The Universe may have more ways of forging heavy elements than we thought.
The creation of heavy elements in the Universe, such as gold, silver, thorium, and uranium, has long been thought to require specific and energetic conditions, such as a supernova explosion, or a collision between neutron stars. However, a recent study has proposed that the r-process (rapid neutron capture process) responsible for the creation of heavy elements could occur in environments surrounding newborn black holes. These environments are characterized by a dense and hot ring of material, known as an accretion disk, which surrounds the black hole and feeds it with gas and dust from the surrounding space.
“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 Oliver Just, an astrophysicist from the GSI Helmholtz Centre for Heavy Ion Research in Germany.
According to the study, neutrinos play a crucial role in the r-process that creates heavy elements in the vicinity of newborn black holes. In these extreme environments, the high emission rate of neutrinos facilitates the conversion of protons to neutrons, leading to an excess of the latter, which is necessary for the process that produces heavy elements. The study suggests that the ideal conditions for the formation of heavy elements occur when the total mass of the accretion disk ranges from 1 to 10 percent of the mass of the Sun.
“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. However, if the mass of the disk is too high, the inverse reaction plays an increased role so that more neutrinos are recaptured by neutrons before they leave the disk.
These neutrons are then converted back to protons, which hinders the r-process,” said Just.
Although the r-process is known to occur in explosive events such as supernovae and neutron star collisions, the study opens up the possibility that the process could also occur in other scenarios. Specifically, when two neutron stars merge, or when the core of a massive star collapses to form a black hole, which is surrounded by a dense, hot ring of material.
“The well-coordinated interplay of theoretical models, experiments, and astronomical observations will enable us researchers in the coming years to test neutron star mergers as the origin of the r-process elements,” said Andreas Bauswein, an astrophysicist from the GSI Helmholtz Centre for Heavy Ion Research.