The first stars made a surprising amount of calcium. New doubts have been raised over how.

June 8, 2021
A hydrodynamic simulation of the hydrogen and helium shells in an early-universe star, which was used to investigate whether this kind of star could create large amounts of calcium without exploding in a faint supernova. (P. Woodward and F. Herwig; NSF; Texas Advanced Computing Center)

A hydrodynamic simulation of the hydrogen and helium shells in an early-universe star, which was used to investigate whether this kind of star could create large amounts of calcium without exploding in a faint supernova. (P. Woodward and F. Herwig; NSF; Texas Advanced Computing Center)

Some astrophysicists theorize that, by fusing their contents in a certain way and ending their lives in "faint supernovas," the earliest stars created a large abundance of calcium seen in today's stars — and likely in our bones. But a new study found that the foundation of this explanation is shakier than previously believed.

A team of astrophysicists and nuclear physicists concluded from preexisting experimental evidence that there is large statistical uncertainty over whether a nuclear fusion reaction crucial to forming calcium in faint supernovas would occur often enough in the core of an early star.

The physicists, who published their work May 26 in Physical Review C, said the results could change the origin story of the universe's stars and its calcium content. One author has already proposed an alternative explanation, which suggests that the first generation of stars, also called Population III stars, created calcium in a different way and never underwent supernovas.

"Understanding how the Pop III stars, these very massive first stars, actually evolved — whether they actually exploded as a faint supernova — that is a key challenge," said senior author Falk Herwig, a professor of physics and astronomy at the University of Victoria. "This is an important step towards solving that mystery."

At the start of the universe, the Big Bang created only a few elements: hydrogen, helium and traces of lithium and beryllium. All other elements have been primarily created within stars across their lifetime, such as during a supernova, the explosive death of a high-mass star. The matter they create often lives on in other forms, including other stars, planets and life.

The first stars are believed to have contained primarily hydrogen and helium and practically none of these heavier elements, which are more prevalent in recently formed stars, such as the metal-rich sun.

But the 2014 discovery of one of the oldest-known stars in the Milky Way Galaxy complicated the heavy-element history and sparked interest in the origins of calcium, according to Herwig.

In a first, the star SMSS J0313−6708 contained no observable traces of iron, but was rich in calcium. Such a combination is unexpected for the supernova remnants from which the star would have formed. Herwig said this raised the question of what kind of fusion had to have taken place within the Population III star that preceded it.

The authors behind the old star's discovery and other scientists have said that the creation of the calcium excess started in the CNO cycle, a nuclear fusion process in which carbon, nitrogen and oxygen catalyze the fusion of hydrogen into helium. A "breakout" from this cycle would lead to the formation of calcium in Population III stars, and they would eventually experience calcium-rich faint supernovas, which are much fainter and briefer than regular supernovas.

The new study focuses on the fluorine nuclei that would follow CNO breakout, and two possible reactions it could undergo in a Population III star. By capturing a proton, fluorine could either grow and become neon — getting one step closer to calcium — or become the smaller element of oxygen and move away from calcium. How frequent each reaction is relative to another determines how much calcium is made.

"That is the bottleneck reaction that determines whether through the CNO breakout ... you can actually get to calcium or not," Herwig said.

The astrophysicist and his colleagues said in their study that little attention has been paid to the fluorine-to-neon reaction, which rarely occurs in the sun and is much more prevalent in metal-poor stars.

The physicists aggregated past experimental results of both fluorine fusion reactions and re-analyzed them collectively. The statistical uncertainty on the fluorine-to-neon reaction rate was found to be much higher than previously understood — about 10 times larger than in previous results at some energy ranges.

This change in uncertainty is a "pretty significant" increase, according to first author James deBoer, a research assistant professor of physics at the University of Notre Dame. 

"It turned out that there was a larger uncertainty than had been estimated previously," deBoer said. "There's just a lot of additional modern information that's known that hadn't been applied to this before."

Much of the uncertainty stemmed from extrapolating observations of high-energy nuclear physics down to lower energy levels found in the stars — despite the core of stars being extremely hot and high-pressure. Experiments at these lower energies would improve precision on the rate of the fluorine-to-neon reaction, deBoer said.

He and his co-authors wrote that the result "cast[s] doubt" on the faint-supernova scenario's ability to explain the observed calcium abundance.

Other work from these authors has raised questions about the CNO breakout process that theoretically drives these early faint supernovas. An earlier study led by a former Ph.D. student under Herwig, Ondrea Clarkson, found that the fluorine-to-neon reaction's efficiency over the fluorine-to-oxygen reaction in their simulations was one-tenth of what would be needed to create enough calcium in Population III stars.

Clarkson and Herwig also developed an alternative explanation for calcium abundance, in which Population III stars did not undergo a faint supernova and instead collapsed directly into a black hole. Interactions between concentric shells of hydrogen and helium within a star would cause carbon in the helium shell to capture neutrons. The heavier element that is created would also capture neutrons, and the process would continue until calcium is made.

Some of the calcium would be ejected into space by the star, which would later implode into a black hole and absorb the remaining heavy elements closer to the core, including iron and the rest of the calcium.

The scenario could explain why the old star discovered in 2014 had undetectable levels of iron but significant amounts of calcium, but more computer simulations are needed to ensure that this process would play out as expected in early stars, according to Herwig.

The study,"19F(p,γ)20Ne and 19F(p,α)16O reaction rates and their effect on calcium production in Population III stars from hot CNO breakout," published May 26 in Physical Review C, was authored by R. J. deBoer, J. Görres, P. Scholz and M. Wiescher, University of Notre Dame; O. Clarkson and F. Herwig, University of Victoria, Michigan State University and Los Alamos National Laboratory; and I. Lombardo, INFN, Sezione di Catania.

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