Mystery of star-made heavy elements solved by breakthrough US research

Researchers at the Argonne National Laboratory studied a nuclear reaction and measured conditions that can explain how heavy elements are formed inside stars.

Stars are mainly composed of light elements hydrogen and helium. However, deep within their cores they also undergo a process called nucleosynthesis that leads to the formation of heavy elements from the fusion of hydrogen and helium nuclei.

Stellar nucleosynthesis has been a mystery for scientists because it is still unclear what specific conditions and mechanisms cause the production of various heavy metals inside stars. The findings from the new study can provide answers to such questions.

The significance of neutron capture and i-process

One of the methods or important parts of nucleosynthesis is neutron capture, a process during which neutrons are added to existing nuclei. These nuclei then evolve and transform into heavy elements.

There are three types of neutron capture processes; the s-process (slow neutron capture, the r-process (rapid neutron capture, and the i-process (intermediate neutron capture. They explain the formation of heavy elements at different stages of a star’s lifecycle.

For instance, the first two occur in the case of red giants (giant luminous stars approaching the end of their life cycle) and supernovae (exploding stars) respectively. Whereas the i-process is particularly useful for explaining the production of specific heavy elements such as lanthanum and barium in growing white dwarfs (the core of a dead star with no nuclear fuel)

The i-process holds great importance because it occurs in environments with moderate neutron densities, unlike the extreme conditions required for the r-process or the lower neutron densities typical of the s-process.

The study authors also “reported on the measurement of a nuclear reaction that affects the production of lanthanum in the i-process.” They claim that “with the new constraints, scientists can confirm the neutron density required for an i-process. They can also confirm that rapidly accreting white dwarf stars are a viable site for the i-process.”

Decoding the formation of lanthanum in stars

The current study focuses on the nuclear reaction that leads to the formation of lanthanum in stars. This process involves the neutron capture and beta decay of barium isotopes.

“We provide the first experimental constraints on the 139 Ba (𝑛,𝛾)⁢ 140 Ba reaction rate, which is the dominant source of uncertainty for the production of lanthanum, a key indicator of 𝑖-process conditions,” the study authors said .

Lanthanum plays an important role in understanding the i-process because its abundance in stars, along with elements like barium and europium , helps scientists deduce the conditions under which the i-process occurs.

The study authors analyzed how the barium-139 nucleus captured neutrons — an important step in forming lanthanum in stars. This allowed them to estimate the neutron density required for the process.

The analysis could further improve the models used to predict where and how the i-process happens in stars, especially in accreting white dwarfs . “This is an important step towards identifying the exact astrophysical site of stars carrying the 𝑖-process signature,” the study authors added.

The study is published in the journal Astronomy & Astrophysics .