Meteorites remember the conditions of stellar explosions

The question of which astronomical events can host the rapid neutron-capture process, r-process in short, that produces the heaviest elements in the Universe such as iodine, gold, platinum, uranium, plutonium, and curium has been a mystery for decades. Presently, it is 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 of 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 did 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 first author of the study.

The team calculated the iodine-129 to curium-247 ratios created by collisions between neutron stars and black holes, and 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 be too high, otherwise too much curium is 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 to which astronomical object was the exact source is still elusive. However, “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 Dr. Marius Eichler, who was also part of the investigating team and postdoc in the group of Prof. Almudena Arcones. Following this work, future astrophysical simulations of stellar mergers and explosions combined with nuclear experiments such as those planned at GSI and FAIR can now also be tested against meteoritic constraints to reveal the source of the heaviest elements of the Solar System.

The work of Dr. Marius Eichler and Prof. Almudena Arcones was supported in part by the ERC Starting Grant EUROPIUM and the DFG Collaborative Research Center 1245.

FRIB / NSCL / TU Darmstadt