The nature of the lithium enrichment in the most Li-rich giant star

Abstract

About one percent of giants[1] are detected to have anomalously high lithium (Li) abundances in their atmospheres, conflicting directly with the prediction of the standard stellar evolution models [2] , and making the production and evolution of Li more intriguing, not only in the sense of the Big Bang nucleosynthesis [3,4] or the Galactic medium [5] , but also the evolution of stars. [6,7,8] , yet the origins of Li-rich giants are still being debated. Here we report the discovery of the most Li-rich giant known to date, with a super-high Li abundance of 4.51. This rare phenomenon was snapshotted together with another short-term event that the star is experiencing its luminosity bump on the red giant branch. Such high Li abundance indicates that the star might be at the very beginning of its Li-rich phase, which provides a great opportunity to investigate the origin and evolution of Li in the Galaxy. A detailed nuclear simulation is presented with up-to-date reaction rates to recreate the Li enriching process in this star. Our results provide tight constraints on both observational and theoretical points of view, suggesting that low-mass giants can produce Li inside themselves to a super high level via 7 Be transportation during the red giant phase. Decades of efforts have been put into explaining why such outliers existLithium is too fragile to survive in deeper layers of a stellar atmosphere due to the high temperature. Thus the first dredge up (FDU) process can sharply dilute the surface Li abundance in red giants. That explains why the first discovery [9] of a Li-rich giant evoked great interests on exploring and understanding the Li-rich objects. However, only about 150 Li-rich giants have been found [1,10,11,12,13,14] in the past three decades, and ∼ 20 of them were found to be super Li-rich with Li abundances higher than 3.3. Considering the NLTE corrections, three [12,15,16] stars were found to be at a level of A(Li) > 4.0. Such rare objects could provide a great opportunity to reveal the nature of the phenomenon of Li-richness because high Li abundance cannot be maintained for a long time due to frequent convection activity. Taking advantage of the powerful ability for spectral collection with the Large Sky Area Multi-Object * sjr@nao.cas.cn † gzhao@nao.cas.cn 1 Fiber Spectroscopy Telescope (LAMOST), we have obtained a large sample of Li-rich candidates by measuring the equivalent width of the Li I line at λ = 6707.8 Å. One of our candidates, TYC 429-2097-1, has a super strong Li absorption line (see Fig. 1, panel a). We then made a follow-up high-resolution observation with the 2.4-m Automated Planet Finder Telescope (APF) located at Lick Observatory on June 23, 2015. The spectrum covers a wavelength range of 374 nm − 970 nm with a resolution of ∼ 80, 000. The total integration time was 1.5 hours and was divided into three single exposures (30 minutes each) for a better subtraction of cosmic-rays. The spectrum of TYC 429-2097-1 obtained from APF is presented in Fig. 1, panels (b) and (e), where the spec...

Survey of Li-rich Giants among Kepler and LAMOST Fields: Determination of Li-rich Giants’ Evolutionary Phase

“…This illustrates the difficulty of determining their precise evolutionary phase. We also note another recent study by Yan et al (2018) in which they reported TYC 429-2097-1 as the most Li-rich giant with A(Li) = 4.51 dex and located at the bump based on their location in HR-diagram. This star is not in the Kepler field.…”

Discovery of ubiquitous lithium production in low-mass stars

“…This is a very large discrepancy between models and observations. This observed RC Li distribution also highlights the rarity of the so-called 'Li-rich giants', with A(Li) > 1.5 dex, which have been the focus of the majority of the lithium in giants literature to date 1,2,[4][5][6]11 . In our RC sample we find that 3.0% of stars are above this limit (275 out of 9284 stars).…”

“…The origin of giant stars highly enriched in lithium has been, and still is, a long-standing mystery 1,2,5,10,11 . Stellar lithium abundance is defined by A(Li), which is the logarithmic abundance of lithium: A(Li) = log 10 (n(Li)/n(H)) + 12, where n(Li) and n(H) are the number densities of Li and H atoms.…”

The Li–age correlation: the Sun is unusually Li deficient for its age

“…The work of Charbonnel & Lagarde (2010) presented a non-standard stellar evolution model considering thermohaline instability and rotational induced mixing and they were able to reproduce the Li behaviour in red giants. The non-standard stellar nucleosynthesis presented in Yan et al (2018) might explain the observations of Li-rich giants in a particular short stellar evolution phase. See also recent papers by Deepak & Reddy (2019) and Casey et al (2019).…”