Recent chemical abundance studies for Milky Way halo stars have been revealing the existence of at least two populations, having different abundance ratios like [α/Fe]. Differences of formation time scale between these two populations are expected from the chemo-dynamical models of the Galaxy formation. Although stellar ages between the two populations in the Solar neighborhood are, in fact, different, the direct measurements of stellar ages for distant stars are very difficult, preventing us from getting a global understanding of the Milky Way halo. Observations by the Kepler enabled us to infer stellar ages for red giants, by providing their masses from stellar oscillations through asteroseismology. Thanks to the recent multi-object spectroscopy with LAMOST, tens of candidates of halo stars in the Kepler field have been identified from metallicity and radial velocity. We propose observation of ∼ 30 candidates with the Subaru/HDS to examine the age difference between the two populations in the halo. High-resolution high-S/N spectra provide precise elemental abundances to divide the sample into two populations. Our observation will provide high-resolution optical spectra of a large number of halo stars to which asteroseismology is applied for the first time.
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