MATHEMATICAL SCIENCES

The researchers used the LAMOST data to discover nine rare ultra-lithium-rich dwarfs


Recently, a scientific research team led by shi Jianrong of the National Astronomical Observatory based on the LAMOST medium-resolution spectrum has discovered nine unevolved stars with extremely high lithium content, namely super-lithium-rich dwarfs. One of the ultra-lithium-rich dwarfs had a lithium content of 31 times that of the Sun (4.8 dex), which set a new record for lithium content in such stars. Prior to this, astronomers had only discovered four similar ultra-lithium-rich dwarfs. The research results are of great scientific significance in revealing the origin and evolution mechanism of lithium elements in ultra-lithium-rich dwarfs, and are also an important discovery made by LAMOST in cutting-edge basic research. The results have been published in the international astronomical journal Astrophysical Journal Letters (ApJL, 2022, 929, L14).

Lithium is a key element connecting the Big Bang, interstellar matter and stars, and its evolution in the universe and stars has always been an important topic in the field of astronomy, but the understanding of lithium in contemporary astronomy is still very limited. Previously, astronomers found that there are abnormally high levels of lithium in a very small number of stars that have evolved, that is, lithium-rich giant stars. Over the past few years, LAMOST has made a series of breakthroughs in the study of lithium-rich giant stars, discovering more than 10,000 lithium-rich giant stars, deepening human understanding of such rare objects and the evolution of lithium in giant stars. The presence of lithium-rich giants suggests that there are unknown mechanisms in the evolution of stars that can significantly alter the lithium content on the surface of stars. However, the nine peculiar ultra-lithium-rich dwarfs in the study suggest that not only giant stars, but also a very small number of dwarfs that have not yet evolved have shown abnormally high lithium content, and the lithium content of these nine ultra-lithium-rich dwarfs is at least three times higher than that in the Sun, which means that these stars may form in an environment with higher lithium content than near the Sun, or it may be some special mechanism that increases the lithium content on the surface of the star.

Schematic diagram of the Sun belonging to an unevolved yellow dwarf

Such superlim-rich dwarfs, which have not yet evolved to the giant stage, are so rare that astronomers have discovered only four in the past, so little is known about their formation. There has been great controversy about the formation mechanism of such super-lithium-rich dwarfs that have not yet evolved to giant stars, and astronomers believe that there are several possibilities: 1. atomic diffusion; 2. stars have accretized the surrounding lithium-rich material; 3. Stars are in binary star systems, and interactions with companion stars have caused abnormally high lithium content.

This time, the researchers gave full play to the advantages of lamost multi-objective medium-resolution spectral sky survey and discovered nine new ultra-lithium-rich dwarfs at once. In the course of the study, the research team considered the effect of non-local thermodynamic equilibrium effect on the determination of lithium abundance, combined with the LAMOST spectrum and the timing lighting data of the US TESS space telescope, the results showed that seven of the ultra-lithium-rich dwarfs have high rotation speeds, all of which are above 9 km per second, three of which have periodic light changes, and one is in a binary star system. These observational evidence suggests that for most superlim-rich dwarfs, the accretion of surrounding lithium-rich material may be the main mechanism for their abnormally high lithium content, but it is not excluded that a few are the result of binary interactions.

The figure shows a schematic diagram of the lithium content of nine newly discovered super-lithium-rich dwarfs

This work was led by the National Astronomical Observatory and jointly completed by Nanjing Tianguang Institute, Beijing Normal University and Peking University. (Source: National Astronomical Observatories, Chinese Academy of Sciences)

Related paper information:https://doi.org/10.3847/2041-8213/ac63a5

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