  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.
