Search

Nuclear astrophysics is an interdisciplinary research field of nuclear physics and astrophysics, seeking for the answer to a question, how to understand the evolution of the universe with the nuclear processes which we learn. We review the research activities of nuclear astrophysics in east and southeast Asia which includes astronomy, experimental and theoretical nuclear physics, and astrophysics. Several hot topics such as the Li problems, critical nuclear reactions and properties in stars, properties of dense matter, r-process nucleosynthesis, and ν-process nucleosynthesis are chosen and discussed in further details. Some future Asian facilities, together with physics perspectives, are introduced. [ABSTRACT FROM AUTHOR]

We discuss effects of shock propagation on neutrino (v) oscillation in supernova environment, which affects directly v-induced nucleosynthesis. Since electron number density varies rapidly behind and in front of the shock, multiple resonances of mixing angles by adiabatic and/or nonadiabatic electron density change can occur during the shock propagation. In this work, we report detailed analysis of the v oscillation in matter and the v-induced nucleosynthesis by the shock propagation. The shock effects increase abundances by about 16% of 138La, 14% of 92Nb and 7% of 98Tc, whose main synthesized region is O-Ne-Mg layer in our supernova model. Here, we adopt a simple thermal bomb hydrodynamic model and the normal v-mass hierarchy. [ABSTRACT FROM AUTHOR]

Neutrino-nucleus reaction processes play important roles in the synthesis of 7Li, 11B, 138La, 180Ta and other elements in explosive environments realized in supernovae. We have constructed new shell-model Hamiltonians based on recent progress in the physics of exotic nuclei. The shell evolutions toward drip-lines and the spin responses of nuclei are found to be remarkably improved by the new Hamiltonians. Proper tensor components in the Hamiltonians are shown to be important for the improvements. We then applied these Hamiltonians to calculate the neutrino-nucleus reaction cross sections, and obtained more precise theoretical estimates of supernova nucleosynthesis of 7Li, 11B and 55Mn etc, including the neutrino processes. Finally, we propose a new method for determining the neutrino oscillation parameter, the mixing angle θ13, and the mass hierarchy, by making use of the strong dependence of the yields of 7Li and 11B on these parameters. [ABSTRACT FROM AUTHOR]

The existence of a primordial magnetic field (PMF) would affect both the temperature and polarization anisotropies of the cosmic microwave background (CMB) and the formation of the large scale structure(LSS). It also provides a plausible explanation for the disparity between observations and theoretical fits to the CMB power spectrum and the LSS. Here we report on calculations of not only the numerical power spectrum of the PMF, but also the correlations between the PMF power spectrum and the primary curvature perturbations. [ABSTRACT FROM AUTHOR]

The chemically most primitive stars provide constraints on the nature of the first stellar objects that formed in the Universe; elements other than hydrogen, helium and traces of lithium present within these objects were generated by nucleosynthesis in the very first stars. The relative abundances of elements in the surviving primitive stars reflect the masses of the first stars, because the pathways of nucleosynthesis are quite sensitive to stellar masses. Several models have been suggested to explain the origin of the abundance pattern of the giant star HE0107-5240, which hitherto exhibited the highest deficiency of heavy elements known. Here we report the discovery of HE1327-2326, a subgiant or main-sequence star with an iron abundance about a factor of two lower than that of HE0107-5240. Both stars show extreme overabundances of carbon and nitrogen with respect to iron, suggesting a similar origin of the abundance patterns. The unexpectedly low Li and high Sr abundances of HE1327-2326, however, challenge existing theoretical understanding: no model predicts the high Sr abundance or provides a Li depletion mechanism consistent with data available for the most metal-poor stars. [ABSTRACT FROM AUTHOR]

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 a 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 ρBc and the fluctuation parameter σB. 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, ρBc, and σB. We find that the fluctuations of the PMF reduce the 7Be production and enhance 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 ρBc and σB from observed abundances of 4He and D and find that the 7Li 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 7Li occurred, abundances of all light elements can be consistent with observational constraints. [ABSTRACT FROM AUTHOR]

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 the 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 (CC) reaction rates of are enhanced, and yields of proton-rich nuclei, along with 7Be and 11C, are increased. In the inverted hierarchy case, the CC reaction rates of 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. [ABSTRACT FROM AUTHOR]

Observations of type Ia supernovae (SNe Ia) 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 (ECs). 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 EC 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 the 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 SN Ia. We also note that observational constraints on the yields of 54Cr and 54Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SN Ia explosions. [ABSTRACT FROM AUTHOR]

Beta-decay rates for exotic nuclei with neutron magic number of N = 126 relevant to r-process nucleosynthesis are studied up to Z = 78 by shell-model calculations. The half-lives for the waiting-point nuclei obtained, which are short compared to a standard finite-range-droplet model, are used to study r-process nucleosynthesis in core-collapse supernova (CCSN) explosions and binary neutron star mergers. The element abundances are obtained up to the third peak as well as beyond the peak region up to thorium and uranium. The position of the third peak is found to be shifted toward a higher mass region in both CCSN explosions and neutron star mergers. We find that thorium and uranium elements are produced more with the shorter shell-model half-lives and their abundances come close to the observed values in CCSN explosions. In the case of binary neutron star mergers, thorium and uranium are produced consistently with the observed values independent of the half-lives. [ABSTRACT FROM AUTHOR]

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 with 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. [ABSTRACT FROM AUTHOR]

