New Nuclear Reaction Measurements Sharpen the Picture of Supernovae and X-ray Bursts

Researchers from the University of Surrey have measured nuclear reactions at the heart of two of the universe's most powerful explosions — supernovae and X-ray bursts — in a pair of studies published in Physical Review Letters. The measurements replace theoretical estimates with experimental data, letting astronomers test their computer models of exploding stars against what telescopes actually observe.

The first study concerns supernovae, the bright explosions that mark the death of massive stars. One of the best observational clues to how they unfold is radioactive titanium-44, which is produced in the blast and remains detectable by space telescopes long afterwards. Working at Argonne National Laboratory in the United States, the team obtained the first experimental data needed to fix the rate of a nuclear reaction that controls how much titanium-44 a supernova makes. The reaction proceeds much more slowly than previously thought, raising predicted titanium-44 production by up to 35 percent. "A measurement like this would have been considered out of reach only a couple of decades ago," said Christopher Cousins, a postdoctoral researcher in Surrey's Nuclear Physics Group.

The second study examined type I X-ray bursts — the most frequent stellar explosions in the universe — which occur when a dense neutron star pulls material from a nearby companion star, triggering runaway thermonuclear reactions. At the Facility for Rare Isotope Beams in Michigan, the team measured a reaction that powers these bursts with more than ten times less uncertainty than before. The result settles a long-running question about the nickel-copper cycle, a process that can temporarily trap nuclear material during an explosion: material does become trapped, but likely only a small proportion, said Connor O'Shea, a postdoctoral research fellow in nuclear astrophysics.

Gavin Lotay, professor of nuclear astrophysics at Surrey, said the two studies provide experimental evidence where scientists previously had to rely on theory and estimates, giving a clearer basis for models of how the chemical elements are created and spread through the universe.