[Paper Review] Heavy elements and electromagnetic transients from neutron star mergers

Stephan Rosswog, Oleg Korobkin|arXiv (Cornell University)|Aug 30, 2022

Gamma-ray bursts and supernovae4 citations

TL;DR

This paper reviews the role of neutron star mergers as sources of r-process elements and electromagnetic transients (kilonovae), synthesizing theoretical and observational insights from GW170817. It presents a publicly available heating rate library and a fit formula for simulating radioactive decay in ejecta, enabling accurate modeling of kilonova light curves across diverse ejecta conditions.

ABSTRACT

Compact binary mergers involving neutron stars can eject a fraction of their mass to space. Being extremely neutron rich, this material undergoes rapid neutron capture nucleosynthesis, and the resulting radioactivity powers fast, short-lived electromagnetic transients known as kilonova or macronova. Such transients are exciting probes of the most extreme physical conditions and their observation signals the enrichment of the Universe with heavy elements. Here we review our current understanding of the mass ejection mechanisms, the properties of the ejecta and the resulting radioactive transients. The first well-observed event in the aftermath of GW170817 delivered a wealth of insights, but much of today's picture of such events is still based on a patchwork of theoretical studies. Apart from summarizing the current understanding, we also point out questions where no consensus has been reached yet, and we sketch possible directions for the future research. In an appendix, we describe a publicly available heating rate library based on the WinNet nuclear reaction network, and we provide a simple fit formula to alleviate the implementation in hydrodynamic simulations.

Motivation & Objective

• To synthesize current understanding of mass ejection mechanisms, ejecta properties, and resulting electromagnetic transients from neutron star mergers.
• To address the unresolved question of which astrophysical sites produce the heaviest r-process elements, particularly in light of rare, high-yield events.
• To provide a practical tool—via a fit formula and heating rate library—for implementing radioactive decay heating in hydrodynamic simulations.
• To clarify the dependence of r-process abundance patterns on electron fraction $Y_e$, especially the transition at $Y_e \approx 0.25$ between light and heavy r-process.
• To identify open questions in ejecta channels, nucleosynthesis yields, and observational signatures for future research.

Proposed method

• The authors use the WinNet nuclear reaction network to compute heating rates from radioactive decay in r-process nuclei across a range of ejecta conditions.
• They derive a parametric fit formula for the heating rate $\dot{\epsilon}(t)$, modeled as a sum of three exponential terms with adjustable coefficients depending on $Y_e$ and ejecta velocity $v_{\rm ej}$.
• The fit is calibrated using detailed nucleosynthesis simulations and validated across multiple ejecta configurations, including dynamical, wind, and viscous ejecta.
• Coefficients for the fit are provided in tables for $Y_e$ from 0.05 to 0.5 and $v_{\rm ej}$ from 0.05c to 0.5c, enabling direct implementation in hydrodynamic codes.
• The heating rate library is made publicly available to support consistent modeling of kilonova light curves in merger simulations.
• The method enables efficient computation of time-dependent energy deposition without solving full nuclear reaction networks in real-time simulations.

Experimental results

Research questions

• RQ1 What are the dominant mass ejection mechanisms in neutron star mergers, and how do they affect the resulting r-process nucleosynthesis?
• RQ2 How does the electron fraction $Y_e$ in different ejecta components determine the production of light versus heavy r-process elements?
• RQ3 What is the quantitative impact of varying ejecta velocity and $Y_e$ on the time evolution of kilonova luminosity?
• RQ4 How can a simple, accurate parametric fit for radioactive heating rates be constructed to enable efficient hydrodynamic simulations?
• RQ5 What are the key uncertainties in current models of kilonova emission, and where do consensus and open questions remain?

Key findings

• Neutron star mergers eject ~1% of a solar mass per event, with significant variation in $Y_e$ across different ejecta components, leading to distinct r-process abundance patterns.
• The transition between light and heavy r-process occurs sharply around $Y_e \approx 0.25$, with $Y_e < 0.25$ producing elements up to $A \sim 195$ (platinum peak).
• The parametric fit for the heating rate $\dot{\epsilon}(t)$ achieves high accuracy across diverse ejecta conditions, with coefficients provided for $Y_e \in [0.05, 0.5]$ and $v_{\rm ej} \in [0.05c, 0.5c]$.
• The heating rate library enables efficient implementation in hydrodynamic simulations, reducing computational cost while preserving accuracy in kilonova light curve predictions.
• Observational constraints from GW170817 support the presence of both high-$Y_e$ (light r-process) and low-$Y_e$ (heavy r-process) ejecta components, consistent with multi-messenger data.
• Despite progress, uncertainties remain in the relative contributions of different ejection mechanisms (e.g., dynamical vs. wind vs. viscous) and their impact on nucleosynthesis yields.