The rapid neutron-capture process (r-process) is a major process to synthesize elements heavier than iron, but the astrophysical site(s) of r-process is not identified yet. Neutron star mergers (NSMs) are suggested to be a major r-process site from nucleosynthesis studies. Previous chemical evolution studies however require unlikely short merger time of NSMs to reproduce the observed large starto-star scatters in the abundance ratios of r-process elements relative to iron, [Eu/Fe], of extremely metal-poor stars in the Milky Way (MW) halo. This problem can be solved by considering chemical evolution in dwarf spheroidal galaxies (dSphs) which would be building blocks of the MW and have lower star formation efficiencies than the MW halo. We demonstrate that enrichment of r-process elements in dSphs by NSMs using an N -body/smoothed particle hydrodynamics code. Our highresolution model reproduces the observed [Eu/Fe] by NSMs with a merger time of 100 Myr when the effect of metal mixing is taken into account. This is because metallicity is not correlated with time up to ∼ 300 Myr from the start of the simulation due to low star formation efficiency in dSphs. We also confirm that this model is consistent with observed properties of dSphs such as radial profiles and metallicity distribution. The merger time and the Galactic rate of NSMs are suggested to be 300 Myr and ∼ 10 −4 yr −1 , which are consistent with the values suggested by population synthesis and nucleosynthesis studies. This study supports that NSMs are the major astrophysical site of r-process.

Context. The dominant site of production of r-process elements remains unclear despite recent observations of a neutron star merger. Observational constraints on the properties of the sites can be obtained by comparing r-process abundances in different environments. The recent Gaia data releases and large samples from high-resolution optical spectroscopic surveys are enabling us to compare r-process element abundances between stars formed in an accreted dwarf galaxy, Gaia-Enceladus, and those formed in the Milky Way. Aims. Our aim is to understand the origin of r-process elements in Gaia-Enceladus. Methods. We first constructed a sample of stars so that our study on Eu abundance is not affected by the detection limit. We then kinematically selected 76 Gaia-Enceladus stars and 81 in situ stars from the Galactic Archaeology with HERMES (GALAH) DR3, of which 47 and 55 stars, respectively, can be used to study Eu reliably. Results. Gaia-Enceladus stars clearly show higher ratios of [Eu/Mg] than in situ stars. High [Eu/Mg] along with low [Mg/Fe] are also seen in relatively massive satellite galaxies such as the LMC, Fornax, and Sagittarius dwarfs. On the other hand, unlike these galaxies, Gaia-Enceladus does not show enhanced [Ba/Eu] or [La/Eu] ratios suggesting a lack of significant s-process contribution. From comparisons with simple chemical evolution models, we show that the high [Eu/Mg] of Gaia-Enceladus can naturally be explained by considering r-process enrichment by neutron-star mergers with delay time distribution that follows a power-law similar to type Ia supernovae but with a shorter minimum delay time.

The heaviest iron-peak element, Zn has been used as an important tracer of cosmic chemical evolution. Spectroscopic observations of the metal-poor stars in Local Group galaxies show that an increasing trend of [Zn/Fe] ratios toward lower metallicity. However, enrichment of Zn in galaxies is not well understood due to the poor knowledge of astrophysical sites of Zn as well as metal mixing in galaxies. Here we show possible explanations for the observed trend by taking into account electroncapture supernovae (ECSNe) as one of the sources of Zn in our chemodynamical simulations of dwarf galaxies. We find that the ejecta from ECSNe contribute to stars with [Zn/Fe] 0.5. We also find that scatters of [Zn/Fe] in higher metallicity originate from the ejecta of type Ia supernovae. On the other hand, it appears difficult to explain the observed trends if we do not consider ECSNe as a source of Zn. These results come from inhomogeneous spatial metallicity distribution due to the inefficiency of metal mixing. We find that the optimal value of scaling factor for metal diffusion coefficient is ∼ 0.01 in the shear-based metal mixing model in smoothed particle hydrodynamics simulations. These results suggest that ECSNe can be one of the contributors to the enrichment of Zn in galaxies.