Nobuya Nishimura

PRODUCTION OF ALL THE r-PROCESS NUCLIDES IN THE DYNAMICAL EJECTA OF NEUTRON STAR MERGERS

Recent studies suggest that binary neutron star (NS-NS) mergers robustly produce heavy r-process nuclei above the atomic mass number A ~ 130 because their ejecta consist of almost pure neutrons (electron fraction of Y e < 0.1). However, the production of a small amount of the lighter r-process nuclei (A ≈ 90-120) conflicts with the spectroscopic results of r-process-enhanced Galactic halo stars. We present, for the first time, the result of nucleosynthesis calculations based on the fully general relativistic simulation of a NS-NS merger with approximate neutrino transport. It is found that the bulk of the dynamical ejecta are appreciably shock-heated and neutrino processed, resulting in a wide range of Y e (≈0.09-0.45). The mass-averaged abundance distribution of calculated nucleosynthesis yields is in reasonable agreement with the full-mass range (A ≈ 90-240) of the solar r-process curve. This implies, if our model is representative of such events, that the dynamical ejecta of NS-NS mergers could be the origin of the Galactic r-process nuclei. Our result also shows that radioactive heating after ~1 day from the merging, which gives rise to r-process-powered transient emission, is dominated by the β-decays of several species close to stability with precisely measured half-lives. This implies that the total radioactive heating rate for such an event can be well constrained within about a factor of two if the ejected material has a solar-like r-process pattern.

MAGNETOROTATIONALLY DRIVEN SUPERNOVAE AS THE ORIGIN OF EARLY GALAXY r-PROCESS ELEMENTS?

We examine magnetorotationally driven supernovae as sources of r-process elements in the early Galaxy. On the basis of thermodynamic histories of tracer particles from a three-dimensional magnetohydrodynamical core-collapse supernova model with approximated neutrino transport, we perform nucleosynthesis calculations with and without considering the effects of neutrino absorption reactions on the electron fraction (Ye ) during post-processing. We find that the peak distribution of Ye in the ejecta is shifted from ~0.15 to ~0.17 and broadened toward higher Ye due to neutrino absorption. Nevertheless, in both cases, the second and third peaks of the solar r-process element distribution can be reproduced well. The rare progenitor configuration that was used here, characterized by a high rotation rate and a large magnetic field necessary for the formation of bipolar jets, could naturally provide a site for the strong r-process in agreement with observations of the early Galactic chemical evolution.

r-process nucleosynthesis in magnetohydrodynamic jet explosions of core-collapse supernovae

We investigate the r-process nucleosynthesis during a purely magnetohydrodynamic (MHD) explosion in a massive star of 13 M☉. The two-dimensional MHD simulations have been carried out from the onset of the core collapse to the shock propagation to the silicon-rich layers (~500 ms after bounce). Thereafter, using the compositions during the explosion, we calculate the r-process nucleosynthesis in the later phase by employing the two kinds of time extrapolations of the temperature and density. With these computations, we show that the jetlike explosion formed due to the combined effects of rapid rotation and strong magnetic field lowers the electron fraction significantly in the iron core, contrary to the spherical explosion. We demonstrate that the ejected material with low Ye in the jet coming out from the silicon layers is good for reproducing the third peak of the solar r-element pattern. In addition, we investigate the effects of fission using the full nuclear reaction network and the differences of two kinds of mass formulae on the r-process peaks obtained in the above MHD models. As a result, we find that both of them can reproduce the global abundance pattern up to the third peaks, although the detailed distributions are rather different. Finally, we discuss the effects of neutrino absorption reactions, which are not coupled to the above MHD simulations, on the possible reduction of Ye obtained in the above computations. We point out that there should be variations in the r-process nucleosynthesis in the supernova explosion if the MHD effects play an important role.

