arXiv subjects

Takashi Yoshida

Publications and source records attributed to Takashi Yoshida.

At least 55 records · Page 3Linked to original sources

arXiv2018

Black Hole Formation and Explosion from Rapidly Rotating Very Massive Stars

We explore the formation process of a black hole (BH) through the pair-instability collapse of a rotating Population III very massive star in axisymmetric numerical relativity. As the initial condition, we employ a progenitor star which is obtained by evolving a rapidly rotating zero-age main sequence (ZAMS) star with mass $320M_\odot$ until it reaches a pair instability region. We find that for such rapidly rotating model, a fraction of the mass, $\sim 10M_\odot$, forms a torus surrounding the remnant BH of mass $\sim 130M_\odot$ and an outflow is driven by a hydrodynamical effect. We also perform simulations, artificially reducing the initial angular velocity of the progenitor star, and find that only a small or no torus is formed and no outflow is driven. We discuss the possible evolution scenario of the remnant torus for the rapidly rotating model by considering the viscous and recombination effects and show that if the energy of $\sim 10^{52}$ erg is injected from the torus to the envelope, the luminosity and timescale of the explosion could be of the orders of $10^{43}$ erg/s and yrs, respectively. We also point out the possibility for observing gravitational waves associated with the BH formation for the rapidly rotating model by ground-based gravitational-wave detectors.

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arXiv2018

Neutrino-nucleus reactions on $^{16}$O based on new shell-model Hamiltonians

Neutrino-induced reactions on $^{16}$O are investigated by shell-model calculaions with new shell-model Hamiltonians, which can describe well the structure of $p$-shell and $p$-$sd$ shell nuclei. Distribution of the spin-dipole strengths in $^{16}$O, which give major contributions to the $ν$-$^{16}$O reaction cross sections, is studied with the new Hamiltonians. Muon-capture reaction rates on $^{16}$O are also studied to discuss the quenching of the axial-vector coupling in nuclear medium. Charged-current and neutral-current reaction cross sections are evaluated in various particle and $γ$ emission channels as well as the total ones at neutrino energies up to $E_ν\approx$ 100 MeV. Branching ratios for the various channels are obtained by the Hauser-Feshbach statistical model calculations, and partial cross sections for single- and multi-particle emission channels are evaluated. The cross sections updated are compared with previous continuum random phase approximation (CRPA) calculations. Effects of multi-particle emission channels on nucleosynthesis in core-collapse supernova (SN) explosions are investigated. Inclusion of $α$p emission channels is found to lead to an enhancement of production yields of $^{11}$B and $^{11}$C through $^{16}$O ($ν$, $ν$' $α$p) $^{11}$B and $^{16}$O ($ν$, e$^{-}$ $α$p) $^{11}$C reactions, respectively.

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arXiv2018

Stellar Yields of Rotating First Stars. II. Pair Instability Supernovae and Comparison with Observations

Recent theory predicts that a first star is born with a massive initial mass of $\gtrsim$ 100 $M_\odot$. Pair instability supernova (PISN) is a common fate for such a massive star. Our final goal is to prove the existence of PISN and thus the high mass nature of the initial mass function in the early universe by conducting {\it abundance profiling}, in which properties of a hypothetical first star is constrained by metal-poor star abundances. In order to determine reliable and useful abundances, we investigate the PISN nucleosynthesis taking both rotating and non-rotating progenitors for the first time. We show that the initial and CO core mass ranges for PISNe depend on the envelope structures: non-magnetic rotating models developing inflated envelopes have a lower-shifted CO mass range of $\sim$ 70--125 $M_\odot$, while non-rotating and magnetic rotating models with deflated envelopes have a range of $\sim$ 80--135 $M_\odot$. However, we find no significant difference in explosive yields from rotating and non-rotating progenitors, except for large nitrogen production in non-magnetic rotating models. Furthermore, we conduct the first systematic comparison between theoretical yields and a large sample of metal-poor star abundances. We find that the predicted low [Na/Mg] $\sim$ $-1.5$ and high [Ca/Mg] $\sim$ $0.5$--$1.3$ abundance ratios are the most important to discriminate PISN signatures from normal metal-poor star abundances, and confirm that no currently observed metal-poor star matches with the PISN abundance. Extensive discussion on the non-detection is finally made.

