In this letter, we report the discovery of 24 new super Li-rich (A(Li) ≥ 3.2) giants of He-core burning phase at red clump region. Results are based on systematic search of a large sample of about 12,500 giants common to the LAMOST spectroscopic and Kepler time resolved photometric surveys. The two key parameters derived from Kepler data; average period spacing (∆p) between l = 1 mixed gravity dominated g-modes and average large frequency separation (∆ν) l = 0 acoustic p-modes, suggest all the Li-rich giants are in He-core burning phase. This is the first unbiased survey subjected to a robust technique of asteroseismic analysis to unambiguously determine evolutionary phase of Li-rich giants. The results provide a strong evidence that Li enhancement phenomenon is associated with giants of Hecore burning phase, post He-flash, rather than any other phase on RGB with inert He-core surrounded by H-burning shell.
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 ΔΠ1, we find that super-Li-rich giants (SLRs, 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.
In this paper we report two new super Li-rich K giants: KIC 2305930 and KIC 12645107 with Li abundances exceeding that of ISM (A(Li) ≥ 3.2 dex). Importantly, both the giants have been classified as core He-burning red clump stars based on asteroseismic data from Kepler mission. Also, both the stars are found to be low mass (M ≈ 1.0 M ⊙ ) which, together with an evidence of their evolutionary status of being red clump imply that the stars have gone through both the luminosity bump and He-flash during their RGB evolution. Stars' large Li abundance and evolutionary phase suggest that Li enrichment occurred very recently probably at the tip of RGB either during He-flash, an immediate preceding event on RGB, or by some kind of external event such as merger of RGB star with white dwarf. The findings will provide critical constraints to theoretical models for understanding of Li enhancement origin in RGB stars.
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