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The early r-process nucleosynthesis scenarios

Noam Soker

TL;DR

This work tackles which r-process sites contributed to the early Universe's heavy-element inventory by comparing seven active r-process scenarios against metal-poor star observations. It adopts a two-site framework, with a light Eu+Fe site co-producing iron and a separate third-peak site for the heaviest elements, and assesses each scenario against the requirement of short delay times and large per-event yields. The analysis identifies magnetorotational supernovae as the most plausible source for Eu+Fe enrichment and the common envelope jets supernova (CEJSN) r-process as the primary driver of the third-peak production, while magnetar winds, NS winds, and AIC are unlikely major early contributors; NS-NS mergers remain compatible but are unlikely to dominate the earliest enrichment. The study emphasizes that early-Universe r-process enrichment was driven by rare, high-yield events and highlights the need for more CEJSN-focused simulations to fully quantify the diverse pathways to the observed abundance patterns.

Abstract

I compare seven actively studied r-process nucleosynthesis scenarios against observed properties of r-process elements in the early Universe, and conclude that the most likely scenario to contribute to the site of elements below the third r-process peak is the magnetorotational r-process scenario, and that of the third peak is the common envelope jets supernova (CEJSN) r-process scenario. The collapsar and CEJSN r-process scenario might also contribute to the lighter r-process elements, and the binary neutron star (NS-NS) merger r-process scenario might contribute to the third r-process peak. The magnetar, the wind from the newly born NS, and the accretion-induced collapse of a white dwarf r-process scenarios fall short in explaining observations. They might exist, but cannot be major contributors to the r-process in the early Universe. To constrain r-process scenarios in the early Universe, I require that they explain the large scatter in the r-process abundances of very metal-poor stars, account for the correlation between light r-process nucleosynthesis and iron production, and the lack of correlation between the third peak r-process production and iron production, as inferred from very metal-poor stars. I discuss the diversity of the CEJSN r-process scenario and encourage extending its exploration.

The early r-process nucleosynthesis scenarios

TL;DR

This work tackles which r-process sites contributed to the early Universe's heavy-element inventory by comparing seven active r-process scenarios against metal-poor star observations. It adopts a two-site framework, with a light Eu+Fe site co-producing iron and a separate third-peak site for the heaviest elements, and assesses each scenario against the requirement of short delay times and large per-event yields. The analysis identifies magnetorotational supernovae as the most plausible source for Eu+Fe enrichment and the common envelope jets supernova (CEJSN) r-process as the primary driver of the third-peak production, while magnetar winds, NS winds, and AIC are unlikely major early contributors; NS-NS mergers remain compatible but are unlikely to dominate the earliest enrichment. The study emphasizes that early-Universe r-process enrichment was driven by rare, high-yield events and highlights the need for more CEJSN-focused simulations to fully quantify the diverse pathways to the observed abundance patterns.

Abstract

I compare seven actively studied r-process nucleosynthesis scenarios against observed properties of r-process elements in the early Universe, and conclude that the most likely scenario to contribute to the site of elements below the third r-process peak is the magnetorotational r-process scenario, and that of the third peak is the common envelope jets supernova (CEJSN) r-process scenario. The collapsar and CEJSN r-process scenario might also contribute to the lighter r-process elements, and the binary neutron star (NS-NS) merger r-process scenario might contribute to the third r-process peak. The magnetar, the wind from the newly born NS, and the accretion-induced collapse of a white dwarf r-process scenarios fall short in explaining observations. They might exist, but cannot be major contributors to the r-process in the early Universe. To constrain r-process scenarios in the early Universe, I require that they explain the large scatter in the r-process abundances of very metal-poor stars, account for the correlation between light r-process nucleosynthesis and iron production, and the lack of correlation between the third peak r-process production and iron production, as inferred from very metal-poor stars. I discuss the diversity of the CEJSN r-process scenario and encourage extending its exploration.
Paper Structure (6 sections, 2 figures, 1 table)

This paper contains 6 sections, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. R-process in metal-poor stars
  3. Large abundance scatter
  4. Two early r-process sites
  5. Scenarios comparison
  6. Discussion and Summary

Figures (2)

  • Figure 1: A figure adapted from JinSoker2024 presenting log(Ir/Eu) versus log(XLa), where XLa is the ratio between the lanthanides mass and the total mass of r-process elements. Shown are r-enhanced stars, the solar system, and theoretical calculations of the magnetorotational scenario (marked MHDSNe), the collapsar scenario, and the CEJSN r-process scenario. Values for r-enhanced stars are from JINAbase (Abohalimaetal2018) with the two constraints of [Fe/H] $<$ -2.5 and [Ba/Eu] $<$ -0.4. For the collapsar model in Siegeletal2019, the mean values of the $\dot{M}_{1}$ and $\dot{M}_{2}$ accretion regimes are used, representing the strong r-process components. For the magnetorotational scenario from Reichertetal2021, the mean values of the top three strongest r-process traces are presented. The value of $\log{\rm XLa}$ of the NS-NS scenario marked by NSM (pink vertical line) is from Jietal2019. I added, in the lower left, the new estimate by Gillandersetal2026 of the XLa for the kilonova GW170817/AT 2017gfo. The values for the CEJSN scenario are from JinSoker2024. The dashed line is a linear fit to the magnetorotational, collapsar, and CEJSN scenarios, with ${R^2}$ = 0.9926.
  • Figure 2: A figure demonstrating the large scatter in r-process nucleosynthesis at early times with the ratio [Eu/Fe]. The scatter at late times suggests that r-process sites that yield a large mass of r-process elements also dominate at later times (i.e., present). Observational data taken from Cayreletal2004, Hondaetal2004, Hansenetal2012, Hansenetal2014, Roedereretal2014 and Zhaoetal2016 (figure based on the one in Kobayashietal2020 and GrichenerKobayashiSoker2022).