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

¹ Astronomy department, Stockholm University, Roslagstullsbacken 21, 114 21 Stockholm, Sweden
² Department of Physics, North Carolina State University, 2401 Stinson Dr, Box 8202, Raleigh, NC 27695, USA
³ Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements (JINA-CEE), USA
⁴ NSF NOIRLab, Tucson, AZ 85719, USA
⁵ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁶ Department of Physics and Astronomy, University of Notre Dame, Notre Dame, IN 46556, USA
⁷ Department of Astronomy, University of Florida, Bryant Space Science Center, Gainesville, FL 32611, USA
⁸ Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
⁹ Department of Physics and Astronomy, San Francisco State University, San Francisco, CA 94132, USA
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge CB3 0HA, UK
¹¹ Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA
¹² Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712, USA

^★★ Corresponding author: mila.racca@astro.su.se

Received: 22 August 2025
Accepted: 28 October 2025

Abstract

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 dominant r-process sites.

Aims. We present a homogeneous chemical abundance analysis of ten r-process element-enhanced stars. These old and metal-poor stars are strongly enriched in r-process elements with minimal contamination from other nucleosynthetic sources. By focusing on this chemically pure sample, we aim to investigate intrinsic variations in the r-process abundance patterns and explore their implications for the nature and potential diversity of r-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 the MOOG radiative transfer code.

Results. We derived abundances for 54 chemical species, including 29 neutron-capture (n-capture) elements, covering the full mass range of the r-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 independent r-process nucleosynthesis events and computed the intrinsic dispersion of each element relative to Zr and Eu for the light and heavy r-process elements, respectively. This exercise resulted in a remarkably low cosmic scatter across the ten r-process sites enriching these stars; for the rare earth and third peak elements, for example, we find σ_[La/Eu] = 0.08 and σ_[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 independent r-process sites show remarkably small cosmic dispersions. This minimal dispersion suggests a high degree of uniformity in r-process yields across diverse astrophysical environments.