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

TL;DR

This study analyzes ten metal-poor stars with strong r-process element enrichment to understand the uniformity and diversity of r-process nucleosynthesis across different astrophysical sites.

Contribution

It provides a homogeneous abundance analysis of ten r-process-enhanced stars, revealing minimal cosmic scatter and suggesting a highly uniform main r-process across various sites.

Findings

Small dispersion in rare-earth and third-peak element ratios.

Light-to-heavy element ratio shows slightly larger variation.

Stars likely originated from ten distinct progenitor systems.

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.