[Paper Review] Actinide-boosting r Process in Black Hole-Neutron Star Merger Ejecta

Shinya Wanajo, Sho Fujibayashi+4 more|arXiv (Cornell University)|Dec 8, 2022

Pulsars and Gravitational Waves Research4 citations

TL;DR

This study presents the first nucleosynthesis analysis in black hole–neutron star merger ejecta using self-consistent, long-term neutrino-radiation-magnetohydrodynamics simulations. It finds that dynamical and post-merger ejecta together reproduce a solar-like r-process pattern, and actinide boost—evidenced by Th/Eu > 0.9—is achieved only when the electron fraction in dynamical ejecta is Yₑ ≈ 0.05–0.08, providing a critical constraint on nuclear equations of state.

ABSTRACT

We examine nucleosynthesis in the ejecta of black hole-neutron star mergers based on the results of long-term neutrino-radiation-magnetohydrodynamics simulations for the first time. We find that the combination of dynamical and post-merger ejecta reproduces a solar-like r-process pattern. Moreover, the enhancement level of actinides is highly sensitive to the distribution of both electron fraction and the velocity of the dynamical ejecta. Our result implies that the mean electron fraction of dynamical ejecta should be >~ 0.05 in order to reconcile the nucleosynthetic abundances with those in r-process-enhanced, actinide-boost stars. Since the tidal ejecta preserve the neutron-richness in the inner crust of pre-merging neutron stars, this result provides an important constraint for nuclear equations of state, if black hole-neutron star mergers are responsible for actinide-boost stars.

Motivation & Objective

• To determine whether black hole–neutron star mergers can reproduce solar-like r-process abundance patterns observed in metal-poor stars.
• To investigate the conditions under which actinide-boosted stars—characterized by enhanced Th/Eu ratios—can be produced in such mergers.
• To constrain the electron fraction (Yₑ) and velocity structure of dynamical ejecta required to match observed actinide boosts.
• To test the sensitivity of r-process yields to nuclear properties such as β-decay rates and fission recycling.
• To provide a new constraint on nuclear equations of state based on the observed Yₑ range needed for actinide enhancement.

Proposed method

• Utilized self-consistent, long-term neutrino-radiation-magnetohydrodynamics simulations of BH-NS mergers (Hayashi et al. 2022a,b) as the basis for nucleosynthesis modeling.
• Extracted thermodynamic trajectories (Yₑ, velocity, mass) from simulation outputs, generating ~1400 tracer particles per model for post-processing nucleosynthesis.
• Applied the rNET nuclear reaction network code to compute r-process abundances along each trajectory, incorporating fission recycling and β-decay rates from HFB-21 and GT2 models.
• Analyzed the dependence of Th/Eu ratios on Yₑ and outflow velocity, accounting for thermal feedback and equilibrium neutron capture conditions.
• Compared simulated Th/Eu ratios with observed values in actinide-boosted stars (e.g., CS 31082-001, J0954+5246) at 13 Gyr ago, correcting for stellar age.
• Used models Q4B5H-DD2, Q4B5H-SFHo, and Q6B5L-DD2 with different equations of state (DD2, SFHo) and resolution to assess robustness.

Experimental results

Research questions

• RQ1 Can black hole–neutron star mergers reproduce the solar-like r-process abundance pattern observed in metal-poor stars?
• RQ2 What range of electron fraction (Yₑ) in dynamical ejecta is required to produce the actinide boost seen in Th/Eu > 0.5 stars?
• RQ3 How does ejecta velocity influence the Th/Eu ratio, and can this explain the observed spread in actinide-boosted stars?
• RQ4 Is the actinide boost sensitive to the choice of nuclear β-decay rates and fission recycling models?
• RQ5 Can the observed Th/Eu ratios in stars like J0954+5246 be reproduced within the BH-NS merger framework, given observational uncertainties?

Key findings

• The combination of dynamical and post-merger ejecta in BH-NS mergers reproduces a solar-like r-process pattern, indicating these mergers can be galactic r-process sites.
• Actinide boost (Th/Eu > 0.9) is achieved only in models with the DD2 equation of state, specifically when Yₑ in dynamical ejecta is ≈ 0.05–0.08.
• The model with SFHo equation of state fails to produce actinide boost, highlighting the sensitivity of results to nuclear EoS.
• For a fixed Yₑ, slower ejecta velocities yield higher Th/Eu ratios due to increased temperature during r-processing, which shifts the r-process path toward heavier, fissile nuclei.
• The highest observed Th/Eu ratio (J0954+5246, ≈0.76) is consistent with simulations if a 0.2 dex uncertainty in Th abundance is allowed.
• The requirement for Yₑ ≈ 0.05–0.08 in dynamical ejecta provides a critical constraint on nuclear equations of state, particularly symmetry energy at high densities.