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S19B0084abst

S19B0084

Recent studies revealed that there is a population of halo stars on extreme retrograde orbits with high orbital energies and that they have lower α-element abundances on average than the previously known two dominant and distinct halo populations. However, the precision of previous abundance measurements is not sufficiently high to show its distinctness in the chemical abundance space and thus it has been impossible to separate the population from the other two. We here propose HDS observation to obtain first high-precision chemical abundances for kinematically-selected ∼15 candidates of the population through application of a strictly line-by-line differential abundance analysis on high-quality high-resolution spectra. We will identify individual group members using the precise Mg abundances and construct a pure sample of the new population. The origin of the population will be tightly constrained through comparisons between pure samples of different populations in various parameter spaces including neutron-capture element abundances, ages, and kinematics. This study will be the first attempt to extend the study of past high-precision abundance analysis of halo stars over a wider range of stellar kinematics, and provide us with a new insight of the assembly history of the Galactic halo.

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