Cosmic element forges: Stellar black holes could be the long sought-after formation sites for 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, as a model simulation now suggests. Candidates for such cosmic gold factories are therefore black holes from neutron star collisions, but also from hypernovae.
Almost all elements of the periodic table did not emerge until well after the Big Bang – with the formation of the first stars in the cosmos. It was not until nuclear fusion inside that hydrogen and helium melted together to form larger and larger atoms, including iron. For even heavier elements, on the other hand, the process of neutron capture is necessary. Free neutrons have to collide with the atom, sometimes converting into protons and thus forming a new element.
Where are the rest of the element factories?
For gold, platinum, uranium and other particularly heavy elements, the normal, slow neutron capture but not off. They can only arise if the free neutrons have a certain minimum energy – as they are released in the collision of neutron stars, among other things. In 2017, astronomers succeeded for the first time in such a collision first tracks of gold and co. to prove, 2019 this was confirmed with a more detailed analysis.
However, there are too few neutron star collisions in the cosmos to explain the entire amount of heavy elements in the cosmos. Gold, for example, comes ffive times more often in our galaxy than it should with only this mode of formation, as researchers calculated in 2020. They therefore suspected that also the extremely high-energy Hypernovae of a collapsing neutron star and the resulting black holes allow rapid neutron capture.
Black holes in sight
Oliver Just from the GSI Helmholtz Center for Heavy Ion Research in Darmstadt and his colleagues have now determined which black holes can be used as element factories and how their accretion disks must 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,” explains Just.
The result: In fact, the accretion disks of certain black holes have good conditions for the formation of the heaviest elements through rapid neutron capture. Because there are enough fast neutrons in them that can collide with atoms and form new elements, as the researchers report.
Disc mass is crucial
However, there are also restrictions: “What is decisive is the total mass of the pane. The more massive the disk, the more often neutrons are formed from protons by trapping electrons and emitting neutrinos and are therefore available for the synthesis of heavy elements using the r-process, ”explains Just. But if the disk becomes too heavy, this is reversed and more neutrons are converted into protons. Then the neutron capture will not have enough supplies. The team found that the optimal disk mass for element factories is around 0.01 to 0.1 solar masses.
This confirms that the black holes that arise after neutron star collisions can actually be good “factories” for gold, platinum and the like. Because, according to Just and his colleagues, many of them have accretion disks in this mass range. But also the black holes from hypernovae are theoretically possible – the star explosions in which a star first becomes a neutron star and then collapses due to further influx of matter to the black hole. However, the influx of matter has to be relatively high, as the researchers report.
Many questions still unanswered
Black holes and their accretion disks could therefore be the places in the cosmos where the heaviest elements have arisen and continue to arise. The modeling by Just and his team has now helped shed light on at least some features and requirements of such element factories. However, as the researchers also emphasize, the search for the locations of rapid neutron capture is only just beginning and there are still plenty of unanswered questions. (Monthly Notices of the Royal Astronomical Society, 2021; doi: 10.1093 / mnras / stab2861)
Source: GSI Helmholtz Center for Heavy Ion Research