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arXiv · 2510.25500

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

Mila Racca·Terese T. Hansen·Ian U. Roederer·Vinicius M. Placco·Anna Frebel·Timothy C. Beers·Rana Ezzeddine·Erika M. Holmbeck·Charli M. Sakari·Stephanie Monty·Øivind Harket·Joshua D. Simon·Chris Sneden·Ian B. Thompson
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Abstract

The astrophysical origin of the rapid neutron-capture process (r-process), which produces about half of the elements heavier than iron, remains uncertain. The oldest, most metal-poor stars preserve the chemical signatures of early nucleosynthesis events and can reveal the nature of the r-process sites. We present a homogeneous chemical abundance analysis of ten r-process-enhanced, metal-poor stars that show strong enrichment in r-process elements with minimal contamination from other nucleosynthetic sources. Using high-resolution, high signal-to-noise spectra, we examined over 1400 absorption lines per star through equivalent width measurements and spectral synthesis under one-dimensional LTE assumptions with the MOOG radiative transfer code. Abundances for 54 chemical species were derived, including 29 neutron-capture elements spanning the full r-process pattern. We quantified the cosmic scatter of elemental ratios relative to Zr (light) and Eu (heavy) and found remarkably small dispersions for the rare-earth and third-peak elements, {\sigma}[La/Eu] = 0.08 dex and {\sigma}[Os/Eu] = 0.11 dex, while the light-to-heavy ratio shows slightly larger variation, {\sigma}[Zr/Eu] = 0.18 dex. A kinematic study indicates that the stars likely originated from ten distinct progenitor systems, allowing us to probe the intrinsic variation between independent r-process events. These results imply that the main r-process operates under highly uniform conditions across diverse astrophysical sites.

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Mila Racca, Terese T. Hansen, Ian U. Roederer, Vinicius M. Placco, Anna Frebel, Timothy C. Beers, Rana Ezzeddine, Erika M. Holmbeck, Charli M. Sakari, Stephanie Monty, Øivind Harket, Joshua D. Simon, Chris Sneden, Ian B. Thompson. 2025-10-29. The R-Process Alliance: Exploring the cosmic scatter among ten r-process sites with stellar abundances. https://doi.org/10.1051/0004-6361%2F202556947

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