NUCLEOSYNTHESIS IN MAGNETICALLY DRIVEN JETS FROM COLLAPSARS

We have made detailed calculations of the composition of magnetically driven jets ejected from collapsars, or rapidly rotating massive stars, based on long-term magnetohydrodynamic simulations of their core collapse with various distributions of magnetic field and angular momentum before collapse. We follow the evolution of the abundances of about 4000 nuclides from the collapse phase to the ejection phase and through the jet generation phase using a large nuclear reaction network. We find that the r-process successfully operates only in energetic jets (>1051 ergs), such that U and Th are synthesized abundantly, even when the collapsar has a relatively weak magnetic field (1010 G) and a moderately rotating core before the collapse. The abundance patterns inside the jets are similar to those of the r-elements in the solar system. About 0.01-0.06 M☉ of neutron-rich, heavy nuclei are ejected from a collapsar with energetic jets. The higher energy jets have larger amounts of 56Ni, varying from 3.7 × 10−4 to 0.06 M☉. Less energetic jets, which eject small amounts of 56Ni, could induce a gamma-ray burst (GRB) without a supernova, such as GRB 060505 or GRB 060614. Considerable amounts of r-elements are likely to be ejected from GRBs with hypernovae, if both the GRB and hypernova are induced by jets that are driven near the black hole.

Code dependencies of pre-supernova evolution and nucleosynthesis in massive stars: evolution to the end of core helium burning

Massive stars are key sources of radiative, kinetic, and chemical feedback in the universe. Grids of massive star models computed by different groups each using their own codes, input physics choices and numerical approximations, however, lead to inconsistent results for the same stars. We use three of these 1D codes---GENEC, KEPLER and MESA---to compute non-rotating stellar models of 15 M⊙, 20 M⊙, and 25 M⊙ and compare their nucleosynthesis. We follow the evolution from the main sequence until the end of core helium burning. The GENEC and KEPLER models hold physics assumptions used in large grids of published models. The MESA code was set up to use convective core overshooting such that the CO core masses are consistent with those obtained by GENEC. For all models, full nucleosynthesis is computed using the NuGrid post-processing tool MPPNP. We find that the surface abundances predicted by the models are in reasonable agreement. In the helium core, the standard deviation of the elemental overproduction factors for Fe to Mo is less than 30%---smaller than the impact of the present nuclear physics uncertainties. For our three initial masses, the three stellar evolution codes yield consistent results. Differences in key properties of the models, e.g., helium and CO core masses and the time spent as a red supergiant, are traced back to the treatment of convection and, to a lesser extent, mass loss. The mixing processes in stars remain the key uncertainty in stellar modelling. Better constrained prescriptions are thus necessary to improve the predictive power of stellar evolution models.

Impact of new beta-decay half-lives on r-process nucleosynthesis

Abstract We investigate the effects of newly measured β-decay half-lives on r-process nucleosynthesis.These new rates were determined by recent experiments at the radioactive isotope beam factoryfacility in the RIKEN Nishina Center. We adopt an r-process nucleosynthesis environment basedon a magnetohydrodynamic supernova explosion model that includes strong magnetic fields andrapid rotation of the progenitor. A number of the new β-decay rates are for nuclei on or near the r-process path, and hence, they affect the nucleosynthesis yields and timescale of the r-process. Themain effect of the newly measured β-decay half-lives is an enhancement in the calculated abundanceof isotopes with mass number A = 110 – 120 relative to calculated abundances based upon β-decayrates estimated with the finite-range droplet mass model. This effect slightly alleviates, but doesnot fully explain, the tendency of r-process models to underproduce isotopes with A = 110 – 120compared to the solar-system r-process abundances.PACS numbers: 23.40.-s, 25.30.-c, 26.30.Hj, 26.50.+x, 97.60.Bw

Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-Radiation Hydrodynamics Study

We perform long-term general relativistic neutrino radiation hydrodynamics simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a torus, which is a canonical remnant formed after the binary neutron star merger. We take into account the effects of viscosity, which is likely to arise in the merger remnant due to magnetohydrodynamical turbulence. As the initial condition, we employ the azimuthally averaged data of the MNS-torus system derived in a three-dimensional, numerical-relativity simulation for the binary neutron star merger. The viscous effect plays key roles for the remnant evolution and mass ejection from it in two phases of the evolution. In the first

The Intermediate r-process in Core-collapse Supernovae Driven by the Magneto-rotational Instability

t\lesssim10

Uncertainties in s-process nucleosynthesis in massive stars determined by Monte Carlo variations

ms, a differential rotation state of the MNS is changed to a rigidly rotating state, and as a result, a sound wave, which subsequently becomes a shock wave, is formed in the vicinity of the MNS due to the variation of the quasi-equilibrium state of the MNS. The shock wave induces significant mass ejection of mass

Uncertainties in the production of p nuclei in massive stars obtained from Monte Carlo variations

\sim(0.5-2.0)\times 10^{-2}M_\odot