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

Abstract

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.

Figures (6)

• Figure 1: Panel (a): Comparison of theoretical and experimental masses for neodymium ($Z=60$) isotopes as a function of neutron number. The bulk LDM AME2020 contribution is subtracted from all the masses. Panel (b): Comparison of two-neutron separation energies. Panel (c): comparison of two-neutron shell-gap energies as a function of charge number for $N=124$ isotones. In all the panels, black circles represent AME2020 experimental data.
• Figure 2: Mass-integrated abundances as a function of atomic number (top panels) and mass number $A$ (bottom panels) predicted by different mass models at $\tau_{(n, \gamma)} = \tau_\beta$ (left panels) and at 1 Gyr (right panels). Black circles are r-process abundances in the solar system.
• Figure 3: FRDM masses (panel (a)) and two-neutron separation energies (panel (b)) in MeV for different values of $a_\textup{Nsym}$ along the neodymium isotopic chain as a function of neutron number. The red solid line represents the DZ31 predictions. Black circles represent the AME2020 experimental data. In panel (a), the bulk LDM AME2020 contribution is subtracted from all the masses.
• Figure 4: Integrated abundances as a function of atomic number $Z$ (top row) and mass number $A$ (bottom row) predicted by FRDM mass models with different values of $a_\textup{Nsym}$ and DZ31 at three different times: neutron capture freeze-out (left column), when $\tau_{(n, \gamma)} = \tau_\beta$ (middle column), and at 1 Gyr (right column). Black circles are r-process abundances in the solar system.
• Figure 5: Panels (a) and (b): Nuclear masses and two-neutron separation energies (in MeV) for different mixtures of DZ31 and FRDM shell effects (DZ31/FRDM) along the neodymium isotopic chain as a function of neutron number. The bulk LDM contribution extracted from AME2020 experimental data is subtracted from all the masses. The bulk LDM extracted from the FRDM model is subtracted from all the $S_{2n}$. Panel (c) and (d): two-neutron shell-gap energies for the same models as in the top panels along the $N=124$ and $N=94$ isotonic chains as a function of proton number. Black circles represent the AME2020 experimental data.