Implications of a Weakening N = 126 Shell Closure Away from Stability for r-Process Astrophysical Conditions

TL;DR

The paper addresses how a weakening of the $N=126$ shell closure away from stability affects $r$-process nucleosynthesis, focusing on the formation of the third peak near $A\approx195$. It uses the Duflo–Zuker framework with a weakened and a strengthened shell variant, couples them to three $\beta^-$-decay rate prescriptions ($\text{MLR}$, $\text{Ney}$, $\text{MKT}$), and runs hot-$r$-process trajectories spanning $Y_e$ from $0.05$ to $0.35$ with $s/k=20$–40. The results show that strong closures reproduce the solar third peak, while weakened closures broaden and shift the peak unless the environment is extremely neutron-rich and decay rates are slow, thus constraining possible astrophysical sites. This work highlights the need for precise mass measurements near $N=126$ and improved beta-decay data to reliably connect nuclear structure to astrophysical conditions; upcoming experiments and multi-messenger observations will be essential to refine these constraints.

Abstract

The formation of the third r-process abundance peak near A = 195 is highly sensitive to both nuclear structure far from stability and the astrophysical conditions that produce the heaviest elements. In particular, the N = 126 shell closure plays a crucial role in shaping this peak. Experimental data hints that the shell weakens as proton number departs from Z = 82, a trend largely missed by global mass models. To investigate its impact on r-process nucleosynthesis, we employ both standard global models with strong closures and modified Duflo-Zuker (DZ) models that reproduce the weakening, combined with three sets of beta minus decay rates. Strong shell closures generate sharply peaked abundances, whereas weakened closures consistent with the experimental trend produce broader, flatter patterns. Accurately reproducing the solar third peak under weakened shell strength requires highly neutron-rich conditions (Ye <= 0.175) and slower decay rates. These results demonstrate that a weakening N = 126 shell closure away from stability imposes significant constraints on the astrophysical environments of the r-process and underscores the need for precise mass measurements and improved characterization of beta minus decay properties in this region.

Figures (9)

• Figure 1: Strength of the $N = 126$ shell closure from different mass models. The upper panel shows the two-neutron shell gap $D_{2n}$ predicted by HFB (purple), FRDM (pink), DZ (blue), and experimental AME2020 data (black). The lower panel compares the default DZ with its modified variants: WK-DZ (weaker shell closure) and ST-DZ (stronger shell closure). The inset highlights the $S_{2n}$ values for the $Z = 60$ isotopic chain, illustrating the contrast in shell strength across $N = 126$. The shaded gray regions mark the $r$-process path range and, separately, the nuclei proposed for measurement at the $N = 126$ Factory.
• Figure 2: Simulated $r$-process abundance patterns for a single trajectory, with $Y_e = 0.2$ and entropy per baryon $s/k=40$, using different nuclear mass models. The top panel shows the results with various global mass models, while the bottom panel compares results using the default DZ model with its modified versions. Solar $r$-process residuals from Arnould_2007 are shown as points.
• Figure 3: Top: abundance pattern at freeze-out from ($n,\gamma$)–($\gamma,n$) equilibrium with a stronger shell closure, compared to solar $r$-process residuals (black stars). Bottom: isotopic abundances on the nuclear chart, with color indicating relative abundance. Stable nuclei are shown as black squares; black and red contours mark neutron-separation energies and $\beta^-$-decay rates, respectively, and dashed vertical lines indicate neutron closed shells.
• Figure 4: Same as Fig. \ref{['fig:2p_str']}, but with a weaker shell closure applied in the simulation.
• Figure 5: Simulated $r$-process abundance patterns using the default (blue) and modified DZ mass models with weaker (orange) and stronger (green) shell closures. Results are normalized across a set of trajectories to account for varying astrophysical conditions. The upper panel shows isobaric abundances, while the lower panel shows elemental abundances; crosses indicate solar data.