Tracking the evolution of lithium in giants using asteroseismology: Super-Li-rich stars are almost exclusively young red-clump stars

Abstract

We report novel observational evidence on the evolutionary status of lithium-rich giant stars by combining asteroseismic and lithium abundance data. Comparing observations and models of the asteroseismic gravity-mode period spacing ∆Π 1 , we find that super-Li-rich giants (SLR, A(Li) > 3.2 dex) are almost exclusively young red-clump (RC) stars. Depending on the exact phase of evolution, which requires more data to refine, SLR stars are either (i) less than ∼ 2 Myr or (ii) less than ∼ 40 Myr past the main core helium flash (CHeF). Our observations set a strong upper limit for the time of the inferred Li-enrichment phase of < 40 Myr post-CHeF, lending support to the idea that lithium is produced around the time of the CHeF. In contrast, the more evolved RC stars (> 40 Myr post-CHeF) generally have low lithium abundances (A(Li) < 1.0 dex). Between the young, super-Li-rich phase, and the mostly old, Li-poor RC phase, there is an average reduction of lithium by about 3 orders of magnitude. This Li-destruction may occur rapidly. We find the situation to be less clear with stars having Li abundances between the two extremes of super-Li-rich and Li-poor. This group, the 'Li-rich' stars (3.2 > A(Li) > 1.0 dex), shows a wide range of evolutionary states.

The Gaia-ESO survey: Mixing processes in low-mass stars traced by lithium abundance in cluster and field stars

“…The recent discovery of large samples of Li-rich giants indicates that they are not restricted just to the luminosity bump on the RGB for the low-mass stars, or its equivalent on the early-AGB for intermediate mass stars that ignite central He burning in nondegenerate conditions (Charbonnel & Balachandran 2000), but they are also found along the RGB and in the red clump (see, e.g Alcalá et al 2011;Kumar et al 2011;Lebzelter et al 2012;Carlberg et al 2016;Smiljanic et al 2018;Deepak & Reddy 2019;Charbonnel et al 2020;Martell et al 2020;Yan et al 2021). The works of ; Yan et al (2021); Singh et al (2021); Deepak & Lambert (2021), combining the results from asteroseismic and spectroscopic surveys, suggested that a high fraction of the Li-rich giants belong to the red clump central He-burning phase, confirming a previous idea presented by Kumar et al (2011). Similar results were obtained by Casey et al (2019) where 80% of their sample stars have likely helium-burning cores.…”