arXiv subjects

Toshitaka Kajino

Publications and source records attributed to Toshitaka Kajino.

At least 55 records · Page 3Linked to original sources

arXiv2020

Screening Effects on Electron Capture Rates and Type Ia Supernova Nucleosynthesis

Type Ia supernovae (SNe Ia) are believed to be a thermonuclear explosion of a white dwarf, but the mass of their progenitors is still an open problem. In near-Chandrasekhar-mass (near-M_Ch) models of SNe Ia, the central density reaches >10^9 g cm^{-3}. The electron chemical potential becomes higher than the Q-values of electron capture (EC) transitions between fp-shell nuclei, so a portion of the available electrons is captured by iron group elements and thus neutron-rich isotopes are formed. Since EC reaction rates are sensitive to the density, the degree of neutronization is a key to distinguish near- and sub-M_Ch models. In order to compare observations and theoretical models, an accurate treatment of EC reactions is necessary. In previous theoretical works, however, effects of electron screening on ECs are ignored. Screening lowers EC rates and thus leads to a higher electron fraction. We implement electron screening on ECs to calculate explosive SN Ia nucleosynthesis in a near-M_Ch single degenerate model. It is found that some of neutron-rich nuclear abundances, namely those of 46,48Ca, 50Ti, 54Cr, 58Fe, 64Ni and 67,70Zn, decrease when screening effects on ECs are considered. Of these, 50Ti, 54Cr and 58Fe are particularly interesting because a significant portion of the solar abundance of these nuclei is presumed to originate from SNe Ia. We conclude that implementing the screening effect on ECs in modern SN Ia models is desirable to precisely calculate abundances of neutron-rich nuclides.

astro-ph.HE↗

arXiv2020

Elimination of the Blue Loops in the Evolution of Intermediate-mass Stars by the Neutrino Magnetic Moment and Large Extra Dimensions

For searching beyond Standard Model physics, stars are laboratories which complement terrestrial experiments. Massless neutrinos in the Standard Model of particle physics cannot have a magnetic moment, but massive neutrinos have a finite magnetic moment in the minimal extension of the Standard Model. Large extra dimensions are a possible solution of the hierarchy problem. Both of these provide additional energy loss channels in stellar interiors via the electromagnetic interaction and radiation into extra dimensions, respectively, and thus affect stellar evolution. We perform simulations of stellar evolution with such additional energy losses and find that they eliminate the blue loops in the evolution of intermediate-mass stars. The existence of Cepheid stars can be used to constrain the neutrino magnetic moment and large extra dimensions. In order for Cepheids to exist, the neutrino magnetic moment should be smaller than the range ~2x10^{-10} to 4x10^{-11}mu_B, where mu_B is the Bohr magneton, and the fundamental scale in the (4+2)-spacetime should be larger than ~2 to 5 TeV, depending on the rate of the ^{12}C(alpha, gamma)^{16}O reaction. The fundamental scale also has strong dependence on the metallicity. This value of the magnetic moment is in the range explored in the reactor experiments, but higher than the limit inferred from globular clusters. Similarly the fundamental scale value we constrain corresponds to a size of the compactified dimensions comparable to those explored in the torsion balance experiments, but is smaller than the limits inferred from collider experiments and low-mass stars.

astro-ph.SR↗

arXiv2020

New Insights into Uncertainties in the Relic Neutrino Background and Effects from the Nuclear Equation of State

We review the computation of and associated uncertainties in the current understanding of the relic neutrino background due to core-collapse supernovae, black hole formation and neutron-star merger events. We consider the current status of uncertainties due to the nuclear equation of state (EoS), the progenitor masses, the source supernova neutrino spectrum, the cosmological star formation rate, the stellar initial mass function, neutrino oscillations, and neutrino self-interactions. We summarize the current viability of future neutrino detectors to distinguish the nuclear EoS and the temperature of supernova neutrinos via the detected relic supernova neutrino spectrum.

astro-ph.HE↗

arXiv2020

Nuclear Reaction Screening, Weak Interactions, and r-Process Nucleosynthesis in High Magnetic Fields

