Division of Theoretical Astronomy, National Astronomical Observatory of Japan


Research Highlights

Conversion of Hadronic Matter to Quark Matter in Neutron Stars

We study transitions of hadronic matter (HM) to quark matter (QM) in neutron stars.We not only pay attention to the structures inside the conversion front by taking into account what happens during the conversion processes on the time scale of weak interactions. We demonstrated that the combustion will occur in the so-called endothermic regime and there are different characteristics of the mixed state of HM and QM and the appearance of strange quarks.We find that the laminar front is unstable in the exothermic regime but stable in the endothermic regime, which is quite contrary to the ordinary combustion. (2016/3/31)

"Hydrodynamical study on the conversion of hadronic matter to quark matter: I. Shock-induced conversion", Shun Furusawa, Takahiro Sanada, and Shoichi Yamada
Phys. Rev. D 93, 043018 [Journal] [ADS] [arXiv]

"Hydrodynamical study on the conversion of hadronic matter to quark matter: II. Diffusion -induced conversion", Shun Furusawa, Takahiro Sanada, and Shoichi Yamada
Phys. Rev. D 93, 043019 [Journal] [ADS] [arXiv]
Shun Furusawa [personal webpage]

New Model on the Origin of the Heavy Elements:
Solving the Underproduction Problem of the r-Process

The origin of heavy elements like gold and uranium, called r-process elements, is one of the 11 Greatest Unanswered Questions of the Century in Particle & Nuclear Physics and Astronomy (see Discover magazine). Shota Shibagaki and Taka Kajino in COSNAP International Consortium(link)proposed a new theoretical model that the r-process elements were produced in supernova explosions from the early Galactic evolution, followed later by the additional contribution from the binary neutron-star mergers after 100 Myr of cosmic time. In this theoretical model, a long-standing underproduction problem of the isotopic abundances above and below the typical r-process peaks around A = 130, 165 and 195 is resolved, still satisfying the universal r-process abundance pattern between the early generations of metal-deficient stars and the solar-system.
Figure: Solar-system isotopic r-prpcess abundance pattern. Observation (black dots) vs. theoretical calculation which consists of the r-process in magnetohydrodynamic jet supernova model (blue), neutrino-heated supernova model (green), binary neutron-star merger model (red) and total sum (black).

Relative contributions of the weak, main and fission-recycling r-process
S. Shibagaki, T. Kajino, G. J. Mathews, S. Chiba, S. Nishimura, and G. Lorusso, Astrophys. J. 816 (2016), 79. [Journal] [ADS] [arXiv]
Shota Shibagaki
Toshitaka Kajino (personal website)

Galactic Spiral Arms by Swing Amplification

Many galaxies have spiral arms. One of the models to explain the origin of spiral arms is swing amplification mechanism. Because of the differential rotation, the spiral arms wind gradually. Then, the self-gravity of the spiral arms becomes strong relatively. Thus, the amplitude of the spiral arms is enhanced. This mechanism is called swing amplification. The structures formed by the swing amplification has not yet been investigated quantitatively. Therefore using the theoretical analyses and N-body simulations we investigated the amplitude and the wavelengths of the spiral arms formed by the swing amplification. We found that the number of spiral arms, the pitch angle, and the amplitude relate with the velocity shear and the random velocity of the stars. We expect that we can understand the role of the swing amplification in the spiral arm formation by comparing with the observations and numerical simulations.
"Galactic Spiral Arms by Swing Amplification", Shugo Michikoshi, Eiichiro Kokubo, 2016, Astrophysical Journal, [arXiv]
Shugo Michikoshi, Eiichiro Kokubo