A Helium-flash-induced Mixing Event Can Explain the Lithium Abundances of Red Clump Stars
23 Sep 2020This research was featured in AAS Nova.
The Astrophysical Journal Letters, Volume 901, Issue 1, id. L18, 5 pp.
Josiah Schwab
Observations demonstrate that the surface abundance of ${}^{7}\mathrm{Li}$ in low-mass stars changes dramatically between the tip of the red giant branch and the red clump. This naturally suggests an association with the helium core flash, which occurs between these two stages. Using stellar evolution models and a simple, ad hoc mixing prescription, we demonstrate that the ${}^{7}\mathrm{Li}$ enhancement can be explained by a brief chemical mixing event that occurs at the time of the first, strongest He subflash. The amount of ${}^{7}\mathrm{Be}$ already present above the H-burning shell just before the flash, once it mixes into the cooler envelope and undergoes an electron capture converting it to ${}^{7}\mathrm{Li}$ , is sufficient to explain the observed abundance at the red clump. We suggest that the excitation of internal gravity waves by the vigorous turbulent convection during the flash may provide a physical mechanism that can induce such mixing.