Evolution of lithium in low-mass giants: an observational perspective
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J. Astrophys. Astr. (2020)41:49 https://doi.org/10.1007/s12036-020-09660-9
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Evolution of lithium in low-mass giants: an observational perspective YERRA BHARAT KUMAR1, ,*
and BACHAM ESWAR REDDY2
1
Key Laboratory of Optical Astronomy, National Astronomical Observatories, Beijing 100101, China. Indian Institute of Astrophysics, Bengaluru 560 034, India. *Corresponding author. E-mail: [email protected] 2
MS received 10 September 2020; accepted 2 November 2020 Abstract. The overabundance of lithium in low-mass red giants has been a topic of interest for over four decades. Low-mass stars are expected to destroy lithium gradually throughout their lifetimes. Against this expectation, about 1% of red giants in the Galaxy show anomalously large Li which, in the literature, are known as lithium-rich giants. The advent of large-scale stellar surveys (LAMOST, GALAH, Kepler, Gaia) coupled with high-resolution spectra enabled to find important clues about Li enhancement origin in red giants. These new studies suggest Li enhancement is mostly associated with the red clump region, post-Heflash. Here, we will describe our recent results along with current updates in the field. Keywords. Late-type stars—stellar evolution—abundances-lithium.
1. Introduction Lithium (Li) is one of the three elements, apart from Hydrogen and Helium, and the only element among solids known to have produced at the beginning of the universe in the Big Bang nucleosynthesis (BBN). The current best value for Li abundance of BBN, based on critical parameters like baryon-to-photon-ratio measured by WMAP, is A(Li) = 2.7 dex (Cyburt et al. 2016). This is considered as primordial value for Li abundance, which is in disagreement with the long observed value of primordial lithium in metal-poor dwarfs, A(Li) = 2.2 dex, known as Spite-Plateau (Spite & Spite 1982a). The difference between the two is significant, by an order of magnitude. Since the Spite-Plateau breaks down at the extreme metal-poor end ([Fe/H] \ 2.8 dex), and the good agreement between the observed primordial H & He and BBN predictions suggests the discrepancy may be to do with the slow depletion of Li abundance over a star’s lifetime via diffusion or other processes rather than
LAMOST Fellow. This article is part of the Topical Collection: Chemical Elements in the Universe: Origin and Evolution.
BBN (e.g. Richard et al. 2005; Korn et al. 2006; Fu et al. 2015). Irrespective of which value we adapt for the primordial abundance, observations show significant enrichment in Li abundance in the Galaxy since the Big Bang. The present value of Li observed in interstellar clouds or meteorites (Lodders 2003) is A(Li) = 3.28 dex, which is a factor of 4 more than the BBN prediction. Possible sources for Li enrichment (e.g., Matteucci 2010) are cosmic ray spallation, novae, supernovae, and nucleosynthesis in evolved stars. Fresh Li is known to be produced in intermediate-mass Asymptotic Giant Branch (AGB) stars via hot-bottom burning (McKellar 1940; Sm
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