Impact of individual nuclear masses on $r$-process abundances

We have performed for the first time a comprehensive study of the sensitivity of $r$-process nucleosynthesis to individual nuclear masses across the chart of nuclides. Using the latest version (2012) of the Finite-Range Droplet Model, we consider mass variations of $\pm 0.5$ MeV and propagate each mass change to all affected quantities, including $Q$ values, reaction rates, and branching ratios. We find such mass variations can result in up to an order of magnitude local change in the final abundance pattern produced in an $r$-process simulation. We identify key nuclei whose masses have a substantial impact on abundance predictions for hot, cold, and neutron star merger $r$-process scenarios and could be measured at future radioactive beam facilities.

Figure 1

The quantities of neighboring nuclei of importance to the $r$-process that may be altered by a change in the mass of the nucleus with $Z$ protons and $N$ neutrons.

Figure 2

Nuclei that significantly impact final $r$-process abundances for four astrophysical conditions. The intensity of the color denotes the maximum $F$ value, Eq. (4), resulting from individual $\pm 0.5$ MeV mass variations. Light gray denotes the extent of measured masses from the 2012 AME and stable nuclei are colored black. For reference, estimated accessibility limits are shown for the upcoming Facility for Rare Isotope Beams (FRIB) (black line shows the intensity of ${10}^{-4}$ particles per second [62]).

Figure 3

Variances of the ensembles of final abundance patterns (shaded bands) for the four sensitivity studies described in this work, compared to scaled solar $r$-process residuals (black circles) from Ref. [64].