[NTD News November 17, 2021 Beijing time]A new study found that a black hole with a specific mass accretion disk created by the collision of two neutron stars produces a large number of heavy elements in the universe, including gold.
How are heavy elements like gold and uranium produced in the universe? Scientists know that all heavy elements are not native to the earth, but are produced by various violent events in the universe, such as the explosion of stars, the merger of neutron stars, etc., but the specific generation process is not clear.
The new research has theoretically ascertained part of this process: the black hole produced by the collision of two neutron stars, if the mass of the accretion disk around it is not too big or small, it is just between 0.01 and 0.1 times that of the sun. In between, such an accretion disk is an ideal environment for producing heavy elements. Researchers believe that a large part of the heavy elements in the universe should be produced in this type of accretion disk.
A variety of violent astronomical events can generate black holes. People are more familiar with the collapse of stars into black holes after their deaths. From the gravitational wave signals detected in 2017, scientists discovered for the first time that the collision of two neutron stars would also turn into a black hole. Neutron stars are the remains of certain stars after they explode.
Researchers speculate that the accretion disk around the black hole is the key to the production of heavy elements. The accretion disk is a circle of high-temperature, high-density matter surrounding the black hole.
Scientists still don’t know much about the material composition in the accretion disk, especially the source of a large number of neutrons, and a large number of neutrons is a basic condition for the synthesis of heavy elements. So they used the most advanced black hole model to simulate the situation of these celestial bodies, and found that the mass of the accretion disk around the black hole is closely related to the production of heavy elements.
Theoretically, protons capture electrons and emit neutrinos to become neutrinos. These neutrons then trigger the neutron capture process (r-process, also known as R-process) to synthesize heavy elements. However, neutrinos with a mass close to zero play an important role in this process. They can not only promote protons to become neutrons, but also make neutrons return to protons. Researchers have found that the quality of the accretion disk plays a key role in which process is dominant.
Oliver Just of the German Helmholtzzentrum für Schwerionenforschung GmbH, one of the main researchers, said: “Our research has examined for the first time the different masses of accretion disks around multiple black holes. We studied the conversion rate of neutrons and protons inside. We found that a large number of neutrons will appear only when the mass of the accretion disk is within a certain range.”
“The quality of the accretion disk is the decisive factor. The larger the mass of the accretion disk, the higher the ratio of protons to neutrons. The protons capture electrons and release neutrinos to become neutrons. These neutrons synthesize heavy elements through the R-process. However, if the quality of the accretion disk is too high, the proportion of the reverse process will increase, that is, neutrinos have not had time to leave the accretion disk and are captured by neutrons and turned back to protons, thus hindering the R-process.”
Finally, the researchers found that an accretion disk with a mass between 0.01 and 0.1 times that of the sun is the most ideal condition for the synthesis of heavy elements. Accretion disks with such a mass generally appear around black holes where neutron stars merge.
Researchers are currently unclear whether the black hole created by the collapse of a star can produce an accretion disk with a mass in this range, or what proportion.
Research leader Andreas Bauswein said: “The existing data are not sufficient. When the new generation of equipment-the Facility for Antiproton and Ion Research (FAIR) is completed, it will be able to perform accurate measurements. At that time, we will combine theoretical models, experiments, and astronomical observation data. We believe that in the next few years we will be able to verify the theory of how neutron star collision events trigger the R-process (the production of heavy elements).”
The study was published in Monthly Notices of the Royal Astronomical Society on October 8. ◇
(Reposted from The Epoch Times/Editor: Ye Ping)
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