Physicists start to pin down how stars forge heavy atoms

原始链接: https://www.quantamagazine.org/physicists-start-to-pin-down-how-stars-forge-heavy-atoms-20250702/

Researchers at FRIB are unraveling the mysteries of the i-process, a nuclear reaction responsible for creating heavy elements in stars. By using a fragment separator and a detector called SuN, they're measuring gamma-ray production to determine the rate at which isotopes capture neutrons. This data is then used in simulations to predict the abundance of heavy elements, which are compared to observations of metal-poor stars. Initial results suggest the i-process accurately explains the presence of elements like lanthanum, barium, and europium in these stars. The researchers are also investigating the r-process, believed to be the origin of gold, silver, and platinum. While replicating r-process conditions is challenging, recent observations and discoveries are providing clues. The team aims to apply their i-process techniques to study the rarer isotopes involved in the r-process. Spyrou estimates they will fully understand the key i-process reactions within 5-10 years, paving the way for further insights into the origins of heavy elements in the universe.

A Hacker News post links to a Quanta Magazine article about physicists' progress in understanding how stars create heavy atoms. One commenter, ordu, questions an analogy used in the article to describe the difficulty of isolating specific isotopes during the i-process and r-process. The analogy compares the processes to shattering plates with Italian cities painted on them and then either isolating a picture of a house (i-process) or just a window (r-process). Ordu argues that isolating a smaller, specific piece (like a window) should be easier because it has fewer details to preserve compared to a larger piece (like a house). Frontfor responds, clarifying that while obtaining *any* small piece is easier, obtaining a *specific* small piece is much harder. The comment thread discusses the validity and potential flaws of this analogy in representing the complex processes of isotope isolation in astrophysical settings.

原文

The shards flow through a network of pipes into a fragment separator that sorts them into isotopes of interest. These eventually end up at the SuN, a cylindrical detector 16 inches wide. With metal spokes extending out in all directions, “it kind of looks like the sun, which is fun,” said Ellie Ronning, an MSU graduate student.

Just as the nuclei enter, they begin decaying, shedding electrons and emitting flashes of gamma rays that researchers can use to decode the steps of the i-process. “No one’s been able to see these particular processes before,” said Sean Liddick, a FRIB nuclear chemist.

By measuring gamma-ray production, the researchers infer the rate at which the relevant isotopes capture neutrons (how readily barium-139 gains a neutron and becomes barium-140, to name one important example). Theorists then input this reaction rate into a simulation of the i-process, which predicts how abundant different heavy elements will be in the final chemical mixture. Finally, they can compare that ratio to the elements observed in different stars.

So far, the results seem to draw a circle right where Spyrou and her colleagues had hoped: The relative abundances of lanthanum, barium and europium match what was seen in those carbon-enhanced, metal-poor stars that so puzzled astrophysicists in the early 2000s. “We went from having these huge uncertainties to seeing the i-process fit right where we have the observations,” she said.

The i-process, however, would have taken place in the dying stars that came before those metal-poor ones and provided them with material. Right now, the data is compatible with both white dwarfs and red giants as the setting of the i-process. To see which candidate will prevail, if not both, Spyrou will need to study the neutron capture rates of more isotopes. Meanwhile, to distinguish between those candidate stars, Herwig will create better three-dimensional models of the plasma swimming inside them.

Going for Gold

For 60 years, astronomers have theorized that gold, silver and platinum all spawn during the r-process, but the exact birthplaces of these elements remain one of astrochemistry’s most long-standing questions. That’s because “r-process experiments are basically nonexistent,” Cowan said. It’s hard to reproduce the conditions of a neutron-star collision on Earth.

A 2017 observation found traces of gold and other r-process elements in the debris of a neutron-star collision, lending strong support to that origin story. But a tantalizing discovery reported this past April links the r-process to a colossal flare from a highly magnetic star.

After sorting out the i-process, the researchers in Michigan plan to apply the same tactics to the r-process. Its isotopes are even tricker to isolate; if fragmentation during the i-process is like capturing a picture of a house from a shattered plate, then the r-process means picking out only the window. Still, Spyrou is optimistic that her team will soon try out the rarer flavors of isotopes required for the express recipe, which cooks up heavy nuclei in seconds. “With the r-process, we’re close to accessing the nuclei that matter,” she said.

“But with the i-process, we can access them today,” she said. Spyrou estimates that her lab will nail down all the important i-process reactions and rates within five to 10 years. “Ten years ago,” she added, “I didn’t even know the i-process existed.”

This story was supported in part by the Council for the Advancement of Science Writing and The Brinson Foundation.