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Cosmic collisions forge gold
By Louisa Connolly
Posted: 14 September 2011

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The cosmic production site of heavy chemical elements such as gold and lead may have finally been identified in the merger events of two neutron stars thanks to detailed numerical simulations conducted by scientists at the Max Planck Institute for Astrophysics (MPA) and affiliated to the Excellence Cluster Universe and of the Free University of Brussels (ULB).

Most heavy chemical elements are formed as a product of nuclear fusion, a process that for example in our own Sun occurs when two lighter hydrogen nuclei combine, creating a heavier helium nuclei, whilst releasing vast amounts of energy. In more massive stars, once the hydrogen has all been burned, heavier elements are then formed from helium; this process continues through the elements up to iron. Further fusion reactions do not yield any net energy gain therefore there must be another process in which heavier elements form.

"The source of about half of the heaviest elements in the Universe has been a mystery for a long time," says Hans-Thomas Janka, senior scientist at the Max Planck Institute for Astrophysics (MPA) and within the Excellence Cluster Universe. "The most popular idea has been, and may still be, that they originate from supernova explosions that end the lives of massive stars. But newer models do not support this idea."

Computer simulations of the merger of two neutron stars includes both the evolution of the neutron star matter during the cosmic crash and the formation of chemical elements in the tiny fraction of the whole matter that gets ejected during such events, involving the nuclear reactions of more than 5000 atomic nuclei which produce heavy elements. Image: Max Planck Institute for Astrophysics.

Heavier elements are assembled when neutrons are captured onto seed nuclei – heavy nuclei that are the starting point of fusion chain reactions. This involves two processes; the slow neutron capture process (s-process) and the rapid neutron capture process (r-process). Both processes add a neutron to a nucleus to make it heavier, but the length of time it takes depends on factors such as the conditions within the atom and the cause of reaction. For example, the s-process takes place at low neutron densities inside stars during their late evolution stages, while the r-process is responsible for producing large fractions of the elements heavier than iron, such as platinum, gold, thorium, and plutonium, a process which requires very high neutron densities.

Scientists at the MPA and the ULB are concentrating on detailed computer simulations of compact object mergers such as two neutron stars colliding in a binary system to accommodate the high neutron density required by the r-process. "In just a few split seconds after the merger of the two neutron stars, tidal and pressure forces eject extremely hot matter equivalent to several Jupiter masses," explains Andreas Bauswein, who carried out the simulations at the MPA.

Once this plasma-like matter has cooled to less than 10 billion degrees, nuclear reactions take place that enable the production of heavy elements. "The heavy elements are 'recycled' several times in various reaction chains involving the fission of super-heavy nuclei, which makes the final abundance distribution become largely insensitive to the initial conditions provided by the merger model," adds Stephane Goriely, ULB researcher and nuclear astrophysics expert of the team.

These simulations show that the abundance distribution of the heaviest elements is similar to that observed in our Solar System. By combining the results of the simulations with the estimated number of neutron star collisions in the Milky Way in the past, the results strongly indicate that such events could be the main sources of the heaviest chemical elements in the Universe.

The team now plan to refine their models whilst observational astronomers continue to look out for detecting the transient celestial sources that should be associated with the ejection of radioactive matter in neutron star mergers. A discovery such as this would be the first observational look at r-process elements being formed in their source of origin.