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Black holes are like “gold factories” – accretion disks around black holes can produce heavy elements

Formation of cosmic elements: stellar black holes can be the long-awaited formation sites of gold and other heavy elements. Because in the accretion disks around these holes there are enough high-energy free neutrons to generate these elements through rapid neutron capture, a model simulation now suggests. Thus, the candidates for these cosmic gold factories are black holes from the collision of neutron stars, but also from hypernovae.

Almost all of the elements of the periodic table did not appear until long after the Big Bang – with the formation of the first stars in the universe. It wasn’t until nuclear fusion inside melted hydrogen and helium into larger atoms, including iron. Even for heavier elements, on the other hand, the process of capturing neutrons is necessary. The free neutrons must collide with the atom, sometimes turning into protons and thus forming a new element.

When neutron stars collide, heavy elements are formed, as shown by spectroscopic data. © University of Warwick/Mark Garlick/CC-by-sa 4.0.0 Update

Where are the rest of the element factories?

For gold, platinum, uranium and other especially heavy elements, the ordinary, slow neutron capture But not off. They can arise only if free neutrons have a certain minimum energy – they are released, among other things, in the collision of neutron stars. In 2017, astronomers succeeded for the first time in such a collision first tracks From Gold & Co. to Proof, 2019 It’s confirmed With a more detailed analysis.

However, there are too few neutron star collisions in the universe to explain the full amount of heavy elements in the universe. Gold, for example, comes ffive times more In our galaxy too much with just this pattern of formation, researchers calculated in 2020. So they suspect the energy is too high Hypernovae The collapsing neutron star and the resulting black holes allow the neutron to be picked up quickly.

Black holes on the horizon

Oliver Just of the GSI Helmholtz Center for Heavy Ion Research in Darmstadt and his colleagues have now identified which black holes can be considered as element factories and how their accretion disks should be designed. “In our study, we systematically examined the conversion rates of neutrons and protons for a large number of disk configurations using complex computer simulations,” Just explains.

The result: in fact, the accretion disks of some black holes have good conditions for the formation of the heaviest elements through the rapid capture of neutrons. Because there are enough fast neutrons that they can collide with atoms and form new elements, the researchers report.

Disk block is critical

However, there is also a limitation: “What is critical is the total mass of the plate. The larger the disk, the more neutrons that are formed from protons by capturing electrons and emitting neutrinos, and thus are available for the synthesis of heavy elements using the r process.” More neutrons are converted into protons. Then the neutron families would not have an adequate supply. The team found that the optimal disk mass for the element factories ranged from 0.01 to 0.1 solar masses.

This confirms that black holes that arise after colliding neutron stars can actually be good “factories” for gold, platinum and the like. Because, according to Jost and colleagues, many of them have accretion disks in this mass range. But also black holes caused by hypernovae are theoretically possible – stellar explosions in which a star first becomes a neutron star and then collapses due to further flow of matter into the black hole. However, the flow of matter should be relatively high, the researchers reported.

Many questions remain unanswered

So black holes and their accretion disks could be the places in the universe where the heaviest elements originated and continue to appear. The modeling that Just and his team have now done has helped illuminate at least some of the features and requirements for these item factories. However, as the researchers also emphasize, the search for fast neutron capture sites has only just begun and there are still many unanswered questions. (Monthly Notices of the Royal Astronomical Society, 2021; doi: 10.1093/mnras/stab2861)

Source: GSI Helmholtz Center for Heavy Ion Research