Recently, researchers from the Yunnan Observatories have made progress in understanding the formation mechanism of lithium-rich and super lithium-rich red clump (RC) stars. This work provides a new physical perspective to explain the mechanism of lithium (Li) enrichment in Li-rich giants.
The Li enrichment behavior of giants was first observed in 1982, but its physical origin remains a topic of great interest in astronomy. This phenomenon challenges existing theories of stellar structure and evolution. The standard model predicts that the upper limit of giant Li abundance is around 1.5 dex, yet a large number of red giants above this threshold have been observed, with the highest Li abundance reaching about 6.0 dex. In recent years, spectral observations by LAMOST, GALAH, and Gaia-ESO have shown that the proportion of red giants with anomalous Li abundances is about 1%, but these stars do not exhibit significant differences from normal stars in terms of mass, metallicity, effective temperature, and other basic parameters. This suggests that there is a general Li enrichment mechanism inside giants.
Observational evidence indicates that the vast majority of Li-rich giants (with Li abundances greater than 1.5 dex) are RC stars that have experienced helium flashes and are in the core-helium-burning phase. In contrast to red giant branch stars, which are in the shell-hydrogen-burning phase, this particular phase of helium flash is often regarded as the key to Li enrichment in RC stars.
To explain the formation mechanism of Li-rich RC stars, the researchers considered the meridional circulation prevalent in the interiors of rotating stars. The ability of the mixing process inside the star to transport elements is characterized by solving the diffusion coefficient. Given the limitations of the diffusion coefficients traditionally derived from the diffusion equations of microscopic elements, they further derived relevant diffusion coefficients and normalized the radial mixing region of the meridional circulation according to the mass changes caused by large-scale meridional circulation in the radial direction of the star. By simulating stellar evolution, they found that the maximum Li enrichment increment of the meridional circulation model is about 3 dex, which promotes the formation of Li-rich (and super Li-rich) RC stars. The Li enrichment stage mainly occurs in the late helium flash period or the early core-helium-burning period, with a Li enrichment timescale of about 106 yr, consistent with current asteroseismological observations. Additionally, their simulation results confirm the observed behavior of highly rotating stars having higher Li abundances.
This work, by LI Xuefeng of the Stellar Physics Research Group and collaborators, was recently published in The Astrophysical Journal Letters. The research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the Basic Science Center of the Foundation, the National Natural Science Foundation of China, and the National Key R&D Program of China.
Contact:
LI Xuefeng
Yunnan Observatories, CAS
e-mail: lixuefeng@ynao.ac.cn
