Enormous Li-enhancement preceding red giant phases in low-mass stars in the Milky Way halo

Abstract: Li abundances in the bulk of low-mass metal-poor stars are well reproduced by stellar evolution models adopting a constant initial abundance. However, a small number of stars have exceptionally high Li abundances, for which no convincing models have been established. We report on the discovery of 12 very metal-poor stars that have large excesses of Li, including an object having more than 100 times higher Li abundance than the values found in usual objects, which is the the largest excess in metal-poor stars known to date. The sample is distributed over a wide range of evolutionary stages, including five unevolved stars, showing no abundance anomaly in other elements. The results indicate the existence of an efficient process to enrich Li in a small fraction of low-mass stars at the main-sequence or subgiant phase. The wide distribution of Li-rich stars along the red giant branch could be explained by dilution of surface Li by mixing that occurs when the stars evolve into red giants. Our study narrows down the problem to be solved to understand the origins of Li-excess found in low-mass stars, suggesting the presence of unknown process that affects the surface abundances preceding red giant phases.