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© NASA/SAO/F.Seward/ESA/ASU/J.Hester & A.Loll/JPL-Caltech/Univ Minn/R.Gehrz
Not such a versatile atom factory
They are the brightest of stars, but supernovae may not forge the heaviest elements. That's the suggestion arising from analysis of a new model of the particle winds that rush from the cores of supernovae.

The only two elements formed in abundance shortly after the big bang were hydrogen and helium. All the heavier ones must have been forged by fusing these smaller nuclei together. The high pressures and temperatures inside ordinary stars can account for elements up to a certain size, but making elements bigger than iron, which has a nucleus containing 26 protons, requires some other mechanism.

That's where supernovae come in. These exploding stars blast neutrinos from their cores to their surface at close to the speed of light, kicking protons and neutrons out of other atoms as they go. This creates a "wind" within which neutrons and protons fuse to form the nuclei of small atoms. Further protons, neutrons and atoms join in to make larger ones.

But atoms larger than nickel, with 28 protons, won't accept new protons as the mutual repulsion of so many positively charged particles becomes too strong. To make these atoms, neutrons must enter the nucleus and then transform into protons once inside, a process known as rapid neutron capture or r-process.

It was assumed that all the heavy elements could be made in this way. Now Thomas Janka of the Max Planck Institute for Astrophysics in Garching, Germany, and colleagues say the composition of the neutrino-driven wind means it won't work for the largest elements.

Janka's team used the latest data on the energies and interactions of protons, neutrons and neutrinos to produce a computer model of a smallish supernova. The ability to make the large elements depends on the number of neutrons that can enter nuclei, which in turn depends on the number of neutrons that are not attached to protons. Janka's model revealed that the wind contains more protons than neutrons, which means there are not enough unattached neutrons to create elements much larger than tin, which has 50 protons (Physical Review Letters, DOI: link)

"It is a final dead end," says Janka. "It is the gravestone for r-process in this environment." Instead, Janka suggests that the neutron-rich explosions that occur when collapsed stars merge create the heaviest elements, including gold, lead and uranium.

The case isn't closed. Kohsuke Sumiyoshi of Numazu National College of Technology in Japan points out that large supernovae may explode differently, as their cores have a different make-up from the small one Janka modelled, and so will produce a different proportion of protons and neutrons. Janka expects larger supernovae to unfold in a similar way to the one he modelled.