Astrophysical site(s) of r-process elements in galactic chemodynamical evolution model

¹ Department of Astronomy, Graduate School of Science, The University of Tokyo,Research Fellow of Japan Society for the Promotion of Science, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
² Division of Theoretical Astronomy, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³ Department of Material Science, International Christian University, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
⁵ School of Science and Engineering, Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-0042, Japan

^b e-mail: yutaka.hirai@nao.ac.jp

Published online: 12 February 2016

Abstract

Astrophysical site(s) of rapid neutron-capture process (r-process) is (are) not identified yet. Although core-collapse supernovae have been regarded as one of the possible candidates of the astrophysical site of r-process, nucleosynthesis studies suggest that serious difficulties in core-collapse supernovae to produce heavy elements with mass number of ≳110. Recent studies show that neutron star mergers (NSMs) can synthesize these elements due to their neutron rich environment. Some chemical evolution studies of the Milky Way halo, however, hardly reproduce the observed star-to-star scatters of the abundance ratios of r-process elements (e.g., Eu) in extremely metal-poor stars. This is because of their low rate (∼ 10⁻⁴ yr⁻¹ for a Milky Way size galaxy) and long merger time (≳ 100 Myr). This problem might be solved if the stars in the Galactic halo are consisted of the stars formed in dwarf galaxies where the star formation efficiencies were very low. In this study, we carry out numerical simulations of galactic chemo-dynamical evolution using an N-body/smoothed particle hydrodynamics code. We construct detailed chemo-dynamical evolution model for the Local Group dwarf spheroidal galaxies (dSphs) assuming that the NSMs are the major source of r-process elements. Our models successfully reproduce the observed dispersion in [Eu/Fe] as a function of [Fe/H] if we set merger time of NSMs, ≲ 300 Myr with the Galactic NSM rate of ∼ 10⁻⁴ yr⁻¹. In addition, our results are consistent with the observed metallicity distribution of dSphs. In the early phase (≲1 Gyr) of galaxy evolution is constant due to low star formation efficiency of dSphs. This study supports the idea that NSMs are the major site of r-process nucleosynthesis.