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arXiv2017

Neutrino emissions in all flavors up to the pre-bounce of massive stars and the possibility of their detections

This paper is a sequel to our previous one (Kato et al.2015), which calculated the luminosities and spectra of electron-type anti-neutrinos ($\barν_e$'s) from the progenitors of core-collapse supernovae. Expecting that a capability to detect electron-type neutrinos ($ν_e$'s) will increase dramatically with the emergence of liquid-argon detectors such as DUNE, we broaden the scope in this study to include all-flavors of neutrinos emitted from the pre-bounce phase. We pick up three progenitor models of an electron capture supernova (ECSN) and iron-core collapse supernovae (FeCCSNe). We find that the number luminosities reach $\sim10^{57} \mathrm{s^{-1}}$ and $\sim10^{53} \mathrm{s^{-1}}$ at maximum for $ν_e$ and $\barν_e$, respectively. We also estimate the numbers of detection events at terrestrial neutrino detectors including DUNE, taking flavor oscillations into account and assuming the distance to the progenitors to be 200 pc. It is demonstrated that $\barν_e$'s from the ECSN-progenitor will be undetected at almost all detectors, whereas we will be able to observe $\gtrsim$15900 $ν_e$'s at DUNE for the inverted mass hierarchy. From the FeCCSN-progenitors, the number of $\barν_e$ events will be largest for JUNO, 200-900 $\barν_e$'s, depending on the mass hierarchy whereas the number of $ν_e$ events at DUNE is $\gtrsim$2100 for the inverted mass hierarchy. These results imply that the detection of $\barν_e$'s is useful to distinguish FeCCSN- from ECSN-progenitors, while $ν_e$'s will provide us with detailed information on the collapse phase regardless of the type and mass of progenitor.

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arXiv2017

Explosive nucleosynthesis of ultra-stripped Type Ic supernovae: application to light trans-iron elements

We investigate the explosive nucleosynthesis during two dimensional neutrino-driven explosion of ultra-stripped Type Ic supernovae evolved from 1.45 and 1.5 M$_\odot$ CO stars. These supernovae explode with the explosion energy of $\sim 10^{50}$ erg and release $\sim 0.1$ M$_\odot$ ejecta. The light trans-iron elements Ga-Zr are produced in the neutrino-irradiated ejecta. The abundance distribution of these elements has a large uncertainty because of the uncertainty of the electron fraction of the neutrino-irradiated ejecta. The yield of the elements will be less than 0.01 M$_\odot$. Ultra-stripped supernova and core-collapse supernova evolved from a light CO core can be main sources of the light trans-iron elements. They could also produce neutron-rich nuclei $^{48}$Ca. The ultra-stripped supernovae eject $^{56}$Ni of $\sim$ 0.006 - 0.01 M$_\odot$. If most of neutrino-irradiated ejecta is proton-rich, $^{56}$Ni will be produced more abundantly. The light curves of these supernovae indicate sub-luminous fast decaying explosion with the peak magnitude of about $-15$ - $-16$. Future observations of ultra-stripped supernovae could give a constraint to the event rate of a class of neutron star mergers.

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arXiv2017

Gravitational Collapse of Rotating Supermassive Stars including Nuclear Burning Effects

Supermassive stars (SMSs) of mass $\gtrsim 10^5 M_\odot$ are candidates for seeds of supermassive black holes found in the center of many massive galaxies. We simulate the gravitational collapse of a rigidly rotating SMS core including nuclear burning effects in axisymmetric numerical-relativity simulation. We find that for realistic initial conditions, the nuclear burning does not play an important role. After the collapse, a torus surrounding a rotating black hole is formed and a fraction of the torus material is ejected. We quantitatively study the relation between the properties of these objects and rotation. We find that if a SMS core is sufficiently rapidly rotating, the torus and outflow mass have approximately $6\%$ and $1\%$ of the initial mass, respectively. The typical average velocity and the total kinetic energy of the outflow are $0.2~c$ and $10^{54-56}$ erg where $c$ is the speed of light. Finally, we briefly discuss the possibility for observing the torus and outflow.