Coulomb screening and weak interactions in a hot, magnetized plasma are investigated. Coulomb screening is evaluated in a relativistic thermal plasma in which electrons and positrons are in equilibrium. In addition to temperature effects, effects on weak screening from a strong external magnetic field are evaluated. In high fields, the electron transverse momentum components are quantized into Landau levels. The characteristic plasma screening length at high temperatures and at high magnetic fields is explored. In addition to changes to the screening length, changes in weak interaction rates are estimated. It is found that high fields can result in increased $β$-decay rates as the electron and positron spectra are dominated by Landau levels. Finally, the effects studied here are evaluated in a simple r-process model. It is found that relativistic Coulomb screening has a small effect on the final abundance distribution. While changes in weak interaction rates in strong magnetic fields can have an effect on the r-process evolution and abundance distribution, the field strength required to have a significant effect may be larger than what is currently thought to be typical of the r-process environment in collapsar jets or neutron star mergers. If r-process sites exist in fields $\gtrsim 10^{14}$ G effects from fields on weak decays could be significant.

astro-ph.HE↗

arXiv2020

Explaining the Variations in Isotopic Ratios in Meteoritic Amino Acids

Measurements of the isotopic abundances in meteoritic amino acids have found enhancements of $^2$H/H, $^{15}$N/$^{14}$N, and $^{13}$C/$^{12}$C in the amino acids in the meteorites studied. We show that they are consistent with the processing of the constituents of the meteorites by electron anti-neutrinos that would be expected from a core-collapse supernova or neutron-star merger. Using theoretical electron anti-neutrino cross sections we are able to predict these isotopic ratio variations depending on the time-integrated anti-neutrino flux at the site where the amino acids were processed.

astro-ph.EP↗

arXiv2020

Screening corrections to Electron Capture Rates and resulting constraints on Primordial Magnetic Fields

We explore screening effects arising from a relativistic magnetized plasma with applications to Big Bang nucleosynthesis (BBN). %Specifically, due to their small magnetic moments, energies of electrons and positrons can be easily quantized via Landau quantization. The screening potential which depends on the thermodynamics of charged particles in the plasma is altered by the magnetic field. We focus on the impact of screening on the electron capture interaction. Taking into account the correction in BBN arising from a homogeneous primordial magnetic field (PMF), we constrain the epoch at which the PMF was generated and its strength during BBN. Considering such screening corrections to the electron capture rates and using up-to-date observations of primordial elemental abundances, we also discuss the possibility of solving the problem of under-estimation of the deuterium abundance. We find for certain values of the PMF strength predicted D and $^4$He abundances are both consistent with the observational constraints.

astro-ph.CO↗

arXiv2020

ν\barν -Pair Synchrotron Emission in Neutron-Star Matter based on a Relativistic Quantum Approach

We study the neutrino anti-neutrino pair synchrotron emission from electrons and protons in a relativistic quantum approach. This process occurs only under a strong magnetic field, and it is considered to be one of effective processes for neutron star cooling. In this work we calculate the luminosity of the neutrino anti-neutrino pairs emitted from neutron-star-matter with a magnetic field of about 10^{15} G. We find that the energy loss is much larger than that of the modified Urca process. The neutrino anti-neutrino pair emission processes in strong magnetic fields is expected to contribute significantly to the cooling of the magnetars.

hep-ph↗

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 $(ν)$ induced reactions in a core-collapse supernova explosion. We consider the modification by $ν$ self-interaction ($ν$-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 $ν$-SI. In contrast, $^{11}$B is relatively insensitive to the $ν$-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.

astro-ph.HE↗

arXiv2019

Effects of the Metallicity on Li and B Production in Supernova Neutrino Process

The neutrino process ($ν$-process) for the production of 7Li and 11B in core-collapse supernovae (SNe) is extensively investigated. Initial abundances of s-nuclei and other physical conditions are derived from an updated calculation of the SN 1987A progenitor. The nuclear reaction network including neutrino reactions is constructed with the variable order Bader-Deuflhard integration method. We find that yields of 7Li and 11B significantly depend on the stellar metallicity while they are independent of the weak s-process during the stellar evolution. When the metallicity is high, there are more neutron absorbers, i.e., 56Fe, 14N (from initial CNO nuclei), and 54Fe, and the neutron abundance is small during the $ν$-process. Since 7Be is predominantly destroyed via 7Be(n,p)7Li, a change in the neutron abundance results in different 7Be yields. Then, the calculated yield ratio 7Li/11B=0.93 for the solar metallicity is larger than that for the SN 1987A 7Li/11B=0.80 by 16 % in the inverted mass hierarchy case. We analyze contributions of respective reactions as well as abundance evolution, and clarify the $ν$-process of 7Li and 11B.

