In order to better understand how matter inside stars—in the form of hot, ionized gas—moves, an international team led by the Peruvian astronomer Jorge Meléndez, of the Institute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (USP), compared the quantities of the chemical elements beryllium and lithium observed on the surface of the Sun and seven other similar stars in the Milky Way. “We cannot see inside the stars, only the light given off from their outer layers,” explains Marcelo Tucci Maia, one of Meléndez’s PhD students and the first author of the new study, published in March 2015 in the journal Astronomy & Astrophysics. “The abundance of these elements acts like a probe that helps us investigate what is happening inside the star.”
The conclusion of the study is that the matter on the surface of stars like the Sun can mix with that of inner layers to a depth greater than many researchers had imagined, but not as deep as others had proposed. Meléndez and his colleagues used the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Monte Paranal, Chile, to observe the Sun and seven other stars, chosen because their mass and chemical composition are similar to that of the Sun, but their ages are very different. While the Sun is 4.6 billion years old, the youngest star in the study is only 500 million years old and the oldest, 8.2 billion years old. “It was as if we could monitor the evolution of the Sun, from when it was young until an age much older than it is now,” explains Maia.
Meléndez’s team had already published other studies on these same stars and shown that, the older the star, the less lithium there is on its surface. These results confirmed the findings of prior studies that indicated that stars similar to the Sun destroy lithium as they age.
Most of the lithium in the Universe was created at the beginning of time, during the Big Bang, about 13.6 billion years ago. Considered a relatively fragile element, lithium is destroyed by various types of nuclear reactions that take place inside stars at temperatures over 2.5 million degrees Celsius. Inside the Sun, according to the standard models of stellar evolution, such high temperatures only occur at great depths, near the core, in a region called the radiative zone. The temperature in the radiative zone varies from 15 million degrees, near the core, to 1.5 million degrees further from the core. Just outside the radiative zone, in what is called the convection zone, the temperature decreases gradually from 1.5 million degrees to 6,000 degrees on the star’s surface.
In the radiative zone, the energy produced in the core through the fusion of chemical elements is transported to the outer layers via light particles (photons), while the matter in the star remains relatively immobile. In the convection zone, however, energy is transported differently. Matter is heated near the radiative zone and rises to a layer near the surface, where it releases heat and sinks again.
Until recently, astronomers thought that the matter in the radiative zone never mixed with matter in the convection zone. The observations of Meléndez and his colleagues, however, indicate that this must take place in some way; otherwise, there would be no explanation for the disappearance of lithium on the surface of the stars. Other researchers have been modifying the mathematical equations that describe the internal structure of the stars to take into account other physical phenomena that would allow transport of matter from the convection zone to deeper, hotter regions. However, they are still debating what these phenomena could be. Some argue that this additional mixing could be caused by the star’s rotation. Others imagine that other processes, such as the diffusion pattern of atomic nuclei on a microscopic level, could be more important.
In order to shed some light on the debate, Maia, Meléndez and their colleagues decided to analyze the abundance of another fragile chemical element, beryllium. Like lithium, beryllium is destroyed by nuclear reactions. But only by those that take place at 3.5 million degrees. “The chemical element beryllium is one of the most difficult to observe, because it is hard to isolate its signature in the light from the star,” says Maia.
According to the study in Astronomy & Astrophysics, the surface of a star the size of the Sun loses very little beryllium throughout its evolution. According to Maia, this characteristic, measured now by the group, establishes a maximum depth at which the matter in the radiative zone mixes with that in the convection zone. The mixture must be taking place at depths at which the temperature reaches 2.5 million degrees, but must not go much deeper, stopping in the region in which the temperature reaches 3.5 million degrees. This behavior would explain why, during the lifespan of these stars, almost no beryllium is destroyed—since it is consumed at higher temperatures—while a greater proportion of lithium is annihilated.
The result has already provided evidence against one of the astrophysical models for the evolution of stars like the Sun. However, uncertainties in the observations still prevent them from determining which of the several existing models is correct. Meléndez’s team hopes to further clarify the question by including data from nine other stars similar to the Sun in their analyses. These stars were observed in July 2015 using the Japanese telescope Subaru, located on Mauna Kea Mountain, in Hawaii.
High precision spectroscopy: impact on the study of planets, stars, the galaxy and cosmology (nº 2012/24392-2); Grant Mechanism Thematic Project; Principal Investigator Jorge Luiz Meléndez Moreno (IAG-USP); Investment R$337,292.40 (for the entire project).
TUCCI MAIA, M. et al. Shallow extra mixing in solar twins inferred from Be abundances. Astronomy & Astrophysics. V. 576. April 2015.