S21B0067

Four prominent kinematic substructures have been found among halo stars in the solar neighborhood. We here propose to obtain high-precision chemical abundance of stars for one of them, Thamnos. Its stars are tightly bounded in the Milky Way gravitational potential, which is in clear contrast to surviving low-mass satellite galaxies, and thus potentially offering a unique opportunity to investigate the environmental effect on the evolution of low-mass stellar systems. However, it is still unclear whether Thamnos is really a single low-mass disrupted galaxy or a part of another kinematic substructure Gaia-Enceladus. Our proposed observation aims to compare chemical abundance of Thamnos stars with those of Gaia-Enceladus stars with high-precision. As we have shown in our previous closely-related programs, such high-precision chemical abundance comparison will allow us to separate Thamnos stars from Gaia-Enceladus if they have different progenitor galaxies. Such high-precision abundances are not collected for sufficient number of Thamnos stars by large spectroscopic surveys, such as APOGEE or GALAH, and can only be obtained by observing a carefully-selected sample stars for long exposure time with 8m class telescopes.

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