Division of Theoretical Astronomy, National Astronomical Observatory of Japan



Research Highlights

First Global Simulation Yields New Insights into Ring System

A team of researchers in Japan modeled the two rings around Chariklo, the smallest body in the Solar System known to have rings (Figure 1). This is the first time an entire ring system has been simulated using realistic sizes for the ring particles while also taking into account collisions and gravitational interactions between the particles. The team’s simulation revealed information about the size and density of the particles in the rings. By considering both the detailed structure and the global picture for the first time, the team found that Chariklo’s inner ring should be unstable without help. It is possible the ring particles are much smaller than predicted or that an undiscovered shepherd satellite around Chariklo is stabilizing the ring.
(2017/04/28, CfCA Press Release)

Figure: Visualization constructed from simulation of Chariklo’s double ring. (Credit: Shugo Michikoshi, Eiichiro Kokubo, Hirotaka Nakayama, 4D2U Project, NAOJ)

"Simulating the Smallest Ring World of Chariklo"
Shugo Michikoshi and Eiichiro Kokubo, ApJL, 837, L13, 2017. [ApJL]
Eiichiro Kokubo

An Elegant Solution of the Big-Bang Lithium Problem?

The success of a theory of the CMB fluctuation and anisotropies and the observations of cosmic large-scale structure are thought to be a piece of evidence that the Big-Bang theory, which was first proposed by George Gamow in 1948, is the robust theoretical model of the cosmic expansion. However, this model predicts too large primordial abundance of lithium (atomic number Z=3) among all other light elements and contradicts astronomical observations of the early generations of old metal-poor stars. This discrepancy is called "the Big-Bang lithium problem" which might indicate the breakdown of the standard Big-Bang cosmology or suggest the need of a new particle-cosmological theory beyond the standard model. Elementary particles and nuclei in the hot Big-Bang expansion of the early Universe are not necessarily obey the classical Maxwell-Boltzmann distribution but some other class of non-extensive statistics. Tsallis statistics are the one of them which is presumed to describe well the thermodynamic systems showing chaos or fractals. Our international collaboration team demonstrated to solve this long standing Big-Bang lithium problem by applying Tsallis statistics to the primordial nucleosynthesis. If this "elegant solution" is correct, the Big Bang theory is now one step closer to fully describing the formation of our Universe, as referred in AAS NOVA (link).

Figure: Primordial nucleosynthesis in the early hot Big-Bang expansion and the evolution of the Universe (taken from AAS NOVA) (C) NASA.

"Non-extensive Statistics to the Cosmological Lithium Problem"
S. Q. Ho, et al. with T. Kajino 2017, Astrophys. J. 834, 165. [ApJ]
Taka Kajino

Early evolutionary histories of galaxies deduced from r-process elements

Abundances of r-process elements such as Eu and Ba in extremely metal-poor stars may reflect the early evolutionary histories of galaxies. However, how the chemo-dynamical evolution of galaxies affects the abundance of r-process elements is not yet understood. In this study, we perform a series of N-body/hydrodynamic simulations of galaxies with different density and mass. We calculate the evolution of r-process abundances in these galaxies. We find that galaxies with dynamical times around 100 Myr have star formation rates of less than 10-3 Msun/year and they reproduce the observed r-process abundances (Figure A and B). This result does not depend on the mass of halos. On the other hand, r-process elements appear higher metallicity in galaxies with dynamical times around 10 Myr (Figure C and D). We also find that galaxies with lower star formation rates need longer timescale to mix metals, resulting in larger dispersions of r-process abundance ratios. This study demonstrates that future observations of r-process elements in extremely metal-poor stars will be able to constrain the evolutionary histories of galaxies.

Figure: [Eu/Fe] as a function of [Fe/H]. Grey scale represents model prediction. From left to right, we plot models with initial dynamical times of 100 Myr (A), 70 Myr (B), 30 Myr (C and D). Model D has a different density profile with other models. Small and large dots are observed abundances in the Milky Way halo and dwarf galaxies.

“Early chemo-dynamical evolution of dwarf galaxies deduced from enrichment of r-process elements”
Yutaka Hirai, Yuhri Ishimaru, Takayuki R. Saitoh, Michiko S. Fujii, Jun Hidaka and Toshitaka Kajino
2017, Monthly Notices of Royal Astronomical Society, 466, 2474 [ADS] [arXiv]
Yutaka Hirai [personal webpage]