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New framework suggests stars dissolve into neutrons to forge heavy elements
https://phys.org/news/2025-04-framework-stars-dissolve-neutrons-forge.html [phys.org]
Understanding the origin of heavy elements on the periodic table is one of the most challenging open problems in all of physics. In the search for conditions suitable for these elements via "nucleosynthesis," a Los Alamos National Laboratory-led team is going where no researchers have gone before: the gamma-ray burst jet and surrounding cocoon emerging from collapsed stars.
As proposed in an article in The Astrophysical Journal, high-energy photons produced deep in the jet could dissolve the outer layers of a star into neutrons, causing a series of physical processes that result in the formation of heavy elements.
"The creation of heavy elements such as uranium and plutonium necessitates extreme conditions," said Matthew Mumpower, physicist at Los Alamos. "There are only a few viable yet rare scenarios in the cosmos where these elements can form, and all such locations need a copious amount of neutrons. We propose a new phenomenon where those neutrons don't pre-exist but are produced dynamically in the star."
Free neutrons have a short half-life of about 15 minutes, limiting scenarios in which they are available in the abundance required to form heavy elements. The key to producing the heaviest elements on the periodic table is known as the rapid neutron-capture process, or "r process," and it is thought to be responsible for the production of all naturally occurring thorium, uranium and plutonium in the universe.
The team's framework takes on the challenging physics of the r process and resolves them by proposing reactions and processes around star collapses that could result in heavy element formation.
In addition to understanding the formation of heavy elements, the proposed framework helps address critical questions around neutron transport, multiphysics simulations, and the observation of rare events—all of which are of interest for national security applications that can glean insights from the research.
In the scenario Mumpower proposes, a massive star begins to die as its nuclear fuel runs out. No longer able to push up against its own gravity, a black hole forms at the star's center. If the black hole is spinning fast enough, frame-dragging effects from the extremely strong gravity near the black hole wind up the magnetic field and launch a powerful jet. Through subsequent reactions, a broad spectrum of photons is created, some of which are at high energy.
The jet blasts through the star ahead of it, creating a hot cocoon of material around the jet, "like a freight train plowing through snow," Mumpower said. At the interface of the jet with the stellar material, high-energy photons (that is, light) can interact with atomic nuclei, transmuting protons to neutrons.
Existing atomic nuclei may also be dissolved into individual nucleons, creating more free neutrons to power the r process. The team's calculations suggest the interaction with light and matter can create neutrons incredibly fast, on the order of a nanosecond.
Because of their charge, protons get trapped in the jet by the strong magnetic fields. Neutrons, which are chargeless, are plowed out of the jet into the cocoon. Having experienced a relativistic shock, the neutrons are extremely dense compared with the surrounding stellar material, and thus the r process may ensue, with heavy elements and isotopes forged and then expelled out into space as the star is ripped apart.
