A team of international researchers led by scientists at the Konkoly Observatory have gone back to the formation of our Solar System 4.6 billion years ago in order to gain new insights into the cosmic origin of the heaviest elements on the periodic table, as reported in a study published in Science.

The question of which astronomical events can host the rapid neutron-capture process – known as the r-process – that produces the heaviest elements in the Universe such as iodine, gold, platinum, uranium, plutonium and curium has been a mystery for decades. It is currently thought that the r-process can occur during violent collisions between two neutron stars, one neutron star and a black hole, or during rare supernova explosions following the death of massive stars.

Some of the nuclei produced by the r-process are radioactive and take millions of years to decay into stable nuclei. Iodine-129 and curium-247 are two such radioactive nuclei. They were incorporated into meteorites during the formation of the Sun and have an amazing peculiarity: they decay at almost exactly the same rate. This means that the iodine-129 to curium-247 ratio has not changed since their production billions of years ago. “With the iodine-129 to curium-247 ratio being frozen in time like a prehistoric fossil, we can have a direct look into the last wave of heavy element production that built up the composition of the Solar System,” says Benoit Côté, the leader of the study.

The team calculated the iodine-129 to curium-247 ratios created by collisions between neutron stars and black holes and then compared their model predictions to the value found in meteorites. They concluded that the number of neutrons during the last r-process event that preceded the birth of the Solar System cannot have been too high, otherwise too much curium would have been produced relative to iodine. This implies that very neutron rich sources, such as the material ripped off the surface of a neutron star during a collision, likely did not play an important role, while moderately neutron-rich conditions, often found in ejecta from the discs that form around the merging event, are more consistent with the meteoritic value.

Because nucleosynthesis predictions rely on uncertain nuclear and stellar properties, the final answer as to what astronomical object was the exact source remains elusive, though “the ability of the iodine-129 to curium-247 ratio to peer more directly into the fundamental nature of heavy element nucleosynthesis is an exciting prospect,” says Maria Lugaro, who is also part of the investigating team.  Any future astrophysical simulations of stellar mergers and explosions and measurements of nuclear properties will need to be tested again meteoritic constraints to reveal the source of the heaviest elements of the Solar System.

Young Stars Surrounded by Disks of Dust ,Credit: Nasa /StSci (https://hubblesite.org/copyright)

A Baby Binary Star in Formation , Credit:ALMA (ESO/NAOJ/NRAO),  O. Alves et al.

Original publication: https://science.sciencemag.org/content/371/6532/945