Almudena Arcones

Our understanding of the origin of heavy elements beyond iron relies on the rapid neutron capture process ( r process), which accounts for roughly half of their cosmic abundance. However, the extreme neutron-rich conditions required for the r process involve many nuclei that remain experimentally inaccessible, making theoretical predictions essential. We explore the impact of nuclear masses calculated with the valence-space in-medium similarity renormalization group, focusing on the region around the N = 82 shell closure. We show for the first time that such mass calculations can be used to refine r -process predictions compared to global, but more phenomenological mass models. With the masses, the waiting point of the second r -process peak is strengthened, which leads to an overall slower nucleosynthesis flow, lower abundances of nuclei beyond the peak, and a stronger shift of the third r -process peak.

Context . The astrophysical site(s) of the rapid neutron-capture process ( r -process) remains uncertain, with competing scenarios such as neutron star mergers and magneto-rotational supernovae offering different predictions for the timing, frequency, and yield for heavy elements pollution. Galactic chemical evolution models provide an important tool to constrain these properties by comparing model predictions to the observed abundance of neutron-capture elements across different metallicities. Aims . Our aim is to explore, in a systematic and data-driven manner, the range of astrophysical conditions under which the r -process enrichment can reproduce the observed trends of multiple neutron-capture elements in the Milky Way. Rather than assuming a fixed physical site, we adopted a flexible parametric approach to assess whether a common set of r -process parameters can simultaneously explain the chemical evolution of several heavy elements of interest. Methods . We constructed a grid of one-infall homogeneous Galactic chemical evolution models varying four key parameters: the Eu r -process yield per event, the rate of r -process-producing events, the delay time before enrichment, and the progenitor star mass range. For each of the ∼1.5 × 10 ⁵ models, we computed the predicted [X/Fe] versus [Fe/H] trends for several neutron-capture elements, obtained by scaling Eu yields with the solar r -process pattern, and we evaluated the model performance using χ ² statistics. A multi-objective optimisation approach based on Pareto front analysis was used to identify models that best reproduce the observed abundance trends across multiple neutron-capture elements simultaneously. Results . The best models consistently favour short delay times (≤30 Myr), low-mass progenitors (∼20−25 M _⊙ ), and an effective Eu injection rate of ∼2 × 10 ⁻⁷ M _⊙ per event. Stars more massive than ∼80 M _⊙ are too rare to account for the observed r -process enrichment alone, and therefore they cannot be the only source. While heavier neutron-capture elements (Ba, La, Ce) can be simultaneously reproduced within the Pareto-optimal set, the lighter ones (Sr, Y, Zr) present stronger conflicts with Eu, leading to systematically larger distances from the ideal solution. This tension reflects the fact that the solar r -process scaling relation, used to extend Eu yields to other elements, becomes progressively less valid towards lighter neutron-capture elements. Conclusions . Our results suggest that no single class of r -process events under solar-scaled yields can simultaneously account for the chemical evolution of both light and heavy neutron-capture elements. Instead, at least two distinct scaling components appear necessary: one corresponding to the main r -process, similar to the solar and r -process rich star patterns, and another weak component with enhanced production of lighter r -process elements, similar to that observed in r -process poor stars.

We investigate the nucleosynthesis and kilonova emission based on numerical-relativity binary neutron star merger simulations that incorporate a two-moment neutrino-transport scheme. Unlike in previous works with simpler neutrino treatments, a massive, fast (up to v = 0.3 c ), proton-rich neutrino-driven wind develops in the postmerger phase of the simulations as long as the merger remnant does not collapse to a black hole. We evolve the ejecta for 100 days after the merger using 2D ray-by-ray radiation-hydrodynamics simulations coupled in situ to a complete nuclear network. The most abundant nucleosynthesis products are He, ⁵⁶ Ni, and ⁵⁶ Co. We find a total yield of ∼10 ⁻³ M _⊙ of ⁵⁶ Ni for all mergers that produce massive neutron star remnants, independently of the mass ratio and equations of state. After a few days, the decay of ⁵⁶ Ni and later ⁵⁶ Co becomes the primary source of heating in the matter expanding above the remnant. As a result, the kilonova light curve flattens on timescales of days for polar observation angles. This is an important effect that should be included in future models of kilonova light curves. Furthermore, the observation of a ⁵⁶ Ni– ⁵⁶ Co feature could serve as a clear indicator for the presence of a long-lived neutron star remnant in future kilonova observations.

Our understanding of the origin of heavy elements beyond iron relies on the rapid neutron capture process (r-process), which accounts for roughly half of their cosmic abundance. However, the extreme neutron-rich conditions required for the r-process involve many nuclei that remain experimentally inaccessible, making theoretical predictions essential. We explore the impact of nuclear masses calculated with the ab initio valence-space in-medium similarity renormalization group, focusing on the region around the N = 82 shell closure. We show that such ab initio mass calculations can refine the r-process predictions compared to global, but more phenomenological mass models. With the ab initio masses, the waiting point of the second r-process peak is strengthened, which leads to an overall slower nucleosynthesis flow, lower abundances of nuclei beyond the peak, and a stronger shift of the third r-process peak.

