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

Yutaka Hirai,Takayuki R Saitoh,Michiko S Fujii,Jun Hidaka,Yuhri Ishimaru,Toshitaka Kajino,W.-P Liu,Z.-H Li,B Guo,Y.-P Shen,Y.-B Wang

doi:10.1051/epjconf/201610902001

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.

Highlights

Astrophysical site(s) of rapid neutron-capture process (r-process) is one of the open questions throughout half a century
Of the star formation rate are consistent with the observed dwarf spheroidal galaxies (dSphs)
Detailed chemo-dynamical properties of dSph models are discussed in Hirai et al (2015) [26]

Summary

Introduction

Astrophysical site(s) of rapid neutron-capture process (r-process) is (are) one of the open questions throughout half a century. Astronomical high dispersion spectroscopy observations show that extremely metal-poor stars with [Fe/H] −3 have large star-to-star scatters in the ratio of r-process elements to iron [e.g. 1–3]. This observational feature gives us clues to understanding astrophysical site(s) of r-process. Core-collapse supernovae (CCSNe) and neutron star mergers (NSMs) are candidates of astrophysical site(s) of r-process. CCSNe have long been suggested as a promising site of r-process [e.g., 4–7]. Recent studies suggest that it is difficult to find conditions to synthesize r-process elements with mass number heavier than 110 [e.g., 8–11]

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Conclusion