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 $\Delta {\Pi }_1$, 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 $\sim 2$~Myr or (ii) less than $\sim 40$~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.

arXiv comment9 pages, 5 figures, 1 table, accepted in Astrophysical Journal Letters

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