Recently, a team of researchers from the Yunnan Observatories, National Astronomical Observatories, CAS, and South-Western Institute For Astronomy Research, YNU, has made a progress in understanding the formation and evolution of lithium-rich giants. The work was published online in the Monthly Notices of the Royal Astronomical Society. Authors of the paper include LI Xuefeng, Prof. SHI Jianrong, Prof. LI Yan, Prof. YAN Hongliang, and Dr. ZHANG Jinghua.

Lithium-rich giants, which make up about one percent of all giants, have lithium abundances above 1.5 dex. This is not predicted by standard models of stellar evolution, and the physical reason for this anomalous lithium enrichment behaviour of red giants has remained unclear.

The team’s research predicts the formation of lithium-rich giants by convective mixing. During the first dredge-up phase, the surface convective zone of the star expands into the interior, and its convective boundary reaches the region of uneven chemical composition left by the main sequence hydrogen burning phase. The researchers propose that the convective boundary should be re-selected using the Ledoux criterion, adopting different convective boundaries at different evolutionary stages.

The team studied the evolutionary behaviour of the lithium abundance in red giants using a convective model defined by the Ledoux criterion. They found that the convective model can suppress the dilution of lithium by the surface convective zone in the first dredge-up stage, thus showing the characteristics of lithium enrichment.

Modelling results from the parametric grid of population I stars show that most stars can naturally evolve into lithium-rich red giants through convective mixing. In addition, the results of the convective model do not depend much on stellar parameters, so it can be used as a basic model.

However, only about one percent of red giants are rich in lithium. The researchers propose to match the observations of a small number of lithium-rich giants based on the convection model by taking into account processes such as rotation, diffusion and thermohaline mixing that can cause lithium depletion.

This work was funded by the National Natural Science Foundation of China (NSFC) and the Foundation's Basic Science Centre Project, among others.

Contact:
LI Xuefeng
Yunnan Observatories, CAS
e-mail: lixuefeng@ynao.ac.cn