Simple or Complicated, What do Astronomers Think about Lithium

Jun 05, 2019
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Cover Image Copyright & Credit: Particle Data Group at Lawrence Berkeley National Lab

One hundred and fifty years ago, Dmitri Mendeleev gathered chemists all over the world around a ‘table’. Almost at the same period of time, Gustav Kirchhoff and Robert Bunsen showed that ‘dark lines’ in the solar spectrum which confused scientists for decades were actually the atomic lines absorbed by the same elements as found on the Earth1. Those atomic lines were powerful tracers for astronomers to measure ‘matter contents’ of countless distant celestial bodies in our vast universe. I guess since then, astronomers became a group of people who stare at the Periodic Table as much as chemists do.

Fig.1. The ‘dark lines’ in the spectrum of the Sun, also known as Fraunhofer lines in honor of Joseph von Fraunhofer who first discovered them, were shown to be atomic absorption lines, making it possible to analyze the elemental abundance of a star. The picture shows a stamp of Fraunhofer’s first solar spectrum. (Source & Credit: Deutsche Bundespost - scanned by NobbiP, Public Domain)

Unlike chemists, however, astronomers care about element originations rather than their reactions. Yes, we believe that elements were originally generated in various astrophysical processes, e.g., the Big Bang nucleosynthesis, supernovae explosions, neutron star mergers and nuclear fusions inside stars. Those processes dominate the production and evolution of elements, enrich environments through time, and make what our universe looks like today. 

As the third element in the Periodic Table, lithium does not have a complicated structure compared to most of other elements. Meanwhile, as one of the four nuclei that were synthesized during 3-20 minutes immediately after the big bang2, lithium is of great importance as it preserves rich information of the origin and evolution of our universe (and of course, our Galaxy). However, this ‘simple’ element has casted shadows in many aspects of modern astrophysics. 

Fig.2. History of the universe. Lithium was synthesized during 3-20 minutes after the birth of our universe, and then the symphony of matter and energy in the next 13.8 billion years makes what our universe looks like today. (Source & Credit: Particle Data Group at Lawrence Berkeley National Lab)

For example, the lithium abundance derived from the observation to the oldest stars in the Galaxy is significantly lower than the prediction of the Big Bang nucleosynthesis theory3; a few percent of giant stars were found to have anomalously high lithium abundance although they were predicted to be the opposite by the standard stellar evolution model4,5. All those challenges are indicating that the lithium is more complicated than we previously thought, at least from the astrophysical point of view.

Astronomers are trying to deepen our understanding of lithium by taking various approaches. In our study, we aim to search for the lithium-rich giant stars and study where this over-abundance of lithium comes from. Recently, we discovered such an object in which lithium abundance is thousands of times higher (in mole fraction) than in normal giant stars and proposed a possible origin of the lithium over-abundance. 

The study was facilitated by two telescopes, LAMOST and APF. If the universe is a laboratory to astronomers, then giant telescopes are the instruments that let us carry out experiments. LAMOST is a 4-meter (effective aperture) class telescope located at Xinglong, China. With the capability of capturing 4,000 spectra in only one exposure, LAMOST is one of the most powerful spectra collecting facilities in the world. It has acquired more than 10 million spectra by mid-2018. As mentioned in the beginning, by measuring the atomic line in the stellar spectrum, we could derive the abundance of the corresponding element. We checked the strength of lithium resonance line at 6708 angstrom for millions of stars in LAMOST data. Since this line is usually very weak in the spectra for most giant stars, its strength can be used as the criterion of picking out the lithium-rich candidates. Several targets among our candidates were very interesting as they show extremely strong lithium features in the spectra, and we decided to take a further study to them.

Fig. 3. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) under the starry night of Xinglong, China. (Source & Credit: Yingwei Chen, National Astronomical Observatories, Chinese Academy of Sciences)

However, with a telescope aiming to survey as many stars as possible, the resolution of most LAMOST spectra is only ~1,800 (lambda/d_lambda), which is not high enough to precisely derive the lithium abundance. Fortunately, we got some time from APF telescope located at Lick Observatory, USA. This is a 2.4-meter telescope designed to search for extrasolar planets, and it could obtain spectra at a resolution of ~80,000. Those data helped us to evaluate the lithium abundance in an accuracy of ~0.1 dex. After a careful data analysis, we found a giant star with the highest lithium abundance known to date, and also suggested a possible scenario how its lithium was generated.

Fig.4. The Automated Planet Finder Telescope (APF) situated on the summit of Mount Hamilton, USA. (Source & Credit: By Oleg Alexandrov - Own work, Public Domain)

There is still a long way to go for astronomers to reveal the nature of lithium in the universe, a wider discussion and collaboration with scientists from various fields is absolutely essential. 

After our study was published, one of the Chinese news media wrote a report entitled `astronomers found the largest power bank in the universe’6. I liked their interpretation very much as they link it to the lithium-ion batteries in our life. It is also fascinating to think about the future that humans would mine the resources from stars one day. If that comes true and possible, do not forget to mine some materials from the carbon core of the white dwarfs.


Reference

  1. https://en.wikipedia.org/wiki/Joseph_von_Fraunhofer#cite_note-12 
  2. Coc, A., Goriely, S., Xu, Y., Saimpert, M., & Vangioni, E. Standard big bang nucleosynthesis up to CNO with an improved extended nuclear network. Astrophys. J. 744,158 (2012). 
  3. Spite, M. & Spite, F. Lithium abundance at the formation of the Galaxy. Nature 297, 483–485 (1982) 
  4. Iben, I. Jr Stellar evolution. VI. Evolution from the main sequence to the red-giant branch for stars of mass 1 Msun, 1.25 Msun, and 1.5 Msun. Astrophys. J. 147, 624 (1967) 
  5. Brown, J. A., Sneden, C., Lambert, D. L. & Dutchover, E. Jr A search for lithium-rich giant stars. Astrophys. J. Suppl. 71, 293–322 (1989). 
  6. http://news.sciencenet.cn/htmlpaper/201881111443768046997.shtm


I acknowledge Dr. Michal Filus for his advices to improve this post.

Hong-Liang Yan

Assistant Research Fellow, Night Operation Manager of LAMOST, National Astronomical Observatoies, Chinese Academy of Sciences

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