Big Band Nucleosynthesis (BBN) has produced three light elements: hydrogen, nitrogen and lithium. Abundances of these elements are the most reliable way to explore the physical condition in the first few seconds after the BBN. The stellar abundance group at the National Astronomical Observatories collaborated with Japanese astronomers, discovered a dozen super Li-rich metal-poor stars, using LAMOST data and follow-up observations. Their discovery poses challenge to the standard evolution model for low-mass stars, and has been recently published on the Astrophysical Journal Letters.
The atmosphere of low-mass old stars preserves the primordial material produced by the BBN, and thus during their early stage (before the red giant), they tend to have a constant Li abundance. Along with their evolution, the dredge-up procedure will bring up the internal material and mix with the surface, which will significantly reduce the content of surface Li up to more than the order of one magnitude. Systematic observations on globular clusters have proved such evolution, and supported the success of stellar structure and evolution model. With decades of observations on stars, a dozen of Li-rich low-mass red giants were discovered, which could not be well explained by current modeling, while there is neither any systematic observation.
The NAOC team performed the first systematic searching on Li-rich metal-poor stars with LAMOST and follow-up observations with Subaru Telescope. They found twelve metal-poor stars which have less than 1% solar metallicity but 10 times more Li than similar type of stars. These stars are with 0.8 solar mass or so, and much older than previously discovered Li-rich stars. More surprisingly, five of them are sub-giants, i.e., at the evolutionary stage before red giants. This is the first discovery of such kind of stars in the Galactic field, and moreover, one of them contains more than 100 times of Li compared to similar stars, which becomes the new record of Li abundance in stars. Their research provides crucial evidence to improve the mechanism of Li enrichment in low-luminosity stars, as well as the classical stellar evolution model of low-mass stars. Science News has also made a first time report on this important discovery.
Figure 1. Distribution of Li abundance in metal-poor stars. Grey and blue symbols refer to literature observations of field stars, and green to globular cluster. Red dots are LAMOST discovery.