Program: GN-2021A-FT-205

Very metal-poor (VMP, [Fe/H] < -2.0) stars are relics of the formation and evolution of the Galaxy. The classical understanding is that these VMP stars should be distributed throughout the Galactic halo as they are expected to be the remnants of the chaotic early stages of hierarchical assembly of the Milky Way. However, independent efforts have recently demonstrated that a significant fraction (20-30%) of all VMP stars are confined to the Galactic plane, and mostly on strongly prograde orbits indistinguishable from the canonical thick disk. Hence, many questions have emerged. Have these VMP stars been brought into the Milky Way alongside its primordial building blocks? If so, how could they sustain their disk-like kinematics throughout the entire Galactic evolution? Has the accretion history of the Galaxy been exceptionally quiet? Or could later merging events explain the origin of this VMP population instead? In order to answer these questions, we propose high-resolution spectroscopy with GRACES (at Gemini North) of a large sample of VMP stars associated with the Galactic disk. For this Fast Turnaround project, we have selected 5 very bright (V < 13) stars, vetted as genuine VMP ones from our recent analysis of their GMOS spectra. These observations will constitute a pilot study for a future to-be-proposed long/large program. The detailed elemental-abundance profiles of these ancient stars, compared with those from surviving dwarf satellite galaxies and chemical evolution models, will certainly provide valuable constraints to the formation, evolution, and structure of the Milky Way.