Mystery of Gold's Origin Probed by New Research Into Heavy Element Synthesis in Universe

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Gold bars - Sputnik International, 1920, 16.11.2021
While all heavy elements on Earth were formed in extreme astrophysical conditions, such as those generated by stellar explosions and collisions of neutron stars, it is the mystery of how the heaviest elements, such as gold and uranium, were created, that has particularly intrigued researchers.
In a step forward towards unraveling the origin of heavy elements like gold and uranium produced in our Universe, a collaborative team of researchers has carried out breakthrough computer simulations.
Scientists from the GSI Helmholtz Centre for Heavy Ion Research (GSI Helmholtzzentrum für Schwerionenforschung) in Darmstadt, jointly with colleagues from Belgium and Japan, revealed that promising candidates for synthesis of heavy elements are certain black holes with orbiting matter accumulations or “accretion disks”.
© NASA . Swift/Aurore Simonnet, Sonoma State UniversityIn this artist's rendering, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close.
In this artist's rendering, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close.  - Sputnik International, 1920, 16.11.2021
In this artist's rendering, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close.
These systems are formed either after the merger of two massive neutron stars, or during a so-called collapsar. The latter is the collapse and subsequent explosion of a rotating star, stated research, the results of which were published in the journal Monthly Notices of the Royal Astronomical Society.
A high number of neutrons - a basic requirement for the synthesis of heavy elements – facilitates a rapid neutron-capture process. Playing a crucial role in this process are almost massless neutrinos, sub-atomic particles that permeate the Universe, as they enable conversion between protons and neutrons.
"In our study, we systematically investigated for the first time the conversion rates of neutrons and protons for a large number of disk configurations by means of elaborate computer simulations, and we found that the disks are very rich in neutrons as long as certain conditions are met," said Dr. Oliver Just from the Relativistic Astrophysics group of GSI's research division Theory.
Heavy elements present on planet Earth today were all formed under diverse extreme conditions in the astrophysical environment. The appropriate conditions were either generated inside stars, or resulted from stellar explosions and the collision of neutron stars.
Researchers have been probing which of these astrophysical events provided optimal conditions for the formation of the heaviest elements, such as gold or uranium.
While trailblazing observation of gravitational waves and electromagnetic radiation originating from a neutron star merger in 2017 had suggested that heavy elements could be produced as fallout from these cosmic collisions, it is yet to be determined when and why the material is ejected. Furthermore, scientists have been wondering if there might be other potential promising scenarios for the creation of these heavy elements.
"The decisive factor is the total mass of the disk. The more massive the disk, the more often neutrons are formed from protons through capture of electrons under emission of neutrinos, and are available for the synthesis of heavy elements by means of the r-process," claims the study.
With the optimal disk mass for production of heavy elements revealed by research to be about 0.01 to 0.1 solar masses, there is now convincing evidence that neutron star mergers producing accretion disks with these masses could be where the heavy elements originate.
Further research will be needed to investigate whether and how frequently such accretion disks occur in collapsar systems.
The projected abundance of formed heavy elements testifies to the need to push ahead with further studies in future laboratories, such as the Facility for Antiproton and Ion Research (FAIR), currently under construction, emphasised the team of scientists.
Theresearch group, led by Dr. Andreas Bauswein, also observed light signals generated by the ejected matter, which can be used to deduce the mass and composition of the aforementioned matter. However, to do so scientists need accurate knowledge of a spate of properties of these newly formed elements, including their masses.
"With the next generation of accelerators, such as FAIR, it will be possible to measure them with unprecedented accuracy in the future,” stated Bauswein.
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