A group of international researchers have returned to the formation of the solar system 4.6 billion years ago to take a fresh look at the cosmic origin of the heaviest elements. And I discovered how exactly they were formed and during what process.
The heavy elements that we encounter in our daily life, such as iron and silver, did not exist at the beginning of the universe 13.7 billion years ago. They were created in time by nuclear reactions called nucleosynthesis, which brought atoms together. In particular, iodine, gold, platinum, uranium, plutonium and curium – some of the heaviest elements – were created using a special type of nucleosynthesis called the rapid neutron capture process, or r-process.
The question of which astronomical events can produce the heaviest elements has remained a mystery for decades. Today, it is believed that the r-process can occur during violent collisions between two neutron stars, between a neutron star and a black hole, or during rare explosions after the death of massive stars. These high-energy events are very rare in the universe. When this happens, neutrons are incorporated into the nuclei of atoms and then converted into protons. Since the elements in the periodic table are determined by the number of protons in their nuclei, the r process creates heavier nuclei as more neutrons are captured.
Some of the r-process nuclei are radioactive and take millions of years to decay into stable nuclei. Iodine-129 and curium-247 are two such nuclei that were formed before the formation of the sun. They were incorporated into solids that eventually fell to the earth’s surface as meteorites. Inside these meteorites, as a result of radioactive decay, an excess of stable nuclei was formed. Today, this excess can be measured in laboratories to determine the amount of iodine-129 and curium-247 that were present in the solar system just before its formation.
Why are these two cores of the r-process so special? They have the usual property: they disintegrate at almost the same rate. In other words, the ratio between iodine-129 and curium-247 has not changed since their creation billions of years ago.
“This is an amazing coincidence, especially since these nuclei are two of the five radioactive r-process nuclei that can be measured in meteorites. When the ratio of iodine-129 to curium-247 has frozen in time like a prehistoric fossil, we can look directly at the last wave of heavy element production that shaped the composition of the solar system and everything in it.”Benoit Kote, Konkola Observatory
Iodine, with its 53 protons, is easier to create than curium, with its 96 protons. This is because more neutron capture reactions are required to achieve a larger number of curium protons. As a consequence, the ratio of iodine-129 to curium-247 is highly dependent on the number of neutrons that were available at the time of their creation.
The team calculated the ratio of iodine-129 to curium-247, synthesized by collisions of neutron stars and black holes, to find the right set of conditions that mimic the composition of the meteorites. They concluded that the number of neutrons available during the last r-process event before the birth of the solar system could not have been too large. Otherwise, too much curium would be formed compared to iodine. This means that very neutron-rich sources, such as matter detached from the surface of the neutron star during the collision, probably did not play an important role.
So what created these r-process kernels? While the researchers were able to provide new informative information about how they were created, they were unable to determine the nature of the astronomical object that created them. This is because nucleosynthesis models are based on uncertain nuclear properties, and it is still unclear how to relate the availability of neutrons to specific astronomical objects, such as massive explosions of stars and colliding neutron stars.
With this new diagnostic tool, advances in astrophysical modeling and understanding nuclear properties can reveal which astronomical objects create the heaviest elements in the solar system.