Heavy elements are synthesized by the r-process in neutron star mergers and potentially in rare supernovae linked to strong magnetic fields. Expensive hydrodynamic simulations of these extreme environments are usually postprocessed to calculate the nucleosynthesis. In contrast, here we follow a site-independent approach based on three key parameters: electron fraction, entropy, and expansion timescale. Our model reproduces the results based on hydrodynamic simulations. Moreover, the 120,000 astrophysical conditions analyzed allow us to systematically and generally explore the astrophysical conditions of the r-process, also beyond those found in current simulations. Our results show that a wide range of conditions produce very similar abundance patterns explaining the observed robustness of the r-process between the second and third peak. Furthermore, we cannot find a single condition that produces the full solar r-process pattern from first to third peak. Instead, a superposition of at least two or three conditions or components is required to reproduce the typical r-process pattern as observed in the solar system and very old stars. The different final abundances are grouped into eight nucleosynthesis clusters, which can be used to select representative conditions for comparisons to observations and investigations of the nuclear physics input.

The cross section of the ⁸⁶Kr(α, n)⁸⁹Sr reaction was measured at low energies using the activation technique via the detection of the β ⁻ emission from the residual ⁸⁹Sr nuclei. The theoretical analysis of the measured cross sections shows that the new data can be well described in the statistical model using several recent α-nucleus potentials. The resulting astrophysical reaction rate has small uncertainties between about 15% at higher temperatures and up to a factor of 2 at low temperatures. Using this new reaction rate of the ⁸⁶Kr(α, n)⁸⁹Sr reaction for the nucleosynthesis in neutrino-driven outflows in core-collapse supernovae leads to well-constrained production ratios for the nuclei slightly above krypton (Z = 36) like Sr/Zr, Y/Zr, and Mo/Zr. A comparison to observational ratios in metal-poor stars allows to constrain the astrophysical conditions for this weak r-process.

Heavy elements are synthesized by the r-process in neutron star mergers and potentially in rare supernovae linked to strong magnetic fields. Expensive hydrodynamic simulations of these extreme environments are usually post-processed to calculate the nucleosynthesis. In contrast, here we follow a site-independent approach based on three key parameters: electron fraction, entropy, and expansion timescale. Our model reproduces the results based on hydrodynamic simulations. Moreover, the 120 000 astrophysical conditions analyzed allow us to systematically and generally explore the astrophysical conditions of the r-process, also beyond those found in current simulations. Our results show that a wide range of conditions produce very similar abundance patterns explaining the observed robustness of the r-process between the second and third peak. Furthermore, we cannot find a single condition that produces the full r-process from first to third peak. Instead, a superposition of at least two or three conditions or components is required to reproduce the typical r-process pattern as observed in the solar system and very old stars. The different final abundances are grouped into eight nucleosynthesis clusters, which can be used to select representative conditions for comparisons to observations and investigations of the nuclear physics input.

Neutron emission probabilities and half-lives of 37 β -delayed neutron emitters from Ni 75 to Br 92 were measured at the RIKEN Nishina Center in Japan, including 11 one-neutron and 13 two-neutron emission probabilities and six half-lives for the first time that supersede theoretical estimates. These nuclei lie in the path of the weak r process occurring in neutrino-driven winds from the accretion disk formed after the merger of two neutron stars synthesizing elements in the A ∼ 80 abundance peak. The presence of such elements dominates the accompanying kilonova emission over the first few days and have been identified in the AT2017gfo event, associated to the gravitational wave detection GW170817. Abundance calculations based on over 17 000 simulated trajectories describing the evolution of matter properties in the merger outflows show that the new data lead to an increase of 50%–70% in the abundance of Y, Zr, Nb, and Mo. This enhancement is large compared to the scatter of relative abundances observed in old very metal poor stars and thus is significant in the comparison with other possible astrophysical processes contributing to the light-element production. These results underline the importance of including experimental decay data for very neutron-rich β -delayed neutron emitters into r -process models. Published by the American Physical Society 2025

Context. The third r-process peak (Os, Ir, Pt) is poorly understood due to observational challenges, with spectral lines located in the blue or near-ultraviolet region of stellar spectra. These challenges need to be overcome for a better understanding of the r-process in a broader context. Aims. To understand how the abundances of the third r-process peak are synthesised and evolve in the Universe, it is necessary to carry out a homogeneous chemical analysis of metal-poor stars using high-quality data observed in the blue region of the electromagnetic spectrum (Methods. We performed a classical 1D, local thermodynamic equilibrium (LTE) analysis of four elements (Hf, Os, Ir, Pt) using ATLAS model atmospheres to fit synthetic spectra on high signal-to-noise-ratio spectra of 52 red giants observed with UVES/VLT in high resolution (>40,000). Due to the heavy line blending involved, we carefully determined upper limits and uncertainties. The observational results are compared with state-of-the-art nucleosynthesis models. Results. Our sample displays larger abundances of Ir (Z=77) in comparison to Os (Z=76), both of which have been measured in a few stars in the past. The results also suggest decoupling between the abundances of third r-process peak elements with respect to Eu (a rare earth element) in Eu-poor stars. This seems to contradict a co-production scenario of Eu and the third r-process peak elements Os, Ir, and Pt in the progenitors of these objects. Our results are challenging to explain from a nucleosynthetic point of view: the observationally derived abundances indicate the need for an additional early, primary formation channel (or a non-robust r-process).