The Secrets of Lithium in Sun-Like Stars: Destruction Has a New Life

Is high levels of lithium rare in sun-like stars? Do sun-like stars produce lithium? What stage did it occur at the stage of stellar evolution? The discovery solves the mystery of how the sun-like stars can produce lithium after helium flashes, using spectral data from China’s major scientific and technological infrastructure, the Guo Shoujing Telescope (LAMOST) and international GALAH sky-watching data, led by researcher Zhao Gang of the National Observatory of the Chinese Academy of Sciences and Dr. Kumar.

The Secrets of Lithium in Sun-Like Stars: Destruction Has a New Life

Helium nuclear combustion diagram.Source:China National Observatory

The findings were published on July 6, 2020 in the internationally renowned astronomical journal Nature Astronomy.

Lithium is commonly used in modern communications equipment and transportation. Mobile phones, tablets, electric vehicles, etc. are powered by lithium batteries. In addition, lithium has been widely used in aerospace, defense engineering and other fields. But have you ever wondered where lithium comes from?

The vast majority of lithium origins can be traced back to the same event, the Big Bang, the origin of the universe, about 13.8 billion years ago. Lithium is one of the first three known elements to be produced in the early Big Bang (the other two are hydrogen and helium). Lithium has long been a 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 small increase in lithium during the Big Bang was mainly due to high-energy cosmic rays bombarding heavier nuclei in interstellar media, such as carbon and oxygen, splitting them into smaller atoms, such as lithium. Unlike other elements, researchers generally believe that lithium will gradually disappear from stars.

The Secrets of Lithium in Sun-Like Stars: Destruction Has a New Life

Lithium elements in stars evolve from the main sequence through the red giant, helium flash (the upper end of the red giant) to the burning phase of the helium nucleus of the red cluster. The dashed line is a model prediction. The red symbol band represents the helium nuclear combustion phase of the red cluster star.

This is because lithium participates in a nuclear reaction at a relatively low temperature inside the star (2.5 million degrees, or a million degrees), and then, after mixing with the outer atmosphere, the initial lithium disappears during the star’s life cycle. For example, the sun and earth’s constituent elements are highly similar and are thought to form almost simultaneously, but the amount of lithium in the sun is 100 times lower than that of the Earth.

With advances in observational techniques, it has been found that some solar-like stars (about 1/100 in the Milky Way) have very high levels of lithium in the atmosphere, and in some cases, even 100,000 times higher than the theoretical model predicted. What causes abnormally high levels of lithium in sun-like stars? The problem has plagued researchers for the past four decades.

Using GALAH, LAMOST, and GAIA sky-watching data, the team solved the mystery by discovering that solar-like stars are commonly able to produce lithium after helium flashes. The team systematically studied the abnormal increase in lithium abundance in late-stage solar stars, said Dr. Kumar, the paper’s first author.

Surprisingly, unusually high levels of lithium abundance in solar-like stars are extremely common after they pass through helium flashes. Helium flash is an iconic event in sun-like stars, which accumulates helium at its core late in the star’s evolution and causes temperature and pressure to continue to rise. The giant helium nucleus was eventually ignited, burning it out of control, like detonating a helium bomb inside a star, releasing the equivalent of the entire Milky Way in minutes.

Theoretical models predicted that the lithium content of stars going through this stage should be very low, but in fact, observations found that the average lithium content of these stars was more than 200 times higher than the theoretical prediction, suggesting that sun-like stars produced new lithium elements through helium flashes. Because helium flash is an inevitable process in the evolution of sun-like stars, solar-like stars generally produce lithium after helium flash. LAMOST data play an important role in identifying helium flash stars.

In addition, the study proposes a new standard for identifying objects known as lithium-rich giants, which have been discovered over the past 40 years as just the tip of the iceberg in the universe.

According to the research team leader, co-author Zhao Gang said: For us, the key to the next study is to understand the nuclear fusion of lithium between helium and mixing mechanisms, which still contain many unsolved mysteries.

Note: The original initial value of the current Big Bang model prediction is A (Li) – 2.7dex (measured on a log scale as A (Li) – log n (Li) / n(H) , 12, wherein n is the density of atomic s. In interstellar medium (ISM) or very young stars about A (Li) – 3.3dex, four times more than the original value.