arXiv subjects

K. Takahashi

Publications and source records attributed to K. Takahashi.

67 records · Page 4Linked to original sources

arXiv2001

Effects of Neutrino Oscillation on the Supernova Neutrino Spectrum

The effects of three-flavor neutrino oscillation on the supernova neutrino spectrum are studied. We calculate the expected event rate and energy spectra, and their time evolution at the Superkamiokande (SK) and the Sudbury Neutrino Observatory (SNO), by using a realistic neutrino burst model based on numerical simulations of supernova explosions. We also employ a realistic density profile based on a presupernova model for the calculation of neutrino conversion probability in supernova envelopes. These realistic models and numerical calculations allow us to quantitatively estimate the effects of neutrino oscillation in a more realistic way than previous studies. We then found that the degeneracy of the solutions of the solar neutrino problem can be broken by the combination of the SK and SNO detections of a future Galactic supernova.

hep-ph↗

arXiv2001

Tunneling Mechanism due to Chaos in a Complex Phase Space

We have revealed that the barrier-tunneling process in non-integrable systems is strongly linked to chaos in complex phase space by investigating a simple scattering map model. The semiclassical wavefunction reproduces complicated features of tunneling perfectly and it enables us to solve all the reasons why those features appear in spite of absence of chaos on the real plane. Multi-generation structure of manifolds, which is the manifestation of complex-domain homoclinic entanglement created by complexified classical dynamics, allows a symbolic coding and it is used as a guiding principle to extract dominant complex trajectories from all the semiclassical candidates.

nlin.CD↗

arXiv2001

The Earth effects on the supernova neutrino spectra

The Earth effects on the energy spectra of supernova neutrinos are studied. We analyze numerically the time-integrated energy spectra of neutrino in a mantle-core-mantle step function model of the Earth's matter density profile. We consider a realistic frame-work in which there are three active neutrinos whose mass squared differences and mixings are constrained by the present understanding of solar and atmospheric neutrinos. We find that the energy spectra change for some allowed mixing parameters. Especially, the expected number of events at SNO shows characteristic behavior with respect to energy, i.e. a great dip and peak. We show that observations of the Earth effect allow us to identify the solar neutrino solution and to probe the mixing angle $θ_{2}$.

hep-ph↗

arXiv2000

Vertical Boundary at x ~ 0.11 in the Structural Phase Diagram of the La1-xSrxMnO3 System (0.08 <= x <= 0.125)

The structural phase diagram of the La1-xSrxMnO3 system in the compositional range 0.08 <= x <= 0.125 has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between 20-600 K. Recent studies have reported that there is an unusual rentrant-type phase transition in this range involving an abrupt change in lattice parameters but no change in the crystal symmetry, which remains orthorhombic Pbnm. The transition to the reentrant phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with some unusual properties. Our results demonstrate that for samples with x =0.11-0.125 there exist two lower-symmetry structural regions having monoclinic and triclinic symmetry respectively. There is a sharp first-order transition from the monoclinic to the triclinic phase coinciding with the transition to the ferromagnetic insulating phase, and an abrupt crossover from the orthorhombic Pbnm region with a near-vertical phase boundary just below x = 0.11. The new phases indicate the presence of some novel type of orbital ordering unlike that found in LaMnO3.

cond-mat.str-el↗

arXiv2000

Thouless energy in QCD and effects of diffusion modes on level correlations of Dirac operator

The correlations of the QCD Dirac eigenvalues are studied with use of an extended chiral random matrix model. The inclusion of spatial dependence which the original model lacks enables us to investigate the effects of diffusion modes. We get analytical expressions of level correlation functions with non-universal behavior caused by diffusion modes which is characterized by Thouless energy. Pion mode is shown to be responsible for these diffusion effects when QCD vacuum is considered a disordered medium.

hep-th↗

arXiv1999

Evolution of Multi-mass Globular Clusters in Galactic Tidal Field with the Effects of Velocity Anisotropy

