Exploring composition mixing in kilonova ejecta with ray-by-ray simulations

Journal Article · Thu Apr 02 00:00:00 EDT 2026 · Physical Review. D.

DOI:https://doi.org/10.1103/phzv-ldhz· OSTI ID:3028330

Zhai, Ruocheng ^[1]; Radice, David ^[1]; Magistrelli, Fabio ^[2]; Bernuzzi, Sebastiano ^[2]; Perego, Albino ^[3]

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1. The Pennsylvania State University, University Park, PA (United States)
2. Friedrich-Schiller-Universität Jena (Germany)
3. Università di Trento (Italy); INFN-TIFPA, Trento Institute for Fundamental Physics and Applications (Italy)

Binary neutron star merger (BNSM) ejecta are considered a primary repository of r-process nucleosynthesis and a source of the observed heavy-element abundances. We implement composition mixing into ray-by-ray radiation-hydrodynamic simulations of BNSM ejecta, coupled with an online nuclear network (NN). We model mixing via a gradient-based mixing approximation that evolves simultaneously with the hydrodynamics. Here, we find that mixing occurs in regions where the electron fraction changes rapidly. While mixing smooths composition gradients in transition regions, it has a negligible impact on the heavy-element yields. This is because the primary r-process site (the equatorial ejecta) is initially homogeneous in free neutrons, leaving no strong gradients for mixing to act upon. In each angular ray, the abundances of the most produced elements are robust under mixing, while the less abundant ones are more affected. The total global abundances change only slightly from mixing, since each angular ray contributes its most abundant elements. Furthermore, the predicted kilonova light curves show only minor reddening, with differences below the detectability of state-of-the-art telescopes. In general, we do not observe significant effects from mixing in the time span of the r-process. Consequently, mixing only leads to minor variations in abundances and light curves in ray-by-ray simulations.

• Research Organization: The Pennsylvania State University, University Park, PA (United States)
• Sponsoring Organization: Deutsche Forschungsgemeinschaft (DFG); National Energy Research Scientific Computing Center (NERSC); National Science Foundation (NSF); USDOE Office of Science (SC), Nuclear Physics (NP)
• Grant/Contract Number: SC0021177; SC0024388
• Other Award/Contract Number: AST-2108467; PHY-2020275; PHY-2116686; PHY-2407681; PHY-2512802; 406116891; ERCAP0031370
• OSTI ID: 3028330
• Journal Information: Physical Review. D., Journal Name: Physical Review. D. Journal Issue: 8 Vol. 113; ISSN 2470-0010; ISSN 2470-0029
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

Explosive burning
Nucleosynthesis in explosive environments
r process