Impact of Nuclear Mass Uncertainties on the $r$ Process

Nuclear masses play a fundamental role in understanding how the heaviest elements in the Universe are created in the $r$ process. We predict $r$-process nucleosynthesis yields using neutron capture and photodissociation rates that are based on the nuclear density functional theory. Using six Skyrme energy density functionals based on different optimization protocols, we determine for the first time systematic uncertainty bands—related to mass modeling—for $r$-process abundances in realistic astrophysical scenarios. We find that features of the underlying microphysics make an imprint on abundances especially in the vicinity of neutron shell closures: Abundance peaks and troughs are reflected in trends of neutron separation energy. Further advances in the nuclear theory and experiments, when linked to observations, will help in the understanding of astrophysical conditions in extreme $r$-process sites.

Figure 1

Predicted abundance distributions for neutron star mergers (top) and jetlike supernovae (bottom). Dots indicate the Solar System $r$-process abundances. The systematic uncertainties (gray bands) are due to variations of the masses predicted in the six Skyrme-DFT models of Ref. [42]. The mean predicted abundances are marked by the solid line.

Figure 2

Predictions of the ${\mathrm{SkM}}^{*}$, SLy4, and UNEDF0 models. Bottom: (Half of the) two-neutron separation energies along different isotopic chains (gray lines); every fifth isotope chain is plotted with black lines. The red dots correspond to the freeze-out abundances, i.e., the $r$-process path before matter starts to decay to stability. A few dots around $A\sim 135$ lie below the separation energy that is expected from $(n,\gamma )-(\gamma ,n)$ equilibrium. Here, fission of nuclei with $A>240$ populates regions beyond the $r$-process path. Top: The freeze-out (thin red lines) and final (thick black lines) abundances for a neutron star merger scenario.

Figure 3

Left: Half of the two-neutron separation energies predicted by the models SLy4 (a) and UNEDF0 (b) versus the neutron number in the region where the third $r$-process peak forms. The dashed line marks the approximate $r$-process path during $(n,\gamma )-(\gamma ,n)$ equilibrium ($S_{2n}/2=1.5\mathrm{MeV}$). Black arrows indicate neutron captures; red arrows mark beta decays along the path. Right: The corresponding quadrupole deformations ${\beta }_2$ for SLy4 (c) and UNEDF0 (d).