How does the formation of chemical elements in the Universe take place? Where do heavy elements like gold or uranium come from? Using computer simulations, a team of researchers from the Center for the Study of Heavy Ions. Helmholtz in Darmstadt, Germany, together with colleagues from Belgium and Japan, shows that the synthesis of heavy elements is a characteristic process for black holes around which accretion discs form. The calculated number of elements forming allows us to determine which heavy elements will need to be studied in future laboratories – such as the Facility for Antiproton and Ion Research (FAIR) currently under construction – in order to understand the origin of these heavy elements in the Universe.
All the heavy elements present on Earth today were formed under extreme conditions somewhere in space: inside stars, during stellar explosions, and also during collisions between neutron stars. Researchers are interested in which of these events and processes could give rise to the heaviest elements, such as gold or uranium. When scientists were able to observe gravitational waves for the first time along with their corresponding light in the electromagnetic range in 2017 – it was a merger between two neutron stars – they found out that during such events a significant amount of heavy elements can be formed. However, the question of when and why the eruption of material occurs, as well as possible alternative scenarios for the synthesis of heavy elements, continued to remain open.
The most promising candidates for the Universe’s gold mine are black holes surrounded by a spinning disk of matter, or accretion disk. Such a system is formed as a result of the merger of two massive neutron stars, as well as as a result of the so-called collapsar, the collapse and subsequent explosion of a rotating star. The internal composition of such accretion disks has been insufficiently studied until now, in particular, in relation to the conditions under which the formation of an excess of neutrons occurs. A high concentration of neutrons is the basic condition for the reactions of synthesis of heavy elements, since it allows the development of a rapid process of neutron capture, or r-process. Nearly massless neutrinos play a key role in this process, as they allow conversion between protons and neutrons to occur.
“In our work, for the first time, we systematically analyzed the conversion rates of neutrons and protons for a large number of disk configurations using computer simulations, and we found the conditions under which the disks are rich in neutrons,” explains Dr. Oliver Just of the Relativistic Astrophysics Group of the Research Unit called Theory of the Center for the Study of Heavy Ions. Helmholtz. – The decisive factor is the total mass of the disc. The more massive the disk, the more often neutrons are formed from protons as a result of electron capture during neutrino emission, which contributes to the r-process. However, if the disk mass is too large, a reverse reaction develops, as a result of which excess neutrinos are re-captured by neutrons before they can leave the disk. These neutrons are then converted back to protons, which slows down the r-process. ” According to the findings of the study, the optimal mass of a disk surrounding a black hole to produce large quantities of gold and other heavy elements is between 0.01 and 0.1 times the mass of the Sun.