A new scheme for the short baseline electron antineutrino () disappearance study is investigated. We propose to use an intense neutron emitter, 252Cf, which produces a 8Li isotope through the 7Li(n,γ)8Li reaction; 8Li is a emitter via decay. Because this source needs neither accelerator nor reactor facilities, the can be placed on any neutrino detector as closely as possible. This short baseline circumstance with a suitable detector enables us to study the existence of possible sterile neutrinos, in particular, on 1 eV mass scale. Also, complementary comparison studies among different neutrino detectors can become feasible by using from the 8Li source. As an example, applications to hemisphere and cylinder shape scintillator detectors are performed in detail with the expectation signal modification by the sterile neutrino. Sensitivities to mass and mixing angles of sterile neutrinos are also presented for comparison with those of other neutrino experiments. [ABSTRACT FROM AUTHOR]

Current models for the r process are summarized with an emphasis on the key constraints from both nuclear physics measurements and astronomical observations. In particular, we analyze the importance of nuclear physics input such as beta-decay rates; nuclear masses; neutron-capture cross sections; beta-delayed neutron emission; probability of spontaneous fission, beta- and neutron-induced fission, fission fragment mass distributions; neutrino-induced reaction cross sections, etc. We highlight the effects on models for r-process nucleosynthesis of newly measured β-decay half-lives, masses, and spectroscopy of neutron-rich nuclei near the r-process path. We overview r-process nucleosynthesis in the neutrino driven wind above the proto-neutron star in core collapse supernovae along with the possibility of magneto-hydrodynamic jets from rotating supernova explosion models. We also consider the possibility of neutron star mergers as an r-process environment. A key outcome of newly measured nuclear properties far from stability is the degree of shell quenching for neutron rich isotopes near the closed neutron shells. This leads to important constraints on the sites for r-process nucleosynthesis in which freezeout occurs on a rapid timescale. [ABSTRACT FROM AUTHOR]

Recent experimental results have confirmed a possible reduction in the Gamow–Teller (GT+) strengths of pf-shell nuclei. These proton-rich nuclei are of relevance in the deflagration and explosive burning phases of SNe Ia. While prior GT strengths result in nucleosynthesis predictions with a lower-than-expected electron fraction, a reduction in the GT+ strength can result in a slightly increased electron fraction compared to previous shell model predictions, though the enhancement is not as large as previous enhancements in going from rates computed by Fuller, Fowler, and Newman based on an independent particle model. A shell model parametrization has been developed that more closely matches experimental GT strengths. The resultant electron-capture rates are used in nucleosynthesis calculations for carbon deflagration and explosion phases of SNe Ia, and the final mass fractions are compared to those obtained using more commonly used rates. [ABSTRACT FROM AUTHOR]

The rapid neutron-capture process (r-process) is a major process for the synthesis of elements heavier than iron-peak elements, but the astrophysical site(s) of the r-process has not yet been identified. Neutron star mergers (NSMs) are suggested to be a major r-process site according to nucleosynthesis studies. Previous chemical evolution studies, however, required unlikely short merger times of NSMs to reproduce the observed large star-to-star scatters in the abundance ratios of r-process elements to iron: the [Eu/Fe] of extremely metal-poor stars in the Milky Way (MW) halo. This problem can be solved by considering chemical evolution in dwarf spheroidal galaxies (dSphs), which would be building blocks of the MW and have lower star formation efficiencies than the MW halo. We demonstrate the enrichment of r-process elements in dSphs by NSMs using an N-body/smoothed particle hydrodynamics code. Our high-resolution model reproduces the observed [Eu/Fe] due to NSMs with a merger time of 100 Myr when the effect of metal mixing is taken into account. This is because metallicity is not correlated with time ∼300 Myr from the start of the simulation due to the low star formation efficiency in dSphs. We also confirm that this model is consistent with observed properties of dSphs such as radial profiles and metallicity distribution. The merger time and the Galactic rate of NSMs are suggested to be ≲300 Myr and ∼10−4 year−1, respectively, which are consistent with the values suggested by population synthesis and nucleosynthesis studies. This study supports the argument that NSMs are the major astrophysical site of the r-process. [ABSTRACT FROM AUTHOR]

We investigated the in-medium effect by density-dependent axial and weak-vector form factors on muon-neutrino () and anti-muon-neutrino scattering in the quasi-elastic (QE) region from nucleons (N*) bound in a nucleus or immersed in a nuclear medium via neutral current (NC) and charged current (CC). For the density-dependent form factors, we exploited a quark–meson-coupling (QMC) model. We found that the scattering cross sections via NC in the QE region usually decrease with an increased medium density, while those using CC were increased. However, their rate of change was sensitive to the four-momentum transfer given to a bound nucleon through scattering. We compared these results obtained by the elementary process corrected by the in-medium effect to the BNL and MiniBooNE data, which measured scattering cross sections per nucleon through – 12C scattering in 12C composite targets. The incident energy range was 550 3000 MeV. We increased the energy up to 100 GeV to compare our results to the NOMAD experimental data. In order to study the density effects on a nucleon embedded in 12C, we exploited the QMC form factors evaluated at , where the normal density . The strangeness contributions in NC scattering are also incorporated into the form factors for comparison with experimental data. Our numerical results show that most of the experimental data can be explained in a satisfactory manner by the density-dependent elementary process, but there are some remaining deviations resulting from the nuclear structure, particularly in the low and high momentum-transfer regions. [ABSTRACT FROM AUTHOR]