During their long lifetimes stars generate their energies by nuclear fusion in their interiors, which are generally accepted to be the breeding grounds for carbon and heavier elements. The heaviest elements made this way are iron and nickel; heavier elements are thought to be built by slow and/or rapid neutron-capture reactions, the s- and r-processes. Although these mechanisms for nucleosynthesis have been known for some time, the abundances of some heavy elements have remained a mystery. Now Carla Fröhlich of the Universität Basel and Gabriel Martínez-Pinedo of the Gesellschaft für Schwerionenforschung, Darmstadt, and colleagues have proposed a novel nucleosynthesis that might solve these puzzles.
When a massive star forms a supernova, part of the matter in the stellar interior forms a neutron star, and the liberated energy, mainly in the form of neutrinos, contributes to the ejection of the stellar mantle into the interstellar medium. The temperature of the deepest ejected layers is so hot that nuclei are decomposed into free protons and neutrons. The tremendous flux of neutrinos and antineutrinos, which accompanies the birth of the neutron star, can be absorbed by the nucleons and so determines the relative abundance of protons and neutrons and hence the composition of the nuclei that form when the ejected matter reaches cooler regions.
During the later stages of the explosion the matter is expected to become rich in neutrons, so supernovae are believed to be the site of heavy-element production by the r-process. However, it has been realized very recently that during the first second of the explosion the ejected material is rich in protons.
When Fröhlich and colleagues studied the nucleosynthesis in this proton-rich environment they discovered possible solutions to two long-standing problems. First, they could reproduce the abundances of elements such as scandium, copper and zinc, for which calculations had previously fallen notoriously short. More surprisingly, they also noticed the appearance of heavier elements such as strontium, molybdenum, ruthenium and beyond (C Fröhlich et al. 2006).
This heavy-element production can be attributed to a novel nucleosynthesis process, which Fröhlich and colleagues named the νp process after the two main contributors: proton capture, which transports matter sequentially to higher charges, and (anti)neutrinos, which are captured by free protons and so change the protons to neutrons. This presence of neutrons allows the flow in element creation to circumvent long-lived nuclei such as 56Ni and 64Ge, so enabling the synthesis of heavier elements.
The νp process is a primary process, that is, it should occur in all core-collapse supernovae. As a consequence there should already be fingerprints of νp nucleosynthesis in the earliest and most primitive stars. Indeed, finding strontium in the most metal-poor, and hence oldest, star observed so far came as a big surprise last year. This might now be explained as debris from the νp process that had operated in an earlier supernova. Further observations of elemental abundances in metal-poor stars combined with progress in supernova modelling and improved knowledge of the nuclei involved – as expected from future facilities such as the Facility for Antiproton and Ion Research – will help to disentangle the importance of the νp process for the abundances of the elements in the universe.
Further reading
C Fröhlich et al. 2006 Phys. Rev. Lett. 96 142502.
