Kenzo Arai

Nucleosynthesis inside an accretion disk and disk winds related to gamma-ray bursts

We investigate nucleosynthesis inside both a gamma-ray burst accretion disk and a wind launched from an inner region of the disk using one-dimensional models of the disk and wind and a nuclear reaction network. Far from a central black hole, the composition of accreting gas is taken to be that of an O-rich layer of a massive star before core collapse. We find that the disk consists of five layers characterized by dominant elements: O16, Si28, Fe54 (and Ni56), He4, and nucleons, and the individual layers shift inward with keeping the overall profiles of compositions as the accretion rate decreases. Ni56 are abundantly ejected through the wind from the inner region of the disk with the electron fraction \simeq 0.5. In addition to iron group, elements heavier than Cu, in particular Cu63 and Zn64, are massively produced through the wind. Various neutron-rich nuclei can be also produced in the wind from neutron-rich regions of the disk, though the estimated yields have large uncertainties.We investigate nucleosynthesis inside both a gamma-ray burst accretion disk and a wind launched from an inner region of the disk using one-dimensional models of the disk and wind. Far from a central black hole, the composition of accreting gas is taken to be that of an Si-rich or O-rich layer of a massive star before core collapse. We find that the inner region of the disk comprises five layers characterized by dominant elements: 16O, 28Si, 54Fe (and 56Ni), 4He, and nucleons. As the accretion rate decreases, the individual layers shift inward, retaining the overall profiles of compositions. A massive amount of 56Ni, over 0.1 M☉, is ejected through the wind from the disk where the electron fraction 0.5. The amount of 56Ni produced through the disk wind can be responsible for the light curves observed at the late stage in hypernovae (HNe). The yields of elements heavier than Ca produced via the disk wind are comparable to or greater than those of a normal supernova and their composition is similar to that of an aspherical HN explosion. A significant amount of Ga, Ge, and Se is ejected from the disk. A variety of neutron-rich elements, as well as p-nuclei, are also appreciably produced through the disk wind.

Energy extraction from a rotating black hole by magnetic reconnection in the ergosphere

We investigate mechanisms of energy extraction from a rotating black hole in terms of negative energy at infinity. In addition to the Penrose process through particle fission, the Blandford-Znajek mechanism by magnetic tension, and the magnetohydrodynamic Penrose process, we examine energy extraction from a black hole caused by magnetic reconnection in the ergosphere. The reconnection redistributes the angular momentum efficiently to yield the negative energy at infinity. We derive a condition for the process to operate in a simple situation, where the plasma is incompressible and the magnetic energy is converted completely to the plasma kinetic energy locally. Astrophysical situations of magnetic reconnection around the black holes are also discussed.

Effects on the temperatures of a variable cosmological term after recombination

The thermal evolution of the universe with a variable cosmological term λ = λ10 + λ20a-m is investigated, where a is the scale factor. Energy densities of the radiation and matter are included to examine numerically the thermal evolution from the primordial nucleosynthesis to the matter-dominant era. It is found that the radiation temperature predicted from the variable λ models is lowered significantly compared with the constant λ model if λ20 0.0002 and z 104, which implies that the matter temperature decreases drastically. We stress that the strong decreases in both the radiation temperature and matter temperature affect the formation of the first objects at 10 z 1000.

Effects of a decaying cosmological term on the formation of molecules and first objects

We investigate formation of molecules and first objects in a model with a decaying cosmological term (DCT). In particular, we show how the formation of important molecules such as H2 and HD is influenced during the epoch of the redshift z from 104 to 1. We find that for a phenomenological DCT molecular formation is significantly shifted to the earlier epoch by Δz ~ 103 compared to the case in the standard big bang cosmology, as a result of the rapid decrease in the matter temperature. The mass of first objects is estimated using the timescales relevant to their formation. We suggest that the formation scenarios of the objects studied so far could be revised if a DCT is taken into account.

Big-Bang nucleosynthesis: Constraints on nuclear reaction rates, neutrino degeneracy, inhomogeneous and Brans-Dicke models

We review the recent progress in the Big-Bang nucleosynthesis which includes the standard and nonstandard theory of cosmology, effects of neutrino degeneracy, and inhomogeneous nucleosynthesis within the framework of a Friedmann model. As for a nonstandard theory of gravitation, we adopt a Brans–Dicke theory which incorporates a cosmological constant. We constrain various parameters associated with each subject.

Big-Bang Nucleosynthesis in comparison with observed helium and deuterium abundances: possibility of a nonstandard model

Comparing the latest observed abundances of 4He and D, we make a ?2 analysis to see whether it is possible to reconcile primordial nucleosynthesis using up-to-date nuclear data of NACRE II and the mean-life of neutrons. If we adopt the observational data of

On the Role of 14N in Helium Burning Phase in Stellar Evolution

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Brans-Dicke model constrained from the Big Bang nucleosynthesis and magnitude redshift relations of supernovae

He by Izotov et al., we find that it is impossible to get reasonable concordance against the standard Big-Bang nucleosynthesis. However, including degenerate neutrinos, we have succeeded in obtaining consistent constraints between the neutrino degeneracy and the baryon-to-photon ratio from detailed comparison of calculated abundances with the observational data of

Nucleosynthesis inside accretion disks around intermediate-mass black holes

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The revised form of characteristic actions ℏ(s) in the structure of the Universe

He and D: the baryon-to-photon ratio in units of