National Observatory uses LAMOST to discover that solar-like stars can produce lithium

[ Instrument R&D of instrumentation network ] Is the high content of lithium in solar-like stars rare? Will solar-like stars produce lithium? At what stage did it evolve? An international team led by Zhao Gang, a researcher at the National Astronomical Observatories of the Chinese Academy of Sciences, and Dr. Kumar, used China’s major scientific and technological infrastructure Guo Shoujing telescope (LAMOST) spectral data and international GALAH survey data to find that solar-like stars generally produce lithium after helium flashes. Opened the above puzzle. On July 6, this research was published in "Nature Astronomy" (Nature Astronomy).

Lithium is commonly used in modern communications equipment and transportation industries. Mobile phones, tablet computers, electric cars, etc. all use lithium batteries for power supply. In addition, lithium is also widely used in aerospace, defense and other fields. But where does lithium come from? The origin of the vast majority of lithium can be traced back to the same event, that is, the Big Bang that occurred about 13.8 billion years ago, which is the origin of the universe. Lithium is one of the earliest known three elements produced in the early Big Bang (the other two are hydrogen and helium). Lithium has always been the key element connecting the Big Bang, interstellar matter and stars, and the study of lithium is an important topic in the evolution of the universe and stars.

The lithium content increased slightly during the Big Bang, mainly because high-energy cosmic rays bombarded the heavier nuclei in the interstellar medium, such as carbon and oxygen, and split them into smaller atoms, such as lithium. Unlike other elements, researchers generally believe that the lithium element will gradually disappear in the stars. This is because lithium participates in the nuclear reaction at a relatively low temperature inside the star (2.5 million degrees, which is several times a million degrees), and then mixes with the external atmosphere, the initial lithium will disappear in the life cycle of the star. For example, the constituent elements of the sun and the earth are highly similar and are thought to form almost simultaneously, but the lithium content in the sun is 100 times lower than that in the earth. With the advancement of observation technology, people have discovered that some solar-like stars (about 1/100 in the Milky Way) have a very high lithium content in the atmosphere, and in some cases, even 100,000 times higher than the theoretical model predicts . What causes the abnormally high lithium content in solar-like stars? This problem has plagued researchers for the past 40 years.

With the help of GALAH, LAMOST, and GAIA survey data, the research team discovered that solar-like stars generally produce lithium after helium flashes, and solved the mystery above. Kumar, the first author of the paper, said that the research team systematically studied the phenomenon of abnormally increased lithium abundance in late solar-like stars. Surprisingly, the phenomenon of abnormally increased lithium abundance after solar-like stars flashed through helium. Helium flash is an iconic event in solar-like stars. In the late stages of star evolution, helium accumulates continuously in its core, causing temperature and pressure to rise continuously. This huge helium nucleus was finally ignited, and a violent uncontrolled nuclear burning occurred, just like a helium bomb detonated inside the star, releasing energy equivalent to the entire galaxy in a few minutes. The theoretical model predicts that the lithium content of stars undergoing this stage should be very low, but in fact, observations have found that the average lithium content of these stars is more than 200 times the theoretically predicted value, which shows that solar-like stars have produced new Lithium element. Since helium flash is a process that must be experienced during the evolution of solar-like stars, solar-like stars generally produce lithium after helium flash. LAMOST data played an important role in the identification of helium flash stars.

In addition, the study also proposed a new standard to identify objects called lithium-rich giant stars. According to this standard, the lithium-rich giant stars discovered in the past 40 years may be just the tip of the iceberg in the universe.

The head of the research team and the co-corresponding author of the paper, Zhao Gang, said that for us, the key to the next step is to understand the nuclear fusion of lithium between the helium flash and the mixing mechanism. There are still many unsolved mysteries here.