astro-ph.SR↗

arXiv2019

Supernova Neutrino Process of Li and B Revisited

We reinvestigate effects of neutrino oscillations on the production of 7Li and 11B in core-collapse supernovae (SNe). During the propagation of neutrinos from the proto-neutron star, their flavors change and the neutrino reaction rates for spallation of 12C and 4He are affected. In this work corrected neutrino spallation cross sections for 4He and 12C are adopted. Initial abundances involving heavy s-nuclei and other physical conditions are derived in a new calculation of the SN 1987A progenitor in which effects of the progenitor metallicity are included. A dependence of the SN nucleosynthesis and final yields of 7Li and 11B on the neutrino mass hierarchy are shown in several stellar locations. In the normal hierarchy case, the charged current reaction rates of electron neutrinos are enhanced, and yields of proton-rich nuclei, along with 7Be and 11C, are increased. In the inverted hierarchy case, the charged current reaction rates of electron antineutrinos are enhanced, and yields of neutron-rich nuclei, along with 7Li and 11B, are increased. We find that variation of the metallicity modifies the yields of 7Li, 7Be, 11B, and 11C. This effect is caused by changes in the neutron abundance during SN nucleosynthesis. Therefore, accurate calculations of Li and B production in SNe should take into account the metallicity of progenitor stars.

astro-ph.HE↗

arXiv2019

Catching Element Formation In The Act

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions.

astro-ph.HE↗

arXiv2019

Big Bang Nucleosynthesis with an Inhomogeneous Primordial Magnetic Field Strength

We investigate the effect on the Big Bang Nucleosynthesis (BBN) from the presence of a stochastic primordial magnetic field (PMF) whose strength is spatially inhomogeneous. We assume a uniform total energy density and a gaussian distribution of field strength. In this case, domains of different temperatures exist in the BBN epoch due to variations in the local PMF. We show that in such case, the effective distribution function of particle velocities averaged over domains of different temperatures deviates from the Maxwell-Boltzmann distribution. This deviation is related to the scale invariant strength of the PMF energy density $ρ_{\rm Bc}$ and the fluctuation parameter $σ_{\rm B}$. We perform BBN network calculations taking into account the PMF strength distribution, and deduce the element abundances as functions of the baryon-to-photon ratio $η$, $ρ_{\rm Bc}$, and $σ_{\rm B}$. We find that the fluctuations of the PMF reduces the $^7$Be production and enhances D production. We analyze the averaged thermonuclear reaction rates compared with those of a single temperature, and find that the averaged charged-particle reaction rates are very different. Finally, we constrain the parameters $ρ_{\rm Bc}$ and $σ_{\rm B}$ from observed abundances of $^4$He and D, and find that the $^7$Li abundance is significantly reduced. We also find that if the $η$ value during BBN was larger than the present-day value due to a dissipation of the PMF or a radiative decay of exotic particles after BBN or if the stellar depletion of $^7$Li occurred, abundances of all light elements can be consistent with observational constraints.

astro-ph.CO↗

arXiv2019

On the relative velocity distribution for general statistics and an application to big-bang nucleosynthesis under Tsallis statistics

The distribution function of the relative velocity in a two-body reaction of nonrelativistic uncorrelated particles is derived for general cases of given distribution functions of single particle velocities. The distribution function is then used in calculations of thermonuclear reaction rates. As an example, we take the Tsallis non-Maxwellian distribution, and show that the distribution function of the relative velocity is different from the Tsallis distribution. We identify an inconsistency in previous studies of nuclear reaction rates within Tsallis statistics, and derive revised nuclear reaction rates. Utilizing the revised rates, accurate results of big bang nucleosynthesis are obtained for the Tsallis statistics. For this application it is more difficult to reduce the primordial 7Li abundance while keeping other nuclear abundances within the observational constraints. A small deviation from a Maxwell-Boltzmann distribution can increase the D abundance and slightly reduce 7Li abundance. Although it is impossible to realize a 7Li abundance at the level observed in metal-poor stars, a significant decrease is possible while maintaining a consistency with the observed D abundance.

astro-ph.CO↗

arXiv2018

Impacts of the New Carbon Fusion Cross Sections on Type Ia Supernovae

Type Ia supernovae (SNe Ia) are thought to be thermonuclear explosion of white dwarfs (WDs). Their progenitors are not well understood. One popular scenario is the double-degenerate (DD) scenario, which attributes SNe Ia to WD-WD binary mergers. The fates of the WD mergers depend on the rate of $^{12}$C+$^{12}$C reaction. Recently, the $^{12}$C+$^{12}$C cross sections have been measured and the analysis of the data using the Trojan Horse Method suggested that the astrophysical reaction rate is larger than conventional rates at astrophysical temperatures due to possible resonances. The resonance contribution results in a decrease of the carbon burning ignition temperature. Therefore accretion induced collapse occurs more easily and increases the birthrate of Galactic neutron stars with the contribution of the DD scenario to the SNe Ia rate becoming even smaller.

