Neutrino-driven outflows and the first r-process peak

The science question

Where do the lighter heavy elements between strontium and silver come from, and can their abundance patterns tell us about the conditions in early stellar explosions?

The first r-process peak elements, roughly between strontium and silver (Sr-Ag), appear in metal-poor stars with patterns that are not fully explained by the main r-process or the s-process alone. Neutrino-driven ejecta from core-collapse supernovae provide one possible site, but the answer depends sensitively on \(\alpha\)-induced reaction rates on radioactive, neutron-rich nuclei and the relevant astrophysical conditions.

What we do

• Use nucleosynthesis calculations to identify which \((\alpha,n)\) reactions shape the Sr-Ag abundance pattern.
• Connect nuclear uncertainties to abundance ratios that can be compared with metal-poor stellar observations.
• Use new \(\alpha\)-optical model potentials and experimental cross sections to reduce nuclear uncertainties.
• Help define future measurements at radioactive-beam facilities such as TRIUMF, Argonne National Laboratory and FRIB.

Student entry points

• Run reaction rate variation studies for weak r-process trajectories.
• Compare calculated abundance ratios with observations of metal-poor and limited-r stars.
• Analyze how different astrophysical conditions change the origin of Sr-Zr and Mo-Pd.
• Develop visualizations that connect reaction networks to observable abundance patterns.

Selected output

A. Psaltis et al., Astrophys. J. 935, 27 (2022)
A. Psaltis et al., Astrophys. J. 966, 11 (2024)
C. Fougères et al., Astrophys. J. 983 (2025)
M. Williams et al., Phys. Rev. Lett. 134, 112701 (2025)
A. Psaltis, “Connecting experiments with rare isotopes to stellar abundances”, Nuclear Science Seminar FRIB/MSU (2026)