Gold is one of the most coveted metals in the world, but its creation from heavy metals such as gold, thorium and uranium requires energetic conditions, such as stellar explosions or a collision between neutron stars. This means that all the heavy elements on Earth formed under extreme conditions in astrophysical environments.
Today, astrophysicists have an incomplete understanding of how elements heavier than iron are made. Researchers are intrigued by the question in which of these astrophysical events the appropriate conditions exist for the formation of the heavier elements. To surprise, a new study shows that these elements could form in the accretion disks of black holes.
The accretion disk is called the swirling chaos that surrounds an active newborn black hole as it swallows the dust and gas from space around it. In these extreme environments, the high rate of neutrino emission should facilitate the conversion of protons to neutrons, which would give rise to an excess of the latter, just what is required for the process that produces the heavy elements.
“In our study, we systematically investigated for the first time the neutron and proton conversion rates for a large number of disk configurations using elaborate computer simulations, and found that disks are very rich in neutrons as long as certain conditions are met,” Explain Dr. Oliver Just, from the Relativistic Astrophysics group in the Theory research division of the GSI.
Just dice that: the decisive factor is the total mass of the disk. “The more massive the disk, the more often neutrons are formed from protons by electron capture under neutrino emission, and are available for heavy element synthesis via the r-process.”
On the contrary, if the mass of the disk is very high, the reverse reaction plays a more important role, so that neutrinos are recaptured to a greater extent by neutrinos before leaving the disk. These neutrinos are converted back into protons, making the rapid neutron capture process, or r-process, difficult.
The study indicates that the optimal mass of the disk to become a factory for gold and other heavy materials is between 0.01 and 0.1 solar masses. Since it is currently unclear if and how often these accretion disks occur in collapse systems, the research is still inconclusive.
“These data are currently insufficient. But with the next generation of accelerators, such as the Facility for Antiproton and Ion Research (FAIR), it will be possible to measure them with unprecedented precision in the future.” said astrophysicist Andreas Bauswein of the GSI Helmholtz Center for Heavy Ion Research.
It is known that a large number of elements are produced inside stars, but when we go to elements heavier than iron, literally catastrophic events are used. One of the most extreme events occurs during the birth of black holes. However, astrophysicists are not sure that the conditions actually hold, apart from the relative contributions of these to the overall abundance of heavy elements in the universe.
The team has been hard at work, using simulations to determine if this is really the case. Rhetorically we can call it the magical moment in which astrophysics and computing come together to trace the history of objects that are common to us today, but as we have seen, its origins go back to cosmic events which also includes the bizarre holes blacks.
The research was published in Monthly Notices of the Royal Astronomical Society.
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