astro-ph.HE↗

arXiv2018

Explaining low l anomalies in the CMB power spectrum with resonant superstring excitations during inflation

We explore the possibility that both the suppression of the $\ell = 2$ multipole moment of the power spectrum of cosmic microwave background temperature fluctuations and the possible dip for $\ell = 10-30$ can be explained as well as a possible new dip for $\ell \approx 60$ as the result of the resonant creation of sequential excitations of a fermionic (or bosonic) closed superstring that couples to the inflaton field. We consider a D=26 closed bosonic string with one toroidal compact dimension as an illustration of how string excitations might imprint themselves on the CMB. We analyze the existence of successive momentum states, winding states or oscillations on the string as the source of the three possible dips in the power spectrum. Although the evidence of these dips are of marginal statistical significance, this might constitute the first observational evidence of successive superstring excitations in Nature.

astro-ph.CO↗

arXiv2018

Nucleosynthesis Constraints on the Explosion Mechanism for Type Ia Supernovae

Observations of type Ia supernovae include information about the characteristic nucleosynthesis associated with these thermonuclear explosions. We consider observational constraints from iron-group elemental and isotopic ratios, to compare with various models obtained with the most-realistic recent treatment of electron captures. The nucleosynthesis is sensitive to the highest white-dwarf central densities. Hence, nucleosynthesis yields can distinguish high-density Chandrasekhar-mass models from lower-density burning models such as white-dwarf mergers. We discuss new results of post-processing nucleosynthesis for two spherical models (deflagration and/or delayed detonation models) based upon new electron capture rates. We also consider cylindrical and 3D explosion models (including deflagration, delayed-detonation, or a violent merger model). Although there are uncertainties in the observational constraints, we identify some trends in observations and the models. We make a new comparison of the models with elemental and isotopic ratios from five observed supernovae and three supernova remnants. We find that the models and data tend to fall into two groups. In one group low-density cores such as in a 3D merger model are slightly more consistent with the nucleosynthesis data, while the other group is slightly better identified with higher-density cores such as in single-degenerate 1D-3D deflagration models. Hence, we postulate that both types of environments appear to contribute nearly equally to observed SNIa. We also note that observational constraints on the yields of $^{54}$Cr and $^{54}$Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SNIa explosions.

astro-ph.HE↗

arXiv2018

Spectral shape analysis for electron antineutrino oscillation study by using $^{8}$Li generator with $^{252}$Cf source

Existence of hypothetical fourth neutrino, so-called sterile neutrino, is one of open issues in the particle and neutrino physics. This fourth neutrino is a candidate for explaining some anomalies reported in LSND, MiniBoone, reactor experiments, and gallium experiments. To search for the existence of the sterile neutrino, we report detailed analysis of a feasible experiment for short baseline electron antineutrino (${\barν}_{e}$) disappearance study, in which a ${\barν}_{e}$ source from $^{8}$Li generator is considered under non-accelerator system. For $^{8}$Li production, we suggest to use $^{252}$Cf source as an intense neutron emitter, by which one can produce $^{8}$Li isotope through $^{7}$Li(n,$γ$)$^{8}$Li reaction, effectively. Using the $^{8}$Li generator, one does not need any accelerator or reactor facilities because the generator can be placed on any present and/or planned neutrino detectors as closely as possible. For the effect of the possible sterile neutrinos, we estimate expected neutrino flux and event rates from the neutrino source scheme, and show neutrino disappearance features and possible reaction rate changes by the sterile neutrino using the spectral shape analysis.

physics.ins-det↗

arXiv2018

Sites That Can Produce Left-Handed Amino Acids in the Supernova Neutrino Amino Acid Processing Model

The Supernova Neutrino Amino Acid Processing model, which uses electron anti-neutrinos and the magnetic field from a source object such as a supernova to selectively destroy one amino acid chirality, is studied for possible sites that would produce meteoroids having partially left-handed amino acids. Several sites appear to provide the requisite magnetic field intensities and electron anti-neutrino fluxes. These results have obvious implications for the origin of life on Earth.

physics.bio-ph↗