Measurements of heavy form of zinc allow astronomers to model the crust of dead stars

By Andrew Grant

Web edition: January 31, 2013
Print edition: March 9, 2013; Vol.183 #5 (p. 16)

The precise measurement of an exotic atom in the laboratory has refined scientists’ understanding of neutron stars, which are among the universe’s most extreme objects. The study, published January 22 in Physical Review Letters, could help scientists determine whether the crusts of neutron stars serve as the source of dozens of heavy elements such as zinc, silver and gold.

“One of the universe’s overriding mysteries is where heavy elements originate,” says James Lattimer, an astrophysicist at Stony Brook University in New York who was not involved in the study. “These mass measurements allow us to tune our equations so we can work toward settling the debate.”

Neutron stars are not actually stars at all. After a massive star explodes in a supernova, the remnant is a hot, dense ball about 20 kilometers across. It is made up of protons, electrons and lots of neutrons. That sphere packs in a mass larger than that of the sun, with a surface that one study estimates is 10 billion times as strong as steel. Under these extreme conditions, nuclei of atoms that are normally unstable can subsist in the neutron star’s outer layers.

Neutron stars are too far away to study their composition, and scientists cannot re-create the enormous pressures in the lab. But they can plug the measured properties of various neutron-rich atoms into computer simulations that predict neutron star composition. Physicist Robert Wolf of the University of Greifswald in Germany and an international team were particularly interested in determining the mass of zinc-82, which some models predict should occur in the crust of neutron stars. Zinc-82 has a nucleus consisting of 30 protons and 52 neutrons — many more than the 34 neutrons in the most common form of zinc. The challenge was isolating and measuring the rare isotope, most of which would decay in less than a second.

At CERN in Switzerland, Wolf’s team used the On-Line Isotope Mass Separator facility, which consists of a high-energy proton beam that strikes a thick block of uranium carbide. The protons shatter the nuclei in the target, creating a plethora of exotic isotopes that quickly decay into more-stable atoms. The researchers then exposed the atoms to electric and magnetic fields that separated them by mass. Within several tenths of a second, they measured the mass of a pure sample of zinc-82.

The researchers compared this mass with predictions in various computer models and determined that zinc-82 is probably absent from neutron stars. That’s in line with the most well-supported prediction of neutron star composition. Lattimer believes this aspect of the study is not its greatest contribution; previous experiments had already shown that the leading model was more accurate than others. Instead, he is most impressed by the technique’s potential to pin down the characteristics of other exotic nuclei that may exist in neutron stars.

Scientists want to create this compositional profile because neutron stars may be the source of many of the universe’s heavy elements. Fusion reactions in the cores of regular stars produce carbon, oxygen, nitrogen and other elements essential for life. But the heaviest element that fusion can construct is iron. Astrophysicists have long looked for another astronomical process with enough energy to forge heavier elements from protons and neutrons.

One explanation is that these elements form in the midst of the extreme heat and energy of supernovas — no neutron stars required. But simulations show that these explosions have an insufficient quantity of neutrons. That need for neutrons leads to a competing idea: that heavy elements form when two neutron stars collide and some of their crustal material escapes into space.

To evaluate this possibility, theorists need to improve the models of neutron star composition. Mass measurements of heavy atoms like zinc-82, Wolf says, will help them do that. Then astronomers can survey the abundances of heavy elements in various stars and compare them to predictions of what would be produced in neutron star collisions.