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1. Complete Survey of r -process Conditions: The (Un)robustness of the r -process(es)

   https://doi.org/10.3847/1538-4357/adf0f7  

   Kuske, Jan; Arcones, Almudena; Reichert, Moritz (August 2025, The Astrophysical Journal)

   Abstract Heavy elements are synthesized by ther-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 ther-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 ther-process between the second and third peak. Furthermore, we cannot find a single condition that produces the full solarr-process pattern from first to third peak. Instead, a superposition of at least two or three conditions or components is required to reproduce the typicalr-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. 

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2. The R -Process Alliance: Analysis of limited- r stars

   https://doi.org/10.1051/0004-6361/202449376  

   Xylakis-Dornbusch, T; Hansen, T T; Beers, T C; Christlieb, N; Ezzeddine, R; Frebel, A; Holmbeck, E; Placco, V M; Roederer, I U; Sakari, C M; et al (August 2024, Astronomy & Astrophysics)

   Context.In recent years, theR-Process Alliance (RPA) has conducted a successful search for stars that are enhanced in elements produced by the rapid neutron-capture (r-)process. In particular, the RPA has uncovered a number of stars that are strongly enriched in lightr-process elements, such as Sr, Y, and Zr. These so-called limited-rstars were investigated to explore the astrophysical production site(s) of these elements. Aims.We investigate the possible formation sites for light neutron-capture elements by deriving detailed abundances for neutron-capture elements from high-resolution spectra with a high signal-to-noise ratio of three limited-rstars. Methods.We conducted a kinematic analysis and a 1D local thermodynamic equilibrium spectroscopic abundance analysis of three stars. Furthermore, we calculated the lanthanide mass fraction (X_La) of our stars and of limited-rstars from the literature. Results.We found that the abundance pattern of neutron-capture elements of limited-rstars behaves differently depending on their [Ba/Eu] ratios, and we suggest that this should be taken into account in future investigations of their abundances. Furthermore, we found that theX_Laof limited-rstars is lower than that of the kilonova AT2017gfo. The latter seems to be in the transition zone between limited-rX_Laand that ofr-I andr-II stars. Finally, we found that unliker-I andr-II stars, the current sample of limited-rstars is largely born in the Galaxy and is not accreted. 

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3. The R -Process Alliance: Exploring the cosmic scatter among ten r -process sites with stellar abundances

   https://doi.org/10.1051/0004-6361/202556947  

   Racca, M; Hansen, T T; Roederer, I U; Placco, V M; Frebel, A; Beers, T C; Ezzeddine, R; Holmbeck, E M; Sakari, C M; Monty, S; et al (December 2025, Astronomy & Astrophysics)

   Context. The astrophysical origin of the rapid neutron-capture process (r-process), responsible for producing roughly half of the elements heavier than iron, remains uncertain. Detailed chemical signatures from the oldest, most metal-poor stars, which act as fossil records of the earliest nucleosynthesis events, can be used to identify the dominantr-process sites. Aims. We present a homogeneous chemical abundance analysis of tenr-process element-enhanced stars. These old and metal-poor stars are strongly enriched inr-process elements with minimal contamination from other nucleosynthetic sources. By focusing on this chemically pure sample, we aim to investigate intrinsic variations in ther-process abundance patterns and explore their implications for the nature and potential diversity ofr-process sites. Methods. We performed a detailed chemical abundance analysis of high-resolution, high-signal-to-noise spectra. For each star, we inspected over 1400 individual absorption lines using a combination of equivalent width measurements and spectral synthesis. The analysis was conducted under the assumption of 1D local thermodynamic equilibrium and employing theMOOGradiative transfer code. Results. We derived abundances for 54 chemical species, including 29 neutron-capture (n-capture) elements, covering the full mass range of ther-process abundance pattern. A kinematic analysis reveals that stars likely originated from ten kinematically distinct systems. Based on this assumption, we used the sample to probe the maximum variation expected from ten independentr-process nucleosynthesis events and computed the intrinsic dispersion of each element relative to Zr and Eu for the light and heavyr-process elements, respectively. This exercise resulted in a remarkably low cosmic scatter across the tenr-process sites enriching these stars; for the rare earth and third peak elements, for example, we findσ_[La/Eu]= 0.08 andσ_[Os/Eu]= 0.11 dex, while the scatter between light and heavy elements,σ_[Zr/Eu], is slightly higher at 0.18 dex. Conclusions. The elemental abundance patterns across the ten independentr-process sites show remarkably small cosmic dispersions. This minimal dispersion suggests a high degree of uniformity inr-process yields across diverse astrophysical environments. 

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4. The R -Process Alliance: First Magellan/MIKE Release from the Southern Search for R -process-enhanced Stars

   https://doi.org/10.3847/1538-4357/ab9d1a  

   Ezzeddine, Rana; Rasmussen, Kaitlin; Frebel, Anna; Chiti, Anirudh; Hinojisa, Karina; Placco, Vinicius M.; Ji, Alexander P.; Beers, Timothy C.; Hansen, Terese T.; Roederer, Ian U.; et al (August 2020, The Astrophysical Journal)
5. The R -process Alliance: Enrichment of r -process Elements in a Simulated Milky Way–like Galaxy

   https://doi.org/10.3847/1538-4357/adf10a  

   Hirai, Yutaka; Beers, Timothy C; Lee, Young Sun; Wanajo, Shinya; Roederer, Ian U; Tanaka, Masaomi; Chiba, Masashi; Saitoh, Takayuki R; Placco, Vinicius M; Hansen, Terese T; et al (September 2025, The Astrophysical Journal)

   Abstract We study the formation of stars with varying amounts of heavy elements synthesized by the rapid neutron-capture process (r-process) based on our detailed cosmological zoom-in simulation of a Milky Way–like galaxy with anN-body/smoothed particle hydrodynamics code,asura. Most stars with no overabundance inr-process elements, as well as the stronglyr-process-enhanced (RPE)r-II stars ([Eu/Fe] > +0.7), are formed in dwarf galaxies accreted by the Milky Way within the 6 Gyr after the Big Bang. In contrast, over half of the moderately enhancedr-I stars (+0.3 < [Eu/Fe] ≤ +0.7) are formed in the main in situ disk after 6 Gyr. Our results suggest that the fraction ofr-I andr-II stars formed in disrupted dwarf galaxies is larger the higher their [Eu/Fe] is. Accordingly, the most strongly enhancedr-III stars ([Eu/Fe] > +2.0) are formed in accreted components. These results suggest that non-r-process-enhanced stars andr-II stars are mainly formed in low-mass dwarf galaxies that hosted either none or a single neutron star merger, while ther-I stars tend to form in the well-mixed in situ disk. We compare our findings with high-resolution spectroscopic observations of RPE metal-poor stars in the halo and dwarf galaxies, including those collected by theR-Process Alliance. We conclude that observed [Eu/Fe] and [Eu/Mg] ratios can be employed in chemical tagging of the Milky Way’s accretion history. 

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