We study the evolution of globular clusters with mass spectra under the influence of the steady Galactic tidal field, including the effects of velocity anisotropy. Similar to single-mass models, velocity anisotropy develops as the cluster evolves, but the degree of anisotropy is much smaller than isolated clusters. Except for very early epochs of the cluster evolution, nearly all mass components become tangentially anisotropic at the outer parts. We have compared our results with multi-mass, King-Michie models. The isotropic King model better fits to the Fokker-Planck results because of tangential anisotropy. However, it is almost impossible to fit the computed density profiles to the multi-mass King models for all mass components. Thus if one attempts to derive global mass function based on the observed mass function in limited radial range using multi-mass King models, one may get somewhat erratic results, especially for low mass stars. We have examined how the mass function changes in time. Specifically, we find that the power-law index of the mass function decreases monotonically with the total mass of the cluster. This appears to be consistent with the behaviour of the observed slopes of mass functions for a limited number of clusters, although it is premature to compare quantitatively because there are other mechanisms in contributing the evaporation of stars from the clusters. The projected velocity profiles for anisotropic models with the apocenter criterion for evaporation show significant flattening toward the tidal radius compared to isotropic model or anisotropic model with the energy criterion. Such a behaviour of velocity profile appears to be consistent with the observed profiles of collapsed cluster M15.

astro-ph↗

arXiv1999

Two ferromagnetic phases in La1-xSrxMnO3(x ~1/8)

It was discovered in La1-xSrxMnO3(x~1/8) that a field induced phase transition occurs from a ferromagnetic metal(FM) phase to a ferromagnetic insulator (FI) phase. The magnetization shows a sharp jump at the transition field accompanying with a remarkable increase of magnetoresistance. Striction measurements clarified that this transition is associated with the structural change from a Jahn-Teller(JT) distorted orthorhombic phase to a pseudo cubic phase. These results evidently show that the FI phase with a pseudo cubic symmetry is more stable in high fields than the FM phase due to the double exchange interaction. The driving force of this transition is explained by the enhancement of the ferromagnetic superexchange interaction induced by an antiferromagnetic type orbital ordering in the pseudo cubic phase, which was recently found in the anomalous X-ray scattering experiments.

cond-mat.str-el↗

arXiv1999

Nuclear Astrophysics

Nuclear astrophysics is that branch of astrophysics which helps understanding some of the many facets of the Universe through the knowledge of the microcosm of the atomic nucleus. In the last decades much advance has been made in nuclear astrophysics thanks to the sometimes spectacular progress in the modelling of the structure and evolution of the stars, in the quality and diversity of the astronomical observations, as well as in the experimental and theoretical understanding of the atomic nucleus and of its spontaneous or induced transformations. Developments in other sub-fields of physics and chemistry have also contributed to that advance. Many long-standing problems remain to be solved, however, and the theoretical understanding of a large variety of observational facts needs to be put on safer grounds. In addition, new questions are continuously emerging, and new facts endanger old ideas. This review shows that astrophysics has been, and still is, highly demanding to nuclear physics in both its experimental and theoretical components. On top of the fact that large varieties of nuclei have to be dealt with, these nuclei are immersed in highly unusual environments which may have a significant impact on their static properties, the diversity of their transmutation modes, and on the probabilities of these modes. In order to have a chance of solving some of the problems nuclear astrophysics is facing, the astrophysicists and nuclear physicists are obviously bound to put their competence in common, and have sometimes to benefit from the help of other fields of physics, like particle physics, plasma physics or solid-state physics.

astro-ph↗

arXiv1999

Titanium-44: Its Effective Decay Rate in Young Supernova Remnants, and its Abundance in Cas A

Radioactive isotopes such as 44Ti offer probably the most direct probe into nucleosynthesis environments in the interior of exploding stars, when the associated gamma-ray activities in the explosion remnant are detected and translated back to the isotopic abundances at the time of the explosion. In this paper, we assert that the procedure may not necessarily be straightforward, at least in the case of 44Ti, an orbital-electron capture decay isotope. Using the analytic model of McKee and Truelove (1995) for young supernova remnants, and assuming the existence of overdense 56Fe-dominated clumps that contain also 44Ti, we show that a high degree of ionization may be caused by the reverse shock so that the electron-capture rate of 44Ti could be significantly reduced from its laboratory value. When applied to Cas A, this increases under certain conditions the current 44Ti-activity by a factor 1.5 - 2.5, which yields a better compatibility between the COMPTEL observation of the 1.16 MeV line activity associated with the 44Ti decay and the SN model predictions of the initial 44Ti abundance. This possibility is, however, subject to various uncertainties, and in particular to the unknown properties and radial distribution of the the clumps in the ejecta.

