It is possible that the Universe produces heavy elements in more ways than we previously thought.
Metallic elements such as gold, silver, uranium, and thorium can only be created under extreme conditions, such as a supernova explosion or a collision between neutron stars.
Nevertheless, a new study indicates that these elements might form in the swirling chaos ringing an active newborn black hole as it devours dust and gas from the space around it.
In these extreme environments, the relatively high rate of neutrinos should facilitate the conversion of protons to neutrons, resulting in an excess of neutrons, which are required for the production of heavy elements.
"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," says astronomer Oliver Just of the GSI Helmholtz Centre for Heavy Ion Research in Germany.
After the Big Bang, there were few elements in the universe. Before stars were born and started combining atomic nuclei in their cores, the Universe consisted primarily of hydrogen and helium.
When a star dies, its nuclear fusion process imbues the cosmos with heavier elements, from carbon to iron for the most massive stars.
Iron, however, poses a challenge to core fusion. As fusion produces more heat and energy than it generates, the core temperature of the star drops, resulting in a spectacular explosion - the supernova.
That spectacular bang (and the bangs of colliding neutron stars) occurs when heavier elements are fused together. As a result of the explosion, atoms colliding with force can capture neutrons from each other.
It is known as the rapid neutron capture process or r-process; it needs to occur rapidly in order to prevent radioactive decay to occur before the nucleus is replenished with neutrons.
However, it is unclear whether there are other situations in which r-processes can take place, although newborn black holes would seem to be a promising candidate. Specifically, when two neutron stars merge, and their combined mass is sufficient to turn the newly formed object into a black hole.
There is also the possibility of collapsars: the gravitational collapse of the core of a massive star into a black hole of stellar mass.
The baby black hole is believed to be enclosed in a dense, hot ring of material, which spirals around it and feeds into it, like water flowing into a drain. Observers have long speculated that r-capture nucleosynthesis could be taking place in these environments because neutrinos are emitted in abundance.
To determine whether this is the case, Dr. Just and his colleagues conducted a series of simulations. A number of parameters were varied, including the mass of the black hole and its spin, the mass of the material surrounding it, and the effect of different parameters on neutrinos. They discovered that r-process nucleosynthesis is possible in these environments if conditions are just right.
"The decisive factor is the total mass of the disk," Just said.
"The more massive the disk, the more often neutrons are formed from protons through the 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."
A disk mass between 1 and 10 percent of the mass of the Sun is a sweet spot in which heavy elements are produced most abundantly. As a consequence, neutron star mergers with disk masses in this range could be used to manufacture heavy elements. Because it is unclear how common collapsar disks are, the jury is still out on collapsars, the researchers said.
We must now determine how the light emitted from a neutron star collision can be used to calculate the mass of its accretion disk.
"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 accuracy in the future," said GSI Helmholtz Centre for Heavy Ion Research scientist Andreas Bauswein.
"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."
