How are chemical elements produced in our universe? Where do heavy elements like gold and uranium come from? Using computer simulations, a research team at the GSI Helmholtzzentrum fur schwerionenforschung in Darmstadt, along with colleagues in Belgium and Japan, typified a particular black hole, the so-called accretion disc, where the synthesis of heavy elements involves the accumulation of orbital material. The predicted abundance of formed elements requires studying which heavy elements in future laboratories such as the Antiproton and Ion Research Facility (FAIR) currently under construction to elucidate the origin of the heavy elements. The results are published in the monthly report of the Royal Astronomical Society.
Today, all heavy elements on Earth are formed under the extreme conditions of the astrophysical environment: inside a star, during a stellar explosion, and during a neutron star collision. Researchers have been intrigued by the question of which of these astrophysical events have suitable conditions for the formation of the heaviest elements such as gold and uranium. Spectacular first observations of gravitational waves and electromagnetic radiation from neutron star merger in 2017 suggested that these cosmic collisions could produce and emit very heavy elements. However, the question remains when and why the material is released, and if there are other scenarios in which heavy elements can be produced.
A promising candidate for heavy element production are black hole orbited by an accretion disk of dense & hot matter. Such a system is formed both after the fusion of two giant neutron stars & during a so called collapsar, the collapse & subsequent explosion of a rotating star. The internal composition of such accretion disks has not been well understood, especially with respect to the conditions under which excess neutrons are formed. Numerous neutrons are a fundamental requirement for the synthesis of heavy elements, as they enable rapid neutron capture or r-processes. Neutrinos, which have little mass, play an important role in this process as they allow the conversion between protons and neutrons.
“In our study, we used extensive computer simulations for the first time to systematically investigate the conversion of neutrons and protons for large number of disk configurations. As long as certain known conditions are met, the disk is very rich in neutrons. We found out that I was there, “explains Dr. Oliver Just of the Relativist Astrophysics Theory Group of the GSI Research Division. “The decisive factor is the total mass of the disk. The larger the mass of the disk, the more neutrons are formed from protons by electron capture under neutrino emission, which is used for the synthesis of heavy elements via r-process. If the mass of the disk is too large, the inverse reaction will play a more important role and the neutrons will recapture more neutrons before leaving the disk. These neutrons will be converted back-to-protons, which hinders r-process. As study shows The optimal disk mass for prolific production of heavy elements is about 0.01-0.1 solar mass. The result is that neutron fusion fuses that produce accretion disk with these exact masses could be point of origin for large fraction of heavy elements. It provides strong evidence. However, it is currently un-clear if and how often these accretion disk occur in a collapsed system.
In addition to the process of mass ejection, a research group led by Dr. Andreas Bauswein also investigating the light signals generated by ejected matter. It will be used to estimate the mass and composition of the ejected-matter in future observations of colliding Neutron stars. An important building-block for the correct reading of these light signals is accurate knowledge of the mass & other properties of the newly formed element. “These data are currently inadequate, but next-generation accelerators like FAIR, it will allow us to measure them with unprecedented accuracy in the future.” Well-tuned interactions of theoretical models, experiments, and astronomical observations will allow researchers to test neutron nergers as the origin of the elements of the r-process in the coming years, “Bauswein predicts.
The results are published in the monthly report of the Royal Astronomical Society.