arXiv subjects

T. Ohkubo

Publications and source records attributed to T. Ohkubo.

9 recordsLinked to original sources

arXiv2022

Finite element method modeling of expansion of irradiated rocks: focusing on the minerals

Finite element method (FEM) modeling of the volumetric expansion phenomenon associated with the accumulation of irradiation was performed on rocks in a concrete for nuclear power plant. The FEM mesh of sandstone, tuff, and granite was created based on microscopic images, and the volumetric expansion due to irradiation was calculated. The saturated value of the volume expansion due to irradiation accumulation was calculated, and it was shown that the saturated value of the volume expansion was well explained by the experimental value. In addition, the FEM analysis indicates that irradiation-induced degradation of rock propagates through a localized concentration of stress followed by fracture with cracks spreading throughout the rock.

physics.geo-ph↗

arXiv2021

Observation of Nonlinear Spin-Charge Conversion in the Thin Film of Nominally Centrosymmetric Dirac Semimetal SrIrO3 at Room Temperature

Spin-charge conversion via spin-orbit interaction is one of the core concepts in the current spintronics research. The efficiency of the interconversion between charge and spin current is estimated based on Berry curvature of Bloch wavefunction in the linear-response regime. Beyond the linear regime, nonlinear spin-charge conversion in the higher-order electric field terms has recently been demonstrated in noncentrosymmetric materials with nontrivial spin texture in the momentum space. Here we report the observation of the nonlinear charge-spin conversion in a nominally centrosymmetric oxide material, SrIrO3, by breaking inversion symmetry at the interface. A large second-order magnetoelectric coefficient is observed at room temperature because of the antisymmetric spin-orbit interaction at the interface of Dirac semimetallic bands, which is subject to the symmetry constraint of the substrates. Our study suggests that nonlinear spin-charge conversion can be induced in many materials with strong spin-orbit interaction at the interface by breaking the local inversion symmetry to give rise to spin splitting in otherwise spin degenerate systems.

cond-mat.str-el↗

arXiv2018

Searching the weakest link: Demagnetizing fields and magnetization reversal in permanent magnets

Magnetization reversal in permanent magnets occurs by the nucleation and expansion of reversed domains. Micromagnetic theory offers the possibility to localize the spots within the complex structure of the magnet where magnetization reversal starts. We compute maps of the local nucleation field in a Nd2Fe14B permanent magnet using a model order reduction approach. Considering thermal fluctuations in numerical micromagnetics we can also quantify the reduction of the coercive field due to thermal activation. However, the major reduction of the coercive field is caused by the soft magnetic grain boundary phases and misorientation if there is no surface damage.

cond-mat.mtrl-sci↗

arXiv2006

Asymmetric Explosion of Type Ia Supernovae as Seen from Near Infrared Observations

We present near-infrared spectra of late phase (>200 d) Type Ia supernovae (SNe Ia) taken at the Subaru telescope. The [Fe II] line of SN 2003hv shows a clear flat-topped feature, while that of SN 2005W show less prominent flatness. In addition, a large shift in their line center, varying from -3000 to 1000 (km/s) with respect to the host galaxies, is seen. Such a shift suggests the occurrence of an off-center, non-spherical explosion in the central region, and provides important, new constraints on the explosion models of SNe Ia.

astro-ph↗

arXiv2006

Galactic chemical evolution: Carbon through Zinc

We calculate the evolution of heavy element abundances from C to Zn in the solar neighborhood adopting our new nucleosynthesis yields. Our yields are calculated for wide ranges of metallicity (Z=0-Z_\odot) and the explosion energy (normal supernovae and hypernovae), based on the light curve and spectra fitting of individual supernovae. The elemental abundance ratios are in good agreement with observations. Among the alpha-elements, O, Mg, Si, S, and Ca show a plateau at [Fe/H] < -1, while Ti is underabundant overall. The observed abundance of Zn ([Zn/Fe] ~ 0) can be explained only by the high energy explosion models, which requires a large contribution of hypernovae. The observed decrease in the odd-Z elements (Na, Al, and Cu) toward low [Fe/H] is reproduced by the metallicity effect on nucleosynthesis. The iron-peak elements (Cr, Mn, Co, and Ni) are consistent with the observed mean values at -2.5 < [Fe/H] < -1$, and the observed trend at the lower metallicity can be explained by the energy effect. We also show the abundance ratios and the metallicity distribution functions of the Galactic bulge, halo, and thick disk. Our results suggest that the formation timescale of the thick disk is ~ 1-3 Gyr.

astro-ph↗

arXiv2006

Core-Collapse Very Massive Stars: Evolution, Explosion, and Nucleosynthesis of Population III 500 -- 1000 $M_{\odot}$ Stars

