Shin Ichirou Fujimoto

Magnetohydrodynamic simulations of a rotating massive star collapsing to a black hole

We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of the collapse of a rotating star of 40 M☉ in light of the collapsar model of gamma-ray bursts. Considering two distributions of angular momentum, up to ~1017 cm2 s-1, and the uniform vertical magnetic field, we investigate the formation of an accretion disk around a black hole and the jet production near the black hole. After material reaches the black hole with high angular momentum, the disk forms inside a surface of weak shock. The disk reaches a quasi-steady state for stars whose magnetic field is less than 1010 G before the collapse. We find that the jet can be driven by the magnetic fields even if the central core does not rotate as rapidly as previously assumed as long as the outer layers of the star have sufficiently high angular momentum. The magnetic fields are chiefly amplified inside the disk due to the compression and the wrapping of the field. The fields inside the disk propagate to the polar region along the inner boundary near the black hole through the Alfven wave and eventually drive the jet. The quasi-steady disk is not an advection-dominated disk but a neutrino cooling-dominated one. Mass accretion rates in the disks are greater than 0.01 M☉ s-1 with large fluctuations. The disk is transparent for neutrinos. The dense part of the disk, which is located near the black hole, emits neutrinos efficiently at a constant rate of <8 × 1051 ergs s-1. The neutrino luminosity is much smaller than those from supernovae after the neutrino burst.

Heavy-element nucleosynthesis in a collapsar

We have made detailed calculations of the composition of magnetically driven jets ejected from a collapsar, based on long-term, magnetohydrodynamic simulations of a rapidly rotating, massive (40 M☉) star during core 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 two large nuclear reaction networks. We find that the r-process successfully operates in the jets, so that U and Th are synthesized abundantly when the progenitor has a large magnetic field (1012 G) and a rapidly rotating core. The abundance pattern inside the jets is similar to that of the r-elements in the solar system. About 0.01 M☉ of heavy, neutron-rich nuclei can be ejected from the collapsar. The detailed abundances depend on the nuclear properties of the mass model, β-decay rate, and fission, for nuclei near the neutron drip line. Furthermore, we find that p-nuclei are produced without seeds: not only can light p-nuclei, such as 74Se, 78Kr, 84Sr, and 92Mo, be abundantly synthesized in the jets, but also heavy p-nuclei, 113In, 115Sn, and 138La. The amounts of p-nuclei in the ejecta are much greater than those in core-collapse supernovae. In particular, 92Mo, 113In, 115Sn, and 138La, which are deficient in these supernovae, are produced significantly in the collapsar ejecta.

p-process nucleosynthesis inside supernova-driven supercritical accretion disks

We investigate p-process nucleosynthesis in a supercritical accretion disk around a compact object of 1.4 M☉, using the self-similar solution of an optically thick advection-dominated flow. Supercritical accretion is expected to occur in a supernova with fallback material accreting onto a newborn compact object. It is found that an appreciable number of p-nuclei are synthesized via the p-process in supernova-driven supercritical accretion disks (SSADs) when the accretion rate = c2/(16LEdd) > 105, where LEdd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have features similar to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of protons through Rayleigh-Taylor instability during the explosion, significant amounts of 92Mo are produced inside the SSADs. Isotopes 96Ru and 138La are also produced when the fallback gas contains abundant protons, although the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to the chemical evolution of p-nuclei, in particular 92Mo, if several percent of the fallback matter are ejected via jets and/or winds.

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.

Heavy element production in inhomogeneous big bang nucleosynthesis

We present a new astrophysical site of the big bang nucleosynthesis (BBN) that are very peculiar compared with the standard BBN. Some models of the baryogenesis suggest that very high baryon density regions were formed in the early universe. On the other hand, recent observations suggest that heavy elements already exist in high red-shifts and the origin of these elements become a big puzzle. Motivated by these, we investigate BBN in very high baryon density regions. BBN proceeds in proton-rich environment, which is known to be like the p-process. However, by taking very heavy nuclei into account, we find that BBN proceeds through both the p-process and the r-process simultaneously. P-nuclei such as {sup 92}Mo, {sup 94}Mo, {sup 96}Ru, {sup 98}Ru whose origin is not well known are also synthesized.

