The Enigmatic Origins of Li in Sun-like Stars
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The Enigmatic Origins of Li in Sun-like Stars∗ Amanda I. Karakas What will happen to our Sun when it ages? Where do the chemical elements in the Universe come from, and in particular, what is the origin of the rare element Lithium (Li)? These questions are intimately linked to the processes of stellar evolution and nucleosynthesis. The recent study by Kumar, Reddy et al. (Nature Astronomy, 2020) seeks to answer these questions by looking at large samples of stars obtained by on-going astronomical surveys.
create the dominant isotope of lithium, 7 Li, where the amount created depends on the initial supply of 3 He, which itself is also created by proton captures via d + p → 3 He + γ (see Figure 1 for a description of the symbols and reaction chains involved). The lithium created is destroyed again by the reaction 7 Li + p → 24 He. This last reaction occurs at temperatures of only about 3 million degrees Kelvin, much lower than we predict for the central temperature of the Sun, which is closer to 15 million K.
Out of all of the elements in the periodic table, the light element Li is quite special. It is one of the only three elements that was made by the Big Bang, alongside hydrogen and helium. Following the Big Bang, which started our Universe roughly 14 billion years ago, stars formed, aged and then died, releasing their nucleosynthesis products into galaxies. New generations of stars formed from this material, which was slightly more enriched in elements heavier than Li (e.g., carbon, oxygen, iron) than previous generations. While this story of element synthesis with cosmic time— or chemical evolution—works well for most elements, it doesn’t work so well for lithium.
Computer simulations show that Sun-like stars, which are converting hydrogen to helium at their centres, have depleted their initial supply of Li by the reaction described above. Stars in this stage of evolution are known as ‘main sequence stars’ (see Figure 2) [1]. When stars age, they expand to become red giants, where the position of red giant stars is shown on a Hertzsprung-Russell (HR) Diagram in Figure 2. In the process, some of the products of the hydrogen fusion reactions shown in Figure 1—which is devoid of all Li by now—is mixed to the stellar surface by convective currents. Convective currents mix material from deep within the stellar interior to the surface, where they can be observed by us on Earth, using powerful telescopes. [1] For an overview of stellar evolution, we refer to Hansen, Kawaler, and Trimble (2004).
Why is this? Lithium is easily destroyed by proton-capture nuclear reactions at relatively low stellar temperatures, but it is difficult to create. The reaction 7 Be + e− → 7 Li + νe can
What we see is that the atmospheres of red giant stars become devoid of Li as they age. On the red giant branch, the cores of Sunlike stars are compressed by contraction, and
∗ Vol.25, No.8, DOI: https://doi.org/10.1007/s12045020-1032-9
RESONANCE | August 2020
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Figure 1: Reaction rates involved in the proton-proton (pp
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