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

TL;DR

The study tackles the origin and uniformity of the r-process by conducting a homogeneous chemical abundance analysis of ten metal-poor, r-process-enhanced stars, deriving 54 elemental abundances (including 29 neutron-capture species) from high-resolution spectra. Using 1D-LTE MOOG with ATLAS9 models and a careful treatment of stellar parameters, the authors quantify the intrinsic cosmic scatter across ten independent r-process sites, finding extremely small dispersions for rare-earth and third-peak elements, specifically $ abla \,\sigma_{[\mathrm{La/Eu}]} = 0.08$ dex and $\sigma_{[\mathrm{Os/Eu}]} = 0.11$ dex, with $ abla \sigma_{[\mathrm{Zr/Eu}]} = 0.18$ dex for light-to-heavy ratios. A dynamical analysis in $E$–$L_z$ space indicates that the stars originate from ten distinct progenitor systems, enabling a robust upper bound on r-process yield diversity. The results imply that the main r-process operates under highly uniform conditions across diverse environments, constraining the range of viable nucleosynthetic sites and informing models of Galactic chemical evolution and neutron-capture physics.

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, σ[La/Eu] = 0.08 dex and σ[Os/Eu] = 0.11 dex, while the light-to-heavy ratio shows slightly larger variation, σ[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.

Figures (9)

• Figure 1: $[\mathrm{Eu/Fe}]$ as a function of $[\mathrm{Fe/H}]$, with a color gradient representing $[\mathrm{Ba/Eu}]$ for the sample of ten stars. All stars lie well above the canonical $[\mathrm{Eu/Fe}] > +0.3$ threshold typically adopted for r-process-enhanced stars, and their low $[\mathrm{Ba/Eu}]$ values further confirm that the enrichment is consistent with a pure r-process origin. Grey stars in the background are taken from roederer2014a.
• Figure 2: $\mathrm{[X/Fe]}$ abundances of the light elements for the ten sample stars, compared to stellar abundances from the MW halo roederer2014a.
• Figure 3: Examples of spectral synthesis fits used to derive elemental abundances. The observed data (black squares) are shown along with the best-fit synthetic spectra (solid teal lines) and associated uncertainties (shaded regions). The dotted lines correspond to synthetic spectra with no contribution from the indicated element (i.e., $\log \epsilon = -\infty$).
• Figure 4: Comparison of two IrI lines in the spectrum of J1432$-$4125. The black dots represent the observed spectrum, and the turquoise boxes identify the position of the two lines.
• Figure 5: Left panel: Distribution of the sample stars in the orbital energy ($E$) versus angular momentum ($L_z$) plane. The greyscale background shows GALAH DR3 stars Buder2021 with metallicities in the range $-1.3 \leq \mathrm{[Fe/H]} \leq -0.9$. The thick black curve marks the boundary between dynamically in-situ stars (more bound, lower energy) and accreted populations (less bound, higher energy), following the prescription of Monty2024MNRAS.533.2420M. Colored markers indicate the ten stars in our sample, as labeled in the legend. The grey contours identify the approximate region occupied by Gaia-Sausage-Enceladus (GSE) stars from Belokurov2023, adapted to our adopted potential. Right panel: Action-space diagram ("diamond diagram") showing the normalized azimuthal action ($J_\phi / J_{\rm tot}$) on the x-axis and the normalized vertical-minus-radial action ($(J_z - J_R)/J_{\rm tot}$) on the y-axis. Colored markers correspond to the same stars as in the left panel, now color-coded by their orbital eccentricity. The greyscale background shows again the GALAH DR3 comparison sample. The grey rectangular region marks the approximate locations of the Gaia-Sausage accreted substructure, following the definitions by Myeong2019MNRAS.488.1235M.