astro-ph↗

arXiv1997

Dynamical Simulations: Methods and Comparisons

We review recent progress in computational methods for studying the dynamical evolution of large star clusters. After describing some highlights of a new comparison of different models, including N-body models, we mention in detail recent developments in three codes which allow models with anisotropic distributions of velocity to be studied: (i) a Fokker-Planck code using finite differences, (ii) a Monte Carlo code and (iii) a gaseous model code.

astro-ph↗

arXiv1997

Evolution of Tidally Truncated Globular Clusters with Anisotropy

The evolution of tidally truncated globular clusters is investigated by integrating two-dimensional Fokker-Planck equation that allows the development of velocity anisotropy. We start from the isotropic Plummer model with tidal cut off and followed the evolution through the corecollapse. The heating by three-binary is included to obtain the evolution past the corecollapse. The anisotropy in velocity dispersion develops during the precollapse evolution. However, the anisotropy becomes highly depressed during the post-collapse evolution because of rapid loss of radial orbits. Maximum radial anisotropy appears just after the beginning of the expansion, and degree of anisotropy decreases slowly as the total mass of the cluster decreases. Thus it may be possible to determine the evolutionary status of a cluster if the velocity anisotropy can be measured in the sense that the postcollapse clusters always have very little degree of anisotropy. The structure of the post-collapse cluster can be well fitted to King models because the degree of anisotropy is rather small.

astro-ph↗

arXiv1996

Theoretical Response to the Discovery of Deeply Bound Pionic States in 208Pb(d,3He) reactions

Recently, deeply bound pionic states were found experimentally in (d, $^3$He) reactions on $^{208}$Pb. They found an isolated peak structure in the bound region below the pion production threshold. We study theoretically these excitation functions in (d, $^3$He) reactions on $^{208}$Pb at T$_d$=600 MeV. We found very good agreement with the (d, $^3$He) excitation functions and could identify the underlying structures of the pionic states. We study the energy dependence of the (d, $^3$He) reactions and the change of the excitation functions with the incident energy.

nucl-th↗

arXiv1996

Coalescing neutron stars -- a step towards physical models. II. Neutrino emission, neutron tori, and gamma-ray bursts

Three-dimensional hydrodynamical, Newtonian calculations of the coalescence of equal-mass binary neutron stars are performed, including a physical high-density equation of state and a treatment of the neutrino emission of the heated matter. The total neutrino luminosity climbs to a maximum value of 1--$1.5\cdot 10^{53}$~erg/s of which 90--95\% originate from the toroidal gas cloud surrounding the very dense core formed after the merging. When the neutrino luminosities are highest, $ν\barν$-annihilation deposits about 0.2--0.3\% of the emitted neutrino energy in the immediate neighborhood of the merger, and the maximum integral energy deposition rate is 3--$4\cdot 10^{50}$~erg/s. Since the $3\,M_{\odot}$ core of the merged object will most likely collapse into a black hole within milliseconds, the energy that can be pumped into a pair-photon fireball is insufficient by a factor of about 1000 to explain $γ$-ray bursts at cosmological distances with an energy of the order of $10^{51}/(4π)$~erg/steradian. Analytical estimates show that the additional energy provided by the annihilation of $ν\barν$ pairs emitted from a possible accretion torus of $\sim 0.1\,M_{\odot}$ around the central black hole is still more than a factor of 10 too small, unless focussing of the fireball into a jet-like expansion plays an important role. About $10^{-4}$--$10^{-3}$~$M_\odot$ of material lost during the neutron star merging and swept out from the system in a neutrino-driven wind might be a site for nucleosythesis. Aspects of a possible r-processing in these ejecta are discussed.

astro-ph↗