We calculate evolution, collapse, explosion, and nucleosynthesis of Population III very-massive stars with 500$M_{\odot}$ and 1000$M_{\odot}$. Presupernova evolution is calculated in spherical symmetry. Collapse and explosion are calculated by a two-dimensional code, based on the bipolar jet models. We compare the results of nucleosynthesis with the abundance patterns of intracluster matter, hot gases in M82, and extremely metal-poor stars in the Galactic halo. It was found that both 500$M_{\odot}$ and 1000$M_{\odot}$ models enter the region of pair-instability but continue to undergo core collapse. In the presupernova stage, silicon burning regions occupy a large fraction, more than 20% of the total mass. For moderately aspherical explosions, the patterns of nucleosynthesis match the observational data of both intracluster medium and M82. Our results suggest that explosions of Population III core-collapse very-massive stars contribute significantly to the chemical evolution of gases in clusters of galaxies. For Galactic halo stars, our [O/Fe] ratios are smaller than the observational abundances. However, our proposed scenario is naturally consistent with this outcome. The final black hole masses are $\sim 230M_{\odot}$ and $\sim 500M_{\odot}$ for the $500M_{\odot}$ and 1000$M_{\odot}$ models, respectively. This result may support the view that Population III very massive stars are responsible for the origin of intermediate mass black holes which were recently reported to be discovered.

astro-ph↗

arXiv2005

Hypernovae, Black-Hole-Forming Supernovae, and First Stars

Recent studies of core-collapse supernovae have revealed the existence of two distinct classes of massive supernovae (SNe): 1) very energetic SNe (Hypernovae), whose kinetic energy (KE) exceeds $10^{52}$ erg, about 10 times the KE of normal core-collapse SNe, and 2) very faint and low energy SNe (E < 0.5 $\times$ $10^{51}$ erg; Faint supernovae). These two classes of supernovae are likely to be "black-hole-forming" supernovae with rotating or non-rotating black holes. We compare their nucleosynthesis yields with the abundances of extremely metal-poor (EMP) stars to identify the Pop III (or first) supernovae. We show that the EMP stars, especially the C-rich type, are likely to be enriched by black-hole-forming supernovae.

astro-ph↗

arXiv2003

Type Ia Supernovae: Progenitors and Diversities

A key question for supernova cosmology is whether the peak luminosities of Type Ia supernovae (SNe Ia) are sufficiently free from the effects of cosmic and galactic evolution. To answer this question, we review the currently popular scenario of SN Ia progenitors, i.e., the single degenerate scenario for the Chandrasekhar mass white dwarf (WD) models. We identify the progenitor's evolution with two channels: (1) the WD+RG (red-giant) and (2) the WD+MS (near main-sequence He-rich star) channels. The strong wind from accreting WDs plays a key role, which yields important age and metallicity effects on the evolution. We suggest that the variation of the carbon mass fraction $X$(C) in the C+O WD (or the variation of the initial WD mass) causes the diversity of SN Ia brightness. This model can explain the observed dependence of SNe Ia brightness on the galaxy types. We then predict how SN Ia brightness evolves along the redshift (with changing metallicity and age) for elliptical and spiral galaxies. Such evolutionary effects along the redshift can be corrected as has been made for local SNe Ia. We also touch on several related issues: (1) the abundance pattern of stars in dwarf spheroidal galaxies in relation to the metallicity effect on SNe Ia, (2) effects of angular momentum brought into the WD in relation to the diversities and the fate of double degenerates, and (3) possible presence of helium in the peculiar SN Ia 2000cx in relation to the sub-Chandrasekhar mass model.

astro-ph↗

arXiv2003

Nucleosynthesis in Black-Hole-Forming Supernovae and Abundance Patterns of Extremely Metal-Poor Stars

Stars more massive than $\sim$ 20 - 25 \ms form a black hole at the end of their evolution. Stars with non-rotating black holes are likely to collapse "quietly" ejecting a small amount of heavy elements (Faint supernovae). In contrast, stars with rotating black holes are likely to give rise to very energetic supernovae (Hypernovae). We present distinct nucleosynthesis features of these two types of "black-hole-forming" supernovae. Nucleosynthesis in Hypernovae is characterized by larger abundance ratios (Zn,Co,V,Ti)/Fe and smaller (Mn,Cr)/Fe than normal supernovae, which can explain the observed trend of these ratios in extremely metal-poor stars. Nucleosynthesis in Faint supernovae is characterized by a large amount of fall-back. We show that the abundance pattern of the recently discovered most Fe-poor star, HE0107-5240, and other extremely metal-poor carbon-rich stars are in good accord with those of black-hole-forming supernovae, but not pair-instability supernovae. This suggests that black-hole-forming supernovae made important contributions to the early Galactic (and cosmic) chemical evolution. Finally we discuss the nature of First (Pop III) Stars.

astro-ph↗