Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Research Highlights

Large-scale structure of magnetic reconnection by means of kinetic simulations

How plasma is accelarated and heated is one of the important issues in space and astro plasma physics. Magnetic reconnection is a promising phenomenon converting the magnetic field energy into plasma kinetic energy. However, the detailed processes are poorly understood. So far, there has been a significant gap between kinetic (micro-scale) model and magnetohydrodynamics (MHD) model, and how both the models are connected is not clear. The present study revealed how the microscopic process is connected to large-scale structure, by means of large-scale kinetic simulations with a state-of-the-art supercomputer. In particular, it is found that the Hall current, which is neglected in the MHD approximation, appears in broad area of the exhaust, implying that the previous MHD models are not always appropriate. The present study points out that the MHD approximation used widely in astronomical simulations is not always valid even for large-scale processes.
"Ion and electron dynamics generating the Hall current in the exhaust far downstream of the reconnection x-line", Keizo Fujimoto and Makoto Takamoto (2016), Physics of Plasmas, 23, 012903
[doi] Keizo Fujimoto [personal website]

Origin of r-process elements
in galactic chemodynamical evolution

The r-process is one of the main processes to synthesize elements heavier than iron. The r-process elements have been observed in metal-poor stars in dwarf galaxies and the Milky-Way halo, but astrophysical site(s) of r-process is not identified yet. Nucleosynthesis calculations suggest that binary neutron star mergers are the promising astrophysical site of r-process. In contrast, galactic chemical evolution studies without considering the formation process of galaxies pointed out that it is difficult to reproduce the observed r-process abundance in extremely metal-poor stars by neutron star mergers due to their long merger time and low occurrence rate. In this study, we performed a series of hydrodynamical simulations of dwarf galaxies assuming that neutron star mergers are the major astrophysical site of r-process. Our simulations reproduce the observed r-process abundance in extremely metal-poor stars by neutron star mergers with merger time of 100 Myr. In addition, we find that the metallicity is constant over ~ 300 Myr from the onset of star formation due to low star formation efficiency in dwarf galaxies. We moreover find that metal mixing in star-forming region avoids producing extremely r-process rich stars, which are inconsistent with the observation, due to the low rate of neutron star mergers. The r-process elements observed in the Milky-Way halo might originate in accreted dwarf galaxies.

"Enrichment of r-process Elements in Dwarf Spheroidal Galaxies in Chemo-dynamical Evolution Model", Yutaka Hirai, Yuhri Ishimaru, Takayuki R. Saitoh, Michiko S. Fujii, Jun Hidaka and Toshitaka Kajino
2015, The Astrophysical Journal, 814, 41 [ADS] [arXiv]
Yutaka Hirai [personal webpage]