tokens: ejecta, we assume the nucleosynthesis pattern following the recent numerical results for the postmerger ejecta which show significantly low production of lanthanide elements and give the mass fraction of 1.64 × 10 −2 for Y (Fujibayashi et al. 2018 ). We first propose that the degree of Eu enhancement indicated by [Eu/Fe] for individual stars (−0.7 ≲ [Eu/Fe ≲ +2) could be basically determined by the different levels of Eu enrichment of gas via associated merger events (cNSNSs or cBHNSs) with widely varying masses ( M tid ) of tidally driven dynamical ejecta (10 −4 M ⊙ ≲ M tid ≲ 0.1 M ⊙ ). Considering the [Eu/Fe] distribution of halo stars, we identify two components: a Gaussian distribution in the range of −0.7 ≲ [Eu/Fe] ≲ +1.3 with a peak around [Eu/Fe] ≈ + 0.3 and an extending Eu-rich tail up to [Eu/Fe] ≈ + 2. Then, considering the predicted mass range of dynamical ejecta, cNSNS and cBHNS are likely to be responsible for producing the former and latter distributions, respectively. Approximate mass values of the mass of tidally driven dynamical ejecta are shown in the upper part of the figure, implying that the average value for cNSNSs should be a few times 10 −3 M ⊙ . If cBHNS should produce more Eu than cNSNS due to larger tidal mass ejection, the Y/Eu plateau could be connected to such events. Considering very low production of light r -process elements in the tidally dominated dynamical ejecta of cBHNSs, owing to a low Y e (≲0.1) (Roberts et al. 2017 ), we argue that Y in r-II stars originates from the postmerger ejecta. The Y/Eu plateau implies that the production ratio of Y to Eu is quite similar in individual cBHNS events. As each element owes its origin to nontidal postmerger ejecta and tidally driven dynamical ejecta, respectively, the masses of the two type of ejecta are suggested to scale linearly with each other. This linearity may also hold even if the postmerger ejecta of cBHNSs produce Eu due to a smaller Y e resulting from lower neutrino irradiation (Fernández et al. 2017 ), though there are complexities in the influence of the neutrino treatment (e.g., Caballero et al. 2012 ; Wu et al. 2017 ). The required condition of a similar mass ratio of light r -process elements to Eu among the postmerger ejecta of cBHNSs needs to be investigated (see also Kyutoku et al. 2015 ). On the other hand, the trend of an increasing [Y/Eu] with decreasing [Eu/Fe] at the low [Eu/Fe] regime is likely to reflect the diversity of nucleosynthesis in cNSNSs with various mass asymmetry in our scheme. The tidal interaction tends to be efficient for asymmetric binaries, and M tid becomes large. At the same time, shock interactions are not very efficient for dynamical mass ejection in asymmetric binaries. Thus, the average values of Y e become lower for more asymmetric binaries (e.g., Sekiguchi et al. 2016 ). These combined effects could explain the observed Y/Eu trend. According to this view, cNSNSs with massive (≳0.01 M ⊙ ) tidally driven dynamical
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