The surprise of the supernova creates elemental mystery

Researchers at Michigan State University (MSU) have found that one of the most important reactions in the universe can get a huge and unexpected push within exploding stars known as supernovae.

This discovery also challenges the ideas behind what some of Earth’s heavy elements are made of. In particular, it overturns a theory that explains the planet’s unusually large quantities of some forms, or isotopes, of the elements ruthenium and molybdenum.

“It’s amazing,” said Luke Roberts, assistant professor at the Facility for Rare Isotope Beams, FRIB, and the Department of Physics and Astronomy, at MSU. Roberts implemented the computer code that the team used to model the environment inside a supernova. “It certainly took us a long time to make sure the results were correct.”

The results, published online December 2 in the journal Nature, show that the innermost regions of supernovae can forge carbon atoms 10 times faster than previously thought. This carbon creation occurs through a reaction known as the triple alpha process.

“The triple alpha reaction is, in many ways, the most important reaction. It defines our existence,” said Hendrik Schatz, one of Roberts’ collaborators. Schatz is Distinguished Professor in the Department of Physics and Astronomy and at the Facility for Rare Isotope Beams and director of the Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements, or JINA-CEE.

Almost all of the atoms that make up the Earth and everything on it, including people, have been forged in the stars. Fans of the late author and scientist Carl Sagan may recall his famous quote, “We are all made of stars.” Perhaps no stellar matter is more important to life on Earth than the carbon produced in the cosmos by the triple alpha process.

The process begins with alpha particles, which are the nuclei of helium atoms, or nuclei. Each alpha particle is made up of two protons and two neutrons.

In the triple alpha process, the stars fuse three alpha particles together, creating a new particle with six protons and six neutrons. This is the most common form of carbon in the universe. There are other isotopes produced by other nuclear processes, but these make up just over 1% of the Earth’s carbon atoms.

However, fusing three alpha particles together is usually an inefficient process, Roberts said, unless there’s something helping him. The Spartan team revealed that the innermost regions of supernovae may have such fluctuating helpers: excess protons. Therefore, a proton-rich supernova can accelerate the triple alpha reaction.

But speeding up the triple alpha reaction also slows the supernova’s ability to create heavier elements on the periodic table, Roberts said. This is important because scientists have long believed that proton-rich supernovae created Earth’s astonishing abundance of a few isotopes of ruthenium and molybdenum, which contain more than 100 protons and neutrons.

“You don’t make those isotopes in other places,” Roberts said.

But based on the new study, you probably don’t even produce them in proton-rich supernovae.

“What I find fascinating is that now you have to find another way to explain their existence. They shouldn’t be here with this abundance,” Schatz said of the isotopes. “It is not easy to find alternatives”.

“It’s a bit of a disappointment in a way,” said project creator Sam Austin, a distinguished MSU professor emeritus and former director of the National Superconducting Cyclotron Laboratory, the predecessor of the FRIB. “We thought we knew, but we don’t know well enough.”

There are other ideas out there, the researchers added, but none that nuclear scientists find completely satisfactory. Furthermore, no existing theory yet includes this new discovery.

“Whatever happens next, you have to consider the effects of an accelerated triple alpha reaction. It’s an interesting puzzle,” Schatz said.

While the team has no immediate solutions to this conundrum, the researchers said it will impact upcoming experiments at the FRIB, MSU, which was recently designated as the US Department of Energy (DOE-SC) user facility.

Furthermore, MSU provides a fertile ground for new theories to sprout. It hosts the nation’s best graduate program for training the next generation of nuclear physicists. It is also a central institution of JINA promoting collaborations through nuclear physics and astrophysics such as this one, which also included Shilun Jin. Jin worked on the project as an MSU postdoc and has since joined the Chinese Academy of Sciences.

So while Austin has expressed some disappointment that this finding contradicts long-standing notions of the creation of elements, he also knows that it will fuel new science and a better understanding of the universe.

“Progress comes when there is a contradiction,” he said.

“We love progress,” Schatz said. “Even when it’s destroying our favorite theory.”