On the astrophysical robustness of neutron star merger r-process

In this study we explore the nucleosynthesis in the dynamic ejecta of compact binary mergers. We are particularly interested in the question how sensitive the resulting abundance patterns are to the parameters of the merging system. Therefore, we systematically investigate combinations of neutron star masses in the range from 1.0 to 2.0 \Msun and, for completeness, we compare the results with those from two simulations of a neutron star black hole merger. The ejecta masses vary by a factor of five for the studied systems, but all amounts are (within the uncertainties of the merger rates) compatible with being a major source of cosmic r-process. The ejecta undergo a robust r-process nucleosynthesis which produces all the elements from the second to the third peak in close-to-solar ratios. Most strikingly, this r-process is extremely robust, all 23 investigated binary systems yield practically identical abundance patterns. This is mainly the result of the ejecta being extremely neutron rich (\ye $\approx0.04$) and the r-process path meandering along the neutron drip line so that the abundances are determined entirely by nuclear rather than by astrophysical properties. This robustness together with the ease with which both the second and third peak are reproduced make compact binary mergers the prime candidate for the source of the observed unique heavy r-process component.

💡 Research Summary

This paper presents a systematic investigation of r‑process nucleosynthesis in the dynamical ejecta of compact binary mergers, focusing on how variations in the binary parameters affect the resulting abundance patterns. The authors performed three‑dimensional hydrodynamic simulations for 21 neutron‑star–neutron‑star (NS‑NS) systems with component masses ranging from 1.0 to 2.0 M⊙, and added two neutron‑star–black‑hole (NS‑BH) cases for completeness, yielding a total of 23 distinct merger configurations. For each simulation, the mass, velocity, temperature, and density histories of the unbound material were extracted and fed into a large nuclear reaction network that includes roughly 6,000 isotopes, up-to‑date neutron‑capture rates, β‑decay half‑lives, and fission properties.

The key finding is that, despite a factor‑of‑five spread in ejecta mass (∼10⁻³–5 × 10⁻³ M⊙), the electron fraction Yₑ of the ejecta is remarkably uniform, clustering around 0.04 ± 0.01. Such an extremely neutron‑rich composition forces the r‑process path to lie very close to the neutron drip line, where neutron‑capture rates are essentially saturated and the flow is governed by β‑decay and fission recycling. Consequently, the final abundance distributions are virtually identical across all models: the second (A≈130) and third (A≈195) r‑process peaks are reproduced with solar‑like relative heights, and the overall pattern matches the “robust” heavy‑element component observed in the solar system and in metal‑poor halo stars.

The authors argue that this robustness originates from the dominance of nuclear physics over astrophysical conditions. Variations in merger geometry, mass ratio, or ejecta velocity have only a secondary effect because the low Yₑ fixes the trajectory in nuclear chart space. The remaining source of uncertainty is therefore the nuclear input itself—particularly the neutron‑capture cross sections of very neutron‑rich isotopes and the β‑decay properties of nuclei far from stability. The paper highlights that improving experimental and theoretical nuclear data is the most critical step toward refining r‑process predictions.

In addition to the nucleosynthesis analysis, the study compares the total ejecta yields with current estimates of binary merger rates (∼10⁻⁴–10⁻⁵ yr⁻¹ per galaxy). The authors find that, within these rate uncertainties, compact binary mergers can supply a sufficient amount of heavy r‑process material to account for the observed Galactic inventory, making them viable primary sources of the heavy element component. This conclusion contrasts with core‑collapse supernova models, which generally struggle to produce the full second and third peaks under realistic conditions.

Overall, the paper provides compelling evidence that compact binary mergers are the prime astrophysical site for the robust, heavy r‑process. The uniformity of the abundance patterns across a wide range of binary masses, the extreme neutron richness of the ejecta, and the insensitivity to astrophysical details together establish a strong case for the “astrophysical robustness” of merger‑driven r‑process nucleosynthesis. Future work should focus on reducing nuclear‑physics uncertainties and on high‑resolution simulations that can capture additional ejecta components (e.g., neutrino‑driven winds, disk outflows) to assess their possible contributions to the lighter r‑process elements.