Lithium is one of the light elements produced at the early universe. As one of the building blocks of our present-day universe though, the context of lithium observed in many celestial bodies often disaccord with predictions of classic theories. Lithium-rich stars, accounting for only 1% of the total number of the low-mass evolved stars, is one example of such conflict. They preserve up to thousands of times more lithium than the normal stars that account for the rest 99%. Astronomers are wondering what these stars really are and why.
Fig1. Astronomers reveal the secrets of the lithium-rich low-mass evolved stars by astroseismology and analyzing their spectra.
A recent study revealed the real status of lithium-rich stars by the Chinese astronomers is published in the Nature Astronomy on Oct.5. Yutao Zhou, the postdoc from the department of astronomy, is involved in this study as the co-first author. By combining the LAMOST with the Kepler, they find that most lithium-rich stars locate at the evolutionary status of red clump instead of the red giant branch as previously thought. The findings challenge the corresponding stellar theories, which is vital to resolve the problem of lithium origin.
Fig2. The evolution of low-mass stars. The evolutionary track is followed with the yellow arrows. The temperature and luminosity of red clump stars are indistinguishable from some red giants.
The red giant and the red clump are the different statuses after the main sequence, their internal structure and physical process are significantly different because of the distinctive nuclear reactions in the their interiors. However, it is really hard to tell which status the star is by the external characteristic or the H-R diagram.
Previous observations suggested that the lithium is produced in the star at the red giant branch, many researches focused on the extra-mixing processes of the red giant star, trying to settle the issue of lithium enhancement. The skeptical research team obtained a quite distinct result with the help of the asteroseismic information from Kepler and the stellar spectra from LAMOST, Subaru, etc. Because of the different internal oscillation modes, the red giant star and the red clump star can be distinguished by the asteroseismology, though they look alike on the features of luminosity and effective temperature. The research shows that over 80% of lithium-rich stars are the red clump star. Moreover, the lithium-rich stars at these two statuses present different distributions in the lithium abundance, the stellar mass, nitrogen abundance, etc, which contradict the previous studies.
All of these signatures are hard to explain using the current scenarios. The lithium-rich scenarios of red giant star should not be appropriated for the red clump star due to the different internal structures. A few theories were proposed to account for this issue, including helium flash, binary-merger, extra-mixing, etc. The research finds that the binary-merger is a potential scenario by verifying its predictions with the observational results. To check the other theories is a further step to the truth of lithium-rich secret.
The research is done by an international team with scientists from institutes worldwide, including China, Japan, France, Netherlands, United States, Australia, and Denmark. The involved Chinese institutes include National Astronomical Observatories of CAS, Peking University, Beijing Normal University and Yunnan Observatories. Besides LAMOST, Subaru, and Kepler, four other telescopes contributed the data, including the 3.5-meter telescope at Apache Point Observatory, the Automated Planet Finder telescope at Lick Observatory, the 2.4-meter and 1.8-meter telescope at Lijiang Observatory.
Publication link: https://www.nature.com/articles/s41550-020-01217-8
