Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Research Highlights

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]

Pitch Angle of Self-Gravity Wakes in Saturn's rings

In the main rings of Saturn, small spiral structures called self-gravity wakes were discovered. They are inclined with respect to the tangential direction. Observationally it is known that the pitch angle of self-gravity wakes depends on the Saturnocentric distance. However its dependence has not yet been understood theoretically. Thus, we performed N-body simulations of self-gravity wakes and investigated how the properties of the self-gravity wakes depend on the Saturnocentric distance. We found that the pitch angle increases with the Saturnocentric distance in A ring. This is consistent with the observational study. We proposed the simple model and found that the dependence can be explained by considering the self-gravity and tidal interactions.
"Dynamics of Self-Gravity Wakes in Dense Planetary Rings I. Pitch Angle", Shugo Michikoshi, Akihiko Fujii, Eiichiro Kokubo, and Heikki Salo, Astrophysical Journal, 812, 151 [ADS] [arXiv
Shugo Michikoshi, Eiichiro Kokubo



3rd DTA Symposium:
The Origins of Planetary Systems: from the Current View to New Horizons

2015 June 1 (Mon) - June 4 (Thu)
NAOJ Mitaka, Large seminar room
webpage: http://th.nao.ac.jp/meeting/dta2015a_planet/