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arXiv2016

Presupernova neutrino events relating to the final evolution of massive stars

When a supernova explosion occurs in neighbors around hundreds pc, current and future neutrino detectors are expected to observe neutrinos from the presupernova star before the explosion. We show a possibility for obtaining the evidence for burning processes in the central region of presupernova stars though the observations of neutrino signals by current and future neutrino detectors such as KamLAND, JUNO, and Hyper-Kamiokande. We also investigate supernova alarms using neutrinos from presupernova stars in neighbors. If a supernova explodes at ~ 200 pc, future 20 kton size liquid scintillation detectors are expected to observe hundreds neutrino events. We also propose a possibility of the detection of neutrino events by Gd-loaded Hyper-Kamiokande using delayed $γ$-ray signals. These detectors could observe detailed time variation of neutrino events. The neutrino emission rate increases by the core contraction in the final evolution stage. However, the O and Si shell burnings suppress the neutrino emission for a moment. The observed decrease in the neutrino event rate before hours to the explosion is possibly evidence for the shell burnings. The observations of detailed time evolution of presupernova neutrino events could reveal properties of burning processes in the central region of presupernova stars.

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arXiv2016

Mass ejection by pulsational pair instability in very massive stars and implications for luminous supernovae

Massive stars having a CO core of $\sim$40-60 M$_\odot$ experience pulsational pair-instability (PPI) after carbon-burning. This instability induces strong pulsations of the whole star and a part of outer envelope is ejected. We investigate the evolution and mass ejection of metal-poor very massive stars which experience PPI. We use stellar models with initial masses of 140, 200, and 250 M$_\odot$ and the metallicity Z=0.004. Their masses decrease to 54.09, 58.65, and 61.03 M$_\odot$ before the neon-burning owing to mass-loss and He mass fraction at the surface becomes about 20%. During the PPI period of $\sim$1-2000 yr, they experience six, four, and three pulsations, respectively. The larger CO-core model has the longer PPI period and ejects the larger amount of mass. Since almost all surface He has been lost by the pulsations, these stars become Type Ic supernovae if they explode. Light curves during the PPI stage and supernovae are investigated and are implicated in luminous supernovae. The luminosity created by the interaction of different PPI ejecta becomes $M_{\rm bol} \sim -16$ to $-20$. The interaction between the circumstellar shell ejected by PPI and the supernova ejecta can be more luminous. These luminous transients could be an origin of Type I superluminous supernovae and supernovae with precursor.

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arXiv2016

Alpha-constrained QSE Nucleosynthesis in High-entropy and Fast-expanding Material

We investigate the nucleosynthesis process in high-entropy ($s/k_{\rm B}\gtrsim100$) and very fast-expanding ($τ_{\rm exp}\sim10^{-3}\ {\rm s}$) materials. In such a material with the electron fraction near 0.5, an interesting nucleosynthesis process occurs. In this process, the abundance distribution of heavy-nuclei of $A>100$ achieve quasi-statistical equilibrium (QSE) at high temperature and the abundances are frozen at the end of the nucleosynthesis. We explain this abundance distribution using the "alpha-constrained QSE" abundances formulated in this paper. We demonstrate that this nucleosynthesis would occur in neutrino-driven winds from massive proto-neutron stars in hypernovae, where $A\sim140$ $p$-nuclei are synthesized.

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arXiv2015

Exact and approximate expressions of energy generation rates and their impact on the explosion properties of Pair Instability Supernovae

Energetics of nuclear reaction is fundamentally important to understand the mechanism of pair instability supernovae (PISNe). Based on the hydrodynamic equations and thermodynamic relations, we derive exact expressions for energy conservation suitable to be solved in simulation. We also show that some formulae commonly used in the literature are obtained as approximations of the exact expressions. We simulate the evolution of very massive stars of ~100-320 Msun with zero- and 1/10 Zsun, and calculate further explosions as PISNe, applying each of the exact and approximate formulae. The calculations demonstrate that the explosion properties of PISN, such as the mass range, the 56Ni yield, and the explosion energy, are significantly affected by applying the different energy generation rates. We discuss how these results affect the estimate of the PISN detection rate, which depends on the theoretical predictions of such explosion properties.

