Impact of nuclear masses on r-process nucleosynthesis: bulk properties versus shell effects

We investigate the impact of the model estimating the masses of exotic nuclei on r-process nucleosynthesis, assessing the dependence of the abundance distribution on the specific properties of nuclear masses. By decomposing theoretical nuclear mass predictions into a liquid-drop parametrization and local shell effects, we show that r-process abundances are virtually insensitive to large variations of the masses which originate from nuclear bulk properties of the model, such as the symmetry energy. In contrast, the mass component associated with local shell effects is the main driver of r-process abundance variations, despite its relatively minor contribution to the absolute value of neutron separation energies. Our work suggests that experimental and theoretical studies of masses devoted to r-process applications, such as the nucleosynthesis in the ejecta of neutron star mergers, should focus on the physical origin and determination of local changes in mass trends.

💡 Research Summary

The paper investigates how different components of nuclear mass models affect rapid neutron‑capture (r‑process) nucleosynthesis. The authors separate a theoretical mass prediction into a smooth bulk term, described by a liquid‑drop model (LDM), and a residual term that accounts for local shell corrections. They fit the LDM parameters to two widely used global mass models, the Finite‑Range Droplet Model (FRDM) and the Duflo‑Zuker formula (DZ31), and then construct hybrid mass tables: DZ31* (DZ31 bulk + FRDM shell) and FRDM* (FRDM bulk + DZ31 shell).

Using a large nuclear reaction network that includes all nuclei up to Z = 110, they calculate r‑process abundances for 2 015 trajectories representing the dynamical ejecta of neutron‑star mergers. Neutron‑capture rates are derived with the Hauser‑Feshbach code TALYS 1.95, β‑decay rates from FRDM, and fission treated with the FRDM+TF model and ABLA. The calculations are examined at three stages: the neutron‑capture freeze‑out (τ_(n,γ)=τ_β), at the moment the neutron‑to‑seed ratio reaches unity, and after 1 Gyr when all material has decayed to stability.

The results show that when the shell correction is kept the same, large variations in the bulk LDM parameters (including the symmetry‑energy term) have virtually no impact on the final abundance pattern. Even though the absolute binding energies differ significantly, the two‑neutron separation energies S₂n and the derived two‑neutron shell gaps Δ₂n remain almost unchanged, preserving the r‑process path. In contrast, swapping the shell correction between the two models produces noticeable differences, especially a deeper trough before the third r‑process peak (A ≈ 184). This demonstrates that local shell effects, which modify S₂n and Δ₂n, are the primary drivers of abundance features.

To further test the sensitivity to bulk changes, the authors introduce an additional isospin‑dependent term a_Nsym into the LDM and vary its strength. Even with extreme bulk modifications that shift masses by several MeV, the early‑time abundance distribution only shifts slightly; as the system evolves toward stability, the differences are washed out, yielding nearly identical final patterns.

Overall, the study concludes that r‑process nucleosynthesis is largely insensitive to global mass trends such as the symmetry energy, but highly sensitive to local shell‑structure variations. Consequently, future experimental campaigns and theoretical developments should prioritize accurate determination of shell effects and their evolution far from stability, rather than solely reducing the overall rms deviation of global mass models. This insight refines the strategy for constraining nuclear physics inputs in astrophysical models of neutron‑star mergers and other r‑process sites.