Scientists identify flaws in astrophysical models of massive stars and supernovae

An international research team has found evidence that astrophysical models of massive stars and supernovae are inconsistent with observational gamma-ray astronomy.

The research team used a new experimental method to study the unstable isotope iron-60. Iron-60 is of interest to astrophysicists because it occurs inside massive stars and is ejected from supernovae throughout the galaxy. Their findings are published in Nature Communications.

Team members included Artemis Spirou, professor of physics at the Rare Isotope Beam Facility (FRIB) and the Michigan State University (MSU) Department of Physics and Astronomy; Sean Liddick, FRIB associate professor of chemistry and head of MSU’s Department of Chemistry and Experimental Nuclear Sciences, and 11 FRIB graduate students and postdoctoral researchers.

To investigate the isotopes, Spirou’s team developed a new method at the National Superconducting Cyclotron Institute (predecessor to FRIB) in collaboration with the university’s Nuclear and Energy Physics Professor Anne Cecily Larsen and Emeritus Professor Magne Guttomsen. The experiment was conducted using of Oslo, Norway.

“What’s unique about what we bring to this collaboration is that we combine our expertise in nuclear reactions, isotope beams, and beta decay to learn about reactions that can’t be directly measured,” Spirou said. “In this paper, we tried to measure enough of the characteristics surrounding the reaction of interest to be able to suppress the reaction better than before.”

Models are essential for predicting rare astrophysical phenomena

Iron-60 has a long half-life for an unstable isotope, more than 2 million years, so it leaves a lasting imprint of the supernova from which it originated. Specifically, as iron-60 decays, it emits gamma rays, which scientists can measure and analyze to gain clues about the lifespan of stars and the mechanisms of their explosive deaths. Physicists use this data to create and improve astrophysical models.

“One of the most important goals of nuclear science is to achieve a comprehensive predictive model of the nucleus that accurately describes the nuclear properties of any atomic system,” Riddick said. “But we don’t have that yet. Measure these processes experimentally first.”

Scientists must generate and observe these rare isotopes and compare the results with model predictions to check accuracy.

“To study these nuclei, you can’t just find them naturally on Earth,” Spirou says. “We have to make them, and that’s FRIB’s specialty: take the stable isotopes we can find, accelerate them, fragment them, and make those exotic isotopes These isotopes can only live for a few milliseconds, so they can be studied. “

To accomplish this goal, Spirou and her team devised an experiment that served two purposes. One is to limit the neutron capture process that converts the isotope iron-59 to iron-60. Second, they wanted to use the resulting data to investigate long-standing discrepancies between supernova model predictions and observed signatures of these isotopes.

New method allows better study of short-lived isotopes

Iron-60 has a relatively long half-life, but its neighbor iron-59 is less stable and decays with a half-life of 44 days. This makes neutron capture in iron-59 particularly difficult to measure in the laboratory, as it decays before a suitable measurement can be performed. To overcome this problem, scientists have developed a unique indirect method to suppress this reaction experimentally.

Spirou and Riddick worked closely with colleagues at the University of Oslo to develop a new method to study these highly unstable isotopes. The result, called the Beta-Oslo method, is a variation of the Oslo method first developed by Guttoumsen, a co-author of the project at the Oslo Cyclotron Institute.

Guttormsen’s approach uses nuclear reactions to position atomic nuclei so that researchers can measure their properties. For decades, many astrophysical and nuclear structure applications have been demonstrated, but only for (almost) stable isotopes.

By combining their expertise in detection, beta decay, and reactions, the researchers devised a way to locate target nuclei using the process of beta decay itself, rather than reactions. This innovative approach provided a way to more efficiently produce the isotopes they were looking for and suppress neutron capture reactions in short-lived nuclei.

“The Beta-Oslo method is the only technique that can impose these constraints on very exotic nuclei that are far from stable,” Spirou said.

Modifying the model takes time

After suppressing these key uncertainties about the network of nuclear reactions that produce iron-60, Spyroux’s team concluded that the reaction is twice as likely to occur inside a massive star than models predict. I attached it. Researchers now believe that theoretical models of supernovae are flawed and that there are certain star properties that are still inaccurately represented.

“Solutions to this puzzle could include, for example, reducing the star’s rotation, assuming a smaller explodable mass limit for massive stars, or changing other stellar parameters,” the researchers write in their paper’s conclusion. This should be obtained from stellar modeling using methods such as

This discovery not only has far-reaching implications for the theoretical understanding of massive stars and their internal states, but also further demonstrates that the Beta-Oslo method will be a valuable tool for scientists going forward.

“This would not have been possible without our project partners at the University of Oslo. They inspired Artemis and me when we presented the Oslo Method at an MSU seminar in 2014,” Riddick said. spoke.

“We approached them that day with a question about the use of beta decay, and the discussion started from there. We have worked together ever since, and there is no doubt that we will continue to work together for many years to come. there is no.”