Discovery of ubiquitous lithium production in low-mass stars

Abstract

The vast majority of stars with mass similar to that of the Sun are expected to destroy lithium (Li) gradually over the course of their lives, via low-temperature nuclear burning. This has now been supported by observations of hundreds of thousands of red giant stars1,2,3,4,5. Here we perform a large-scale systematic investigation into the Li content of stars in the red clump phase of evolution, which directly follows the red giant branch phase. Surprisingly, we find that all red clump stars have high levels of Li for their evolutionary stage, with an increase of a factor of 40 over the end of the red giant branch stage, on average. This suggests that all low-mass stars undergo an Li production phase between the tip of the red giant branch and the red clump. We demonstrate that our finding is not predicted by stellar theory, revealing a stark tension between observations and models. We also show that the well-studied1,2,4,5,6 very Li-rich giants, with A(Li) > +1.5 dex, represent only the extreme tail of the Li enhancement distribution, comprising 3% of red clump stars. Our findings suggest a new definition limit for Li-richness in red clump stars, A(Li) > −0.9 dex, which is much lower than the limit of A(Li) > +1.5 dex used over many decades1,5,6,7,8,9.

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Fig. 1: Hertzsprung–Russell diagram of the Galah DR2 sample used to locate the RC stars.
Fig. 2: Asteroseismic properties of giant stars.
Fig. 3: Our Galah stellar sample in the Li-luminosity plane.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work made use of the GALAH survey, which includes data acquired through the Australian Astronomical Observatory. It also made use of the astronomical data analysis software TOPCAT, and the NASA Astrophysics Data Service (ADS). Y.B.K. and G.Z. acknowledge the support of the National Science Foundation of China through grant numbers 11988101, 11850410437, 11890694 and the National Key R&D Program of China grant number 2019YFA0405502. B.E.R. thanks NAOC, Beijing, for support through the CAS PIFI grant number 2019VMA0009. S.W.C. acknowledges federal funding from the Australian Research Council through a Future Fellowship (FT160100046) and Discovery Project (DP190102431). Y.-S.T. acknowledges support from the NASA Hubble Fellowship grant HST-HF2-51425.001 awarded by the Space Telescope Science Institute. We also thank L. Spina and C. Doherty for discussions.

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Authors

Contributions

Y.B.K., B.E.R. and G.Z. proposed and designed this study. Y.B.K. analysed the data and prepared the manuscript. B.E.R. assisted in data analysis and manuscript preparation. S.W.C. calculated the stellar models, assisted in the data analysis and prepared the manuscript. S.M. assisted in data analysis and preparing the figures. G.Z. assisted in manuscript preparation. Y.-S.T. made the asteroseismic inference catalogues, and assisted in manuscript preparation.

Corresponding authors

Correspondence to Yerra Bharat Kumar or Gang Zhao.

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The authors declare that they have no competing financial interests.

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Peer review information Nature Astronomy thanks Joleen Carlberg and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Li versus temperature of giants.

Li versus Teff of giants (\(\mathrm{log}\,(g)<3.1\)). The Galah sample is shown without reduction by Li quality flag (dark grey crosses, 50,690 stars), and with Li quality flag as used in manuscript (light grey crosses, 26,751 stars). The respective red clump (RC) sub-samples are highlighted by blue and cyan symbols. Our Li detection curves, computed using spectral synthesis, are shown by the solid lines (see text for details). It can be seen that the vast majority of stars, in all samples, are above the detection limits. See also Extended Data Fig. 2, which provides a cross-section histogram of this diagram in the RC Teff range.

Extended Data Fig. 2 Histograms of giants per A(Li) bin.

Histograms showing number of giant stars per A(Li) bin. The stellar sample has restricted Teff = 4,650 − 4,900 K, which matches our RC Teff range. We have not restricted the samples in any other way, that is, they contain all stars in this range, regardless of evolutionary state (RC or RGB). The largest sample is the full Galah DR2 (35,800 stars), the next largest is the sample for Fig. 1 (27,847 stars), and the smallest is from our Fig. 3 sample (15,469 stars). The vertical lines show our computed detectability limits (see Extended Data Fig. 1, and text for details). It can be seen that the bulk of the distribution is well above the detection limit, and that the location of the peak is unchanged through each sub-sampling.

Extended Data Fig. 3 Li versus luminosity of giants.

Li versus luminosity of giants. Colour gradient shows the temperature distribution. Red circles are Kepler RC stars, with the lower-Li sample36,37 and super-Li-rich sample4 taken from separate studies. Light blue circles are individual RC stars from OCs8. Li detection limits computed for GALAH and the lower-Li Kepler RC sample are shown in blue and red dashed lines, respectively. It can be seen that the peaks of these literature distributions are much higher than the RGB tip value of about −1.0 dex, as found in our Galah sample. The peaks are also well above the respective detectability limits.

Extended Data Fig. 4 Comparison of Li abundances between Galah DR2 and the literature.

Comparison of Li abundances between Galah DR2 and Gaia-ESO survey34 DR3 (‘GES’). A total of 78 stars were found in common (circles). A very slight offset between Gaia-ESO and Galah is found: + 0.059 dex (σ = 0.37). The common sample includes two RC stars, indicated by squares. These RC stars straddle the peak of the Galah RC distribution at A(Li) = 0.71 dex. Also shown are two RC giants found in a cross-match between Galah DR2 and the LAMOST survey13 (triangles). As LAMOST is a low-resolution survey, Li is only detectable in very Li-rich stars. The colour gradient shows the temperature distribution.

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Kumar, Y.B., Reddy, B.E., Campbell, S.W. et al. Discovery of ubiquitous lithium production in low-mass stars. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1139-7

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