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arXiv2015

Neutrino-driven explosions of ultra-stripped type Ic supernovae generating binary neutron stars

We study explosion characteristics of ultra-stripped supernovae (SNe), which are candidates of SNe generating binary neutron stars (NSs). As a first step, we perform stellar evolutionary simulations of bare carbon-oxygen cores of mass from 1.45 to 2.0 $M_\odot$ until the iron cores become unstable and start collapsing. We then perform axisymmetric hydrodynamics simulations with spectral neutrino transport using these stellar evolution outcomes as initial conditions. All models exhibit successful explosions driven by neutrino heating. The diagnostic explosion energy, ejecta mass, Ni mass, and NS mass are typically $\sim 10^{50}$ erg, $\sim 0.1 M_\odot$, $\sim 0.01M_\odot$, and $\approx 1.3 M_\odot$, which are compatible with observations of rapidly-evolving and luminous transient such as SN 2005ek. We also find that the ultra-stripped SN is a candidate for producing the secondary low-mass NS in the observed compact binary NSs like PSR J0737-3039.

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arXiv2015

Nucleosynthesis in neutrino-driven winds in hypernovae

We investigate the nucleosynthesis in the neutrino-driven winds blown off from a $3M_\odot$ massive proto-neutron star (mPNS) temporarily formed during the collapse of a $100M_\odot$ star. Such mPNSs would be formed in hypernovae. We construct steady and spherically symmetric wind solutions. We set large neutrino luminosities of $\sim 10^{53}\ {\rm erg\ s^{-1}}$ and average energies of electron neutrinos and antineutrinos in the ranges of $ε_{ν_e}=9-16\ {\rm MeV}$ and $ε_{\barν_e}=11-18\ {\rm MeV}$ based on a recent numerical relativity simulation. The wind solutions indicate much shorter temperature-decrease timescale than that of the winds from ordinary PNSs and, depending on $ε_ν$, the winds can be both neutron-rich and proton-rich. In the neutron-rich wind, the $r$-process occurs and the abundance distribution of a fiducial wind model of the mPNS gives an approximate agreement with the abundance pattern of metal-poor weak $r$ star HD~122563, although the third-peak elements are produced only when the $\barν_e$ energy is much larger than the $ν_e$ energy. In the proton-rich wind, the strong $νp$-process occurs and $A>100$ nuclides are synthesized. The synthesized nuclei can be neutron-rich in some cases because the large neutrino luminosity of the mPNS supplies a sufficient amount of neutrons.

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arXiv2015

Pre-supernova neutrino emissions from ONe cores in the progenitors of core-collapse supernovae: are they distinguishable from those of Fe cores?

Aiming to distinguish two types of progenitors of core collapse supernovae, i.e., one with a core composed mainly of oxygen and neon (abbreviated as ONe core) and the other with an iron core (or Fe core), we calculated the luminosities and spectra of neutrinos emitted from these cores prior to gravitational collapse, taking neutrino oscillation into account. We found that the total energies emitted as $\barν_e$ from the ONe core are $\lesssim 10^{46}\ {\rm erg}$, which is much smaller than $\sim 10^{47}\ {\rm erg}$ for Fe cores. The average energy, on the other hand, is twice as large for the ONe core as those for the Fe cores. The neutrinos produced by the plasmon decays in the ONe core are more numerous than those from the electron-positron annihilation in both cores but they have much lower average energies $\lesssim 1\ {\rm MeV}$. Although it is difficult to detect the pre-supernova neutrinos from the ONe core even if it is located within 200$\ $pc from the earth, we expect $ \sim 9 - 43$ and $\sim 7 - 61$ events for Fe cores at KamLAND and Super-Kamiokande, respectively, depending on the progenitor mass and neutrino-mass hierarchy. These numbers might be increased by an order of magnitude if we envisage next-generation detectors such as JUNO. We will hence be able to distinguish the two types of progenitors by the detection or non-detection of the pre-supernova neutrinos if they are close enough ($\lesssim1\ {\rm kpc}$).

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arXiv2014

Stellar Yields of Rotating First Stars. I. Yields of Weak Supernovae and Abundances of Carbon-enhanced Hyper Metal Poor Stars

