On the nuclear robustness of the r process in neutron-star mergers

Abstract: We have performed r-process calculations for matter ejected dynamically in neutron star mergers based on a complete set of trajectories from a three-dimensional relativistic smoothed particle hydrodynamic simulation. Our calculations consider an extended nuclear network, including spontaneous, $\beta$- and neutron-induced fission and adopting fission yield distributions from the ABLA code. We have studied the sensitivity of the r-process abundances to nuclear masses by using different models. Most of the trajectories, corresponding to 90% of the ejected mass, follow a relatively slow expansion allowing for all neutrons to be captured. The resulting abundances are very similar to each other and reproduce the general features of the observed r-process abundance (the second and third peaks, the rare-earth peak and the lead peak) for all mass models as they are mainly determined by the fission yields. We find distinct differences in the abundance yields at and just above the third peak, which can be traced back to different predictions of neutron separation energies for r-process nuclei around neutron number $N=130$. The remaining trajectories, which contribute 10% by mass to the total integrated abundances, follow such a fast expansion that the r process does not use all the neutrons. This also leads to a larger variation of abundances among trajectories as fission does not dominate the r-process dynamics. The total integrated abundances are dominated by contributions from the slow abundances and hence reproduce the general features of the observed r-process abundances. We find that at timescales of weeks relevant for kilonova light curve calculations, the abundance of actinides is larger than the one of lanthanides. Hence actinides can be even more important than lanthanides to determine the photon opacities under kilonova conditions. (Abridged)

• The paper demonstrates that robust r-process abundance patterns, including key peaks and lead production, are consistently reproduced across diverse nuclear models.
• The study employs detailed nuclear reaction networks and multi-trajectory simulations to assess the impact of varying neutron separation energies and fission yields.
• Findings indicate that neutron-star mergers reliably produce heavy elements, underscoring their significance in cosmic chemical evolution and kilonova emissions.

On the Robustness of the r-process in Neutron-star Mergers

This paper investigates the robustness of the rapid neutron-capture process (r-process) during neutron-star mergers, highlighting the nuclear physics and astrophysical conditions crucial to the synthesis of heavy elements. The r-process is responsible for forming roughly half of the heavy elements beyond iron in the universe, taking place in extremely neutron-rich environments. Neutron-star mergers, which eject substantial amounts of neutron-rich material, are considered a promising site for this process.

Main Findings

The authors conduct simulations using multiple trajectories that represent different conditions in the matter ejected from neutron-star mergers. A noteworthy aspect of this study is the comprehensive inclusion of nuclear reactions, such as neutron capture, beta decay, and fission, using a detailed nuclear network and various nuclear mass models. The investigation specifically examines how variations in these mass models impact the predictions of the r-process abundance distribution.

Key results from the simulations indicate that:

• The general features of the observed r-process abundance distribution, including the second and third peaks, the rare-earth peak, and the presence of lead, are consistently reproduced across different models and trajectories.
• The production of the heavier r-process elements shows minimal sensitivity to the astrophysical conditions if the initial neutron-to-seed ratio is sufficient for creating fissionable nuclei.
• There are notable differences in the abundance predictions at and above the third peak, attributable to variations in neutron separation energies, particularly around neutron number $N = 130$.

Implications

Practically, these findings suggest that neutron-star mergers can produce a robust and consistent pattern of r-process elements, aligning closely with the solar system's r-process abundance. This uniformity points towards the possible ubiquity of neutron-star mergers as a significant source of heavy elements in the universe, particularly those beyond iron.

Theoretically, the study underscores the importance of fission yields and neutron separation energies from different mass models in shaping the r-process pathway. Since many of these nuclei have yet to be observed experimentally, the predictions rely heavily on theoretical nuclear models. This dependency emphasizes the need for continued refinement and experimental validation of these models.

Additionally, the research offers insights into the potential contributions of actinides and lanthanides, whose photon opacities significantly affect the observed features of kilonovae, electromagnetic transients associated with neutron-star mergers. Notably, the abundance of actinides at timescales relevant to kilonova light curves can surpass that of lanthanides, indicating their potential role in determining opacities under these conditions.

Future Developments

Continued advancements in computational astrophysics and experiments with rare-isotope facilities will be critical in improving the precision of nuclear models involved in r-process studies. Understanding the variations among different mass models and their influence on the r-process abundances will be crucial in producing more accurate predictions and in identifying neutron-star mergers' definitive contributions to galactic chemical evolution. Future work should also explore the sensitivity of r-process nucleosynthesis to different astrophysical parameters and the implications for observed elemental abundances in the universe.