Physicists Start to Pin Down How Stars Forge Heavy Atoms

Summary

The shards flow through a network of pipes into a fragment separator that sorts them into isotopes of interest. These eventually end up at the SuN, a cylindrical detector 16 inches wide. With metal spokes extending out in all directions, “it kind of looks like the sun, which is fun,” said Ellie Ronning, an MSU graduate student. Just as the nuclei enter, they begin decaying, shedding electrons and emitting flashes of gamma rays that researchers can use to decode the steps of the i-process. “No one’s been able to see these particular processes before,” said Sean Liddick, a FRIB nuclear chemist. By measuring gamma-ray production, the researchers infer the rate at which the relevant isotopes capture neutrons (how readily barium-139 gains a neutron and becomes barium-140, to name one important example). Theorists then input this reaction rate into a simulation of the i-process, which predicts how abundant different heavy elements will be in the final chemical mixture. Finally, they can compare that ratio to the elements observed in different stars. So far, the results seem to draw a circle right where Spyrou and her colleagues had hoped: The relative abundances of lanthanum, barium and europium match what was seen in those carbon-enhanced, metal-poor stars that so puzzled astrophysicists in the early 2000s. “We went from having these huge uncertainties to seeing the i-process fit right where we have the observations,” she said. The i-process, however, would have taken place in the dying stars that came before those metal-poor ones and provided them with material. Right now, the data is compatible with both white dwarfs and red giants as the setting of the i-process. To see which candidate will prevail, if not both, Spyrou will need to study the neutron capture rates of more isotopes. Meanwhile, to distinguish between those candidate stars, Herwig will create better three-dimensional models of the plasma swimming inside them. Going for Gold For 60 years, astronomers have ...