We perform stellar evolution simulation of first stars and calculate stellar yields from the first supernovae. The initial masses are taken from 12 to 140 Msun to cover the whole range of core-collapse supernova progenitors, and stellar rotation is included, which results in efficient internal mixing. A weak explosion is assumed in supernova yield calculations, thus only outer distributed matter, which is not affected by the explosive nucleosynthesis, is ejected in the models. We show that the initial mass and the rotation affect the explosion yield. All the weak explosion models have abundances of [C/O] larger than unity. Stellar yields from massive progenitors of > 40-60 Msun show enhancement of Mg and Si. Rotating models yield abundant Na and Al. And Ca is synthesized in non-rotating heavy massive models of > 80 Msun. We fit the stellar yields to the three most iron-deficient stars, and constrain the initial parameters of the mother progenitor stars. The abundance pattern in SMSS 0313-6708 is well explained by 50-80 Msun non-rotating models, rotating 30-40 Msun models well fit the abundance of HE 0107-5240, and both non-rotating and rotating 15-40 Msun models explain HE 1327-2326. The presented analysis will be applicable to other carbon-enhanced hyper metal poor stars observed in the future. The abundance analyses will give valuable information about the characteristics about the first stars.

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arXiv2013

Type Ic core-collapse supernova explosions evolved from very massive stars

We investigate the possibility of a super-luminous Type Ic core-collapse supernovae producing a large amount of 56Ni. Very massive stars with a main-sequence mass larger than 100 Msun and a metallicity 0.001 < Z < 0.004 are expected to explode as super-luminous Type Ic supernovae. Stars with ~ 110 - 150 Msun and Z < 0.001 would explode as Type Ic pulsational pair-instability supernovae if the whole H and He layers has been lost by the mass loss during pulsational pair-instability. We evaluate the total ejecta mass and the yields of 56Ni, O, and Si in core-collapse supernovae evolved from very massive stars. We adopt 43.1 and 61.1 Msun WO stars with Z=0.004 as supernova progenitors expected to explode as Type Ic core-collapse supernovae. These progenitors have masses of 110 and 250 Msun at the zero-age main sequence. Spherical explosions with an explosion energy larger than 2e52 erg produce more than 3.5 Msun 56Ni, enough to reproduce the light curve of SN 2007bi. Asphericity of the explosion affects the total ejecta mass as well as the yields of 56Ni, O, and Si. Aspherical explosions of the 110 and 250 Msun models reproduce the 56Ni yield of SN 2007bi. These explosions will also show large velocity dispersion. An aspherical core-collapse supernova evolved from a very massive star is a possibility of the explosion of SN 2007bi.

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arXiv2013

Evolution of progenitors for electron capture supernovae

We provide progenitor models for electron capture supernovae (ECSNe) with detailed evolutionary calculation. We include minor electron capture nuclei using a large nuclear reaction network with updated reaction rates. For electron captures, the Coulomb correction on the rates is treated and contribution of neutron-rich nuclei is taken into account in the nuclear statistical equilibrium (NSE) composition. We calculate the evolution of the most massive super asymptotic giant branch stars and show that these stars undergo off-center carbon burnings and form ONe cores at the center. These cores get heavier up to the critical mass of 1.367 Msun and keep contracting even after the initiation of O+Ne deflagration. Though inclusion of minor electron capture nuclei causes convective URCA process at the contraction phase, such process will have minor effect on the evolution. On the other hand, electron captures by neutron-rich nuclei in the NSE region have more significant effect. Also, we discuss the uniqueness of the critical core mass for ECSNe and the effect of mass loss on the plausibility of our models for ECSN progenitors.

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arXiv2012

Massive Star Evolution and Nucleosynthesis -Lower End of Fe-Core Collapse Supernova Progenitors and Remnant Neutron Star Mass Distribution-

In order to explore various aspects of stellar evolution, supernovae, gamma ray bursts and nucleosynthesis, we have developed a new efficient stellar evolution code. In this paper we describe this new code and compare the results with the ones calculated by the previous code. Specifically we focus on the progenitor evolution of lower end of the Fe-core collapse supernovae, and mass distribution of remnant neutron stars. We describe how different assumptions will lead different neutron star mass distribution. We also review recent works of our research group.

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arXiv2011

Beta Decays of Isotones with Neutron Magic Number of N=126 and R-process Nucleosynthesis

Beta decays of the isotones with N=126 are studied by shell model calculations taking into account both the Gamow-Teller (GT) and first-forbidden (FF) transitions. The FF transitions are found to be important to reduce the half-lives, by nearly twice to several times, from those by the GT contributions only. Possible implications of the short half-lives of the waiting point nuclei on the r-process nucleosynthesis during the supernova explosions are discussed. A slight shift of the third peak of the element abundances in the r-process toward higher mass region is found.

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