution is inconsistent with any standard initial mass functions. We notice, however, that previous mass estimates are subject to large systematic uncertainties because most of the relative abundances (X/Si) are not really good probe for the progenitor masses. Instead, we propose to rely only on the Fe/Si ratio which is sensitive to the CO core mass ($M_{\rm COcore}$) and $M_{\rm ZAMS}$. Comparing Fe/Si ratios in SNRs in the literature with the newest theoretical model, we estimate 33 $M_{\rm COcore}$ and $M_{\rm ZAMS}$, leading to a revised progenitor mass distribution of $f_{\rm A}: f_{\rm B}: f_{\rm C} = 0.47: 0.32 : 0.21$. This is consistent with the standard Salpeter initial mass function. However, the relation between $M_{\rm COcore}$ and $M_{\rm ZAMS}$ could be affected by binary evolution, which is not taken into account in this study and should be considered in the future work to derive a better progenitor mass distribution estimate.

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arXiv2018

Diagnosing the Structure of Massive Stars with Galactic Supernova Neutrinos

It has been suggested that whether a star explodes or not, and what kind of explosion properties it shows, is strongly dependent on the progenitor's core structure. We present the results from 101 axisymmetric core-collapse supernova simulations performed with progenitors spanning initial masses in the range from 10.8 to 75 solar masses, and focus on their connections to the compactness of the progenitor's core. Our simulations confirm a correlation between the neutrinos emitted during the accretion phase and the progenitor's compactness. We suggest that the ratio of observed neutrino events during the first hundreds of milliseconds can be used to infer the progenitor's inner mass density structure.

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arXiv2018

Intermediate-Mass-Elements in Young Supernova Remnants Reveal Neutron Star Kicks by Asymmetric Explosions

The birth properties of neutron stars yield important information on the still debated physical processes that trigger the explosion and on intrinsic neutron-star physics. These properties include the high space velocities of young neutron stars with average values of several 100 km/s, whose underlying "kick" mechanism is not finally clarified. There are two competing possibilities that could accelerate NSs during their birth: anisotropic ejection of either stellar debris or neutrinos. We here present new evidence from X-ray measurements that chemical elements between silicon and calcium in six young gaseous supernova remnants are preferentially expelled opposite to the direction of neutron star motion. There is no correlation between the kick velocities and magnetic field strengths of these neutron stars. Our results support a hydrodynamic origin of neutron-star kicks connected to asymmetric explosive mass ejection, and they conflict with neutron-star acceleration scenarios that invoke anisotropic neutrino emission caused by particle and nuclear physics in combination with very strong neutron-star magnetic fields.

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arXiv2018

Impact of Neutrino Opacities on Core-Collapse Supernova Simulations

Accurate description of neutrino opacities is central both to the core-collapse supernova (CCSN) phenomenon and to the validity of the explosion mechanism itself. In this work, we study in a systematic fashion the role of a variety of well-selected neutrino opacities in CCSN simulations where multi-energy, three-flavor neutrino transport is solved by the isotropic diffusion source approximation (IDSA) scheme. To verify our code, we first present results from one-dimensional (1D) simulations following core-collapse, bounce, and up to ~ 250 ms postbounce of a 15 $M_{\odot}$ star using a standard set of neutrino opacities by Bruenn (1985). Detailed comparison with published results supports the reliability of our three-flavor IDSA scheme using the standard opacity set. We then investigate in 1D simulations how the individual opacity update leads to the difference from the base-line run with the standard opacity set. By making a detailed comparison with previous work, we check the validity of our implementation of each update in a step-by-step manner. Individual neutrino opacities with the largest impact on the overall evolution in 1D simulations are selected for a systematic comparison in our two-dimensional (2D) simulations. Special emphasis is devoted to the criterion of explodability in the 2D models. We discuss the implications of these results as well as the limitations and requirements for future towards more elaborate CCSN modeling.

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arXiv2018

Diffuse Supernova Neutrino Background from extensive core-collapse simulations of $8$-$100 {\rm M}_\odot$ progenitors

We revisit the diffuse supernova neutrino background in light of recent systematic studies of stellar core collapse that reveal the quantitative impacts of the progenitor conditions on the collapse process. In general, the dependence of the progenitor on the core-collapse neutrino emission is not monotonic in progenitor initial mass, but we show that it can, at first order, be characterized by the core compactness. For the first time, we incorporate the detailed variations in the neutrino emission over the entire mass range $8$-$100 {\rm M}_\odot$, based on (i) a long-term simulation of the core collapse of a $8.8 {\rm M}_\odot$ O-Ne-Mg core progenitor, (ii) over 100 simulations of iron core collapse to neutron stars, and (iii) half a dozen simulations of core collapse to black holes (the "failed channel"). The fraction of massive stars that undergo the failed channel remains uncertain, but in view of recent simulations which reveal high compactness to be conducive to collapse to black holes, we characterize the failed fraction by considering a threshold compactness above which massive stars collapse to black holes and below which the final remnant is a neutron star. We predict that future detections of the diffuse supernova neutrino background may have the power to reveal this threshold compactness, if its value is relatively small as suggested by interpretations of several recent astronomical observations.

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arXiv2017

Estimating the core compactness of massive stars with Galactic supernova neutrinos

We suggest the future detection of neutrinos from a Galactic core-collapse supernova can be used to infer the progenitor's inner mass density structure. We present the results from 20 axisymmetric core-collapse supernova simulations performed with progenitors spanning initial masses in the range 11--30Msun, and focus on their connections to the progenitor compactness. The compactness is a measure of the mass density profile of the progenitor core and recent investigations have suggested its salient connections to the outcomes of core collapse. Our simulations confirm a correlation between the neutrinos emitted during the accretion phase and the progenitor's compactness, and that the ratio of observed neutrino events during the first hundreds of milliseconds provides a promising handle on the progenitor's inner structure. Neutrino flavor mixing during the accretion phase remains a large source of uncertainty.

astro-ph.HE↗