Neutron Capture Processes in the Universe

Dear Colleagues,

Scientists from all around the world will meet to discuss neutron capture processes at the “s, r & i Element Nucleosynthesis (siren)” Conference, which will be held in Giulianova (Italy) from 8th to 13th of June 2025. The aim of this Special Issue is to make available to the scientific community the research presented during this conference and the relevant discussions presenting the current state of the art regarding these processes in the universe through a series of contributions devoted to observational studies, theoretical modeling and laboratory measurements. It is our hope that this will improve our understanding of the complex details characterizing heavy-element nucleosynthesis and its profound implications for galactic chemical evolution.

The production of elements heavier than iron proceeds through neutron capture processes due to the hampering effect of the Coulomb barrier. To date, two main processes are indicated as responsible for the synthesis of heavy elements: the slow neutron capture process (the s-process) and the rapid neutron capture process (the r-process). Their contribution to the cosmic heavy-element budget is roughly 50% each. In recent years, however, a growing number of observational studies have highlighted the presence of a third neutron capture process, the intermediate one (the i-process), which is able to explain some peculiar chemical distributions whose theoretical explanation creates difficulties in the canonical picture.

The s-process, characterized by the gradual accumulation of neutrons within nuclei, occurs during the quiescent burnings of massive star evolution (weak component) and the asymptotic giant branch (AGB) phase of low- and intermediate-mass stars (main component). In these objects, light elements are transformed into heavier ones by means of relatively mild neutron densities (N_n ≈ 10⁶ − 10⁷ cm⁻³).

In stark contrast, the intermediate neutron capture process (i-process) offers a glimpse into the regime of higher neutron densities (N_n ≈ 10¹⁵ cm⁻³) which boost the production of isotopes on the neutron-rich side of the beta stability valley. As we delve into the still rather unexplored environment of i-process nucleosynthesis, we face the challenges of theoretical modeling and observational validation, striving to unravel the enigmatic genesis of this process.

Lastly, the rapid neutron capture process (r-process) stands as an emblem of cosmic extremity, characterized by incredibly high neutron densities (N_n > 10²³ cm⁻³). From the most powerful supernovae to the exotic landscapes of neutron star mergers, the r-process enriches the universe with the remnants of stellar explosions.

Dr. Sergio Cristallo
Dr. Luciano Piersanti
Dr. Diego Vescovi
Guest Editors

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• stellar nucleosynthesis
• stellar evolution
• multimessenger astronomy
• neutron capture processes