A high-energy photonic jet (white and
blue) blasts through a collapsar with a black hole at its center. The red space
around the jet represents the cocoon where free neutrons may be captured
causing the r process, the nucleosynthesis that results in the formation of
heavy elements. Credit: Los Alamos National Laboratory
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.
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, such as
their atomic weight, half-life, etc.
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. Kilonovas 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 under the sea.
To more fully understand the proposed framework, Mumpower and his team hope to run simulations on their models, including the complex microphysics interactions.
by Los Alamos National Laboratory
Source: New framework suggests stars dissolve into neutrons to forge heavy elements
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