Reply to “Comment on ‘Heavy element production in inhomogeneous big bang nucleosynthesis’”

This is a reply to Rauscher [Phys. Rev. D 75, 068301 (2007)]. We studied heavy element production in the high baryon density region in the early universe [Phys. Rev. D 72, 123505 (2005)]. However, it is claimed by Rauscher [Phys. Rev. D 75, 068301 (2007)] that a small scale but high baryon density region contradicts observations for the light element abundance or, in order not to contradict the observations, the high density region must be so small that it cannot affect the present heavy element abundance. In this paper, we study big bang nucleosynthesis in the high baryon density region and show that in certain parameter spaces it is possible to produce enough of the heavy element without contradiction to cosmic microwave background and light element observations.

P-process nucleosynthesis inside supernova-driven supercritical accretion disks

Abstract We investigate p-process nucleosynthesis in a supercritical accretion disk around a compact object of 1.4 M ⊙ . Supercritical accretion is expected to occur in a supernova (SN) with fallback material accreting onto a nascent compact object. We find that appreciable amounts of p-nuclei are synthesized via the p-process in the disk when the accretion rate M > 1.6 × 10 6 L Edd c 2 , where L Edd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSASs have similar features to those of oxygen/neon layers in the Type II SNe when composition of the fallback gas is initially those of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in SSADs as in Type II SNe. If the fallback gas is mixed with a small fraction of proton, significant amounts of 92 Mo and 138 La are produced inside SSADs. 96 Ru, 113 Sn, and 114 Sn are also abundantly produced when the fallback gas contains abundant proton though the overall abundance profile of p-nuclei is rather different from that of the solar system.

Nucleosynthesis inside accretion disks around intermediate-mass black holes

Abstract Nucleosynthesis is investigated inside an accredition disk around a black hole of 50 – 10 3 M ⊙ located in a circumnuclear region of starburst galaxies. Nuclear reactions proceed through the hot CNO cycle to produce large amounts of 14 O and 15 O, which decay to 14 N and 15 N. The calculated N/O − O/H relation is found to be consistent with that observed in the most metal-poor galaxies. The accretion disk may be one of the promising sites of nitrogen production.

Heavy element nucleosynthesis in jets from collapsars

We investigate nucleosynthesis in collapsars, based on long‐term, magnetohydrodynamic simulations of a rapidly rotating massive star of 40M⊙ during the core collapse. We have calculated detailed composition of magnetically driven jets ejected from the collapsars, in which the magnetic fields before the collapse, are uniform and parallel to the rotational axis of the star and the magnitudes of the fields, B0, are 1010 G or 1012 G. We follow the evolution of chemical composition up to about 4000 nuclides inside the jets from the collapse phase to the ejection phase through the jet generation phase with use of a large nuclear reaction network. We find that the r‐process successfully operates in the jets from the collapsar of B0 = 1012 G, so that U and Th are synthesized abundantly. Abundance pattern inside the jets is similar to that of r‐elements in the solar system. Furthermore, we find that p‐nuclei are produced without seed nuclei: not only light p‐nuclei, such as 74Se, 78Kr, 84Sr, and 92Mo, but also hea...

Nucleosynthesis inside Magnetically‐Driven Jets in A Gamma‐Ray Burst

We investigate nucleosynthesis inside jets in a gamma ray bursts (GRB). We have calculated detailed composition of magnetically driven jets related to GRBs with post‐processing calculation, which is based on long‐term, magneto‐hydrodynamic simulations of a rapidly rotating massive star of 40M⊙ during core collapse. We follow abundance evolution of about 4000 nuclides inside the jets from the collapse phase to the ejection phase through the jet generation phase with a large nuclear reaction network. We find that the r‐process successfully operates inside the jets, so that U and Th are synthesized abundantly. Abundance pattern inside the jets is similar compared to that of r‐elements in the solar system. Heavy neutron‐rich nuclei ∼ 0.001M⊙ can be ejected through the jets. Furthermore, we find that p‐nuclei are produced without seed nuclei: not only light p‐nuclei, such as 74Se, 78Kr, 84Sr, and 92Mo, but also heavy p‐nuclei, 113In, 115Sn, and 138La, can be abundantly synthesized in the jets. The amounts of p...