New theory suggests stars dissolve into neutrons to forge heavy elements

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 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 results 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 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.

Like a freight train plowing through snow

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.

The process of protons converting into neutrons, along with free neutrons escaping into the surrounding cocoon to form heavy elements, involves a broad range of physics principles and encompasses all four fundamental forces of nature: a true multiphysics problem, combining areas of atomic and nuclear physics with hydrodynamics and general relativity. Despite the team’s efforts, more challenges remain as the heavy isotopes created during the r-process have never been made on Earth. Researchers know little about their properties including their atomic weight, half-life and so on.

An explanation for unusual phenomena?

The high-energy jet framework proposed by the team may help explain the origination of kilonova — a glow of optical and infrared electromagnetic radiation — associated with long-duration gamma-ray bursts. Kilonova have been primarily associated with the collision of two neutron stars or the merger of a neutron star and a black hole. These intense collisions are one possible method for confirming with observations the cosmic factories of heavy-element formation. Star dissolution via high-energy photon jet offers an alternative origin for the production of heavy elements and the kilonova they may manufacture, a possibility not previously thought to be associated with collapsing stars.

Relatedly, scientists have observed iron and plutonium in deep-sea sediment. These deposits, after study, are confirmed to be from extraterrestrial sources, though as with the phenomena producing kilonova, the specific location or cosmic event remains elusive. The collapsar high-energy jet scenario represents an intriguing possibility as the source of these heavy elements found undersea.

To more fully understand the proposed framework, Mumpower and his team hope to run simulations on their models, including the complex microphysics interactions.

Paper: “Let there be neutrons! Hadronic photoproduction from a large flux of high-energy photons.” The Astrophysical Journal. DOI: 10.3847/1538-4357/adb1e3

Funding: The work was supported in part by the Laboratory Directed Research and Development program at Los Alamos.

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