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Peer-reviewedCC BY 4.0The Astrophysical Journal

Non-LTE Ionization Modeling for Helium and Strontium in Neutron Star Merger Ejecta

  • Koya Chiba1
  • Masaomi Tanaka1
  • Shinya Wanajo1,2
  • Sho Fujibayashi1
  • Kyohei Kawaguchi2,3
  • Kenta Hotokezaka4
4 affiliations
  1. 1Tohoku University
  2. 2Kyoto University
  3. 3Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
  4. 4The University of Tokyo
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Abstract

The material ejected from a binary neutron star merger produces a “kilonova,” a radioactively powered emission at ultraviolet, optical, and infrared wavelengths. The early-phase spectra of the kilonova AT2017gfo, following the gravitational wave event GW170817, exhibit a strong absorption feature around 1 μ m. Helium (He) and strontium (Sr) have been proposed as the candidate elements contributing to this feature. However, due to the lack of consistent modeling including these two elements simultaneously, the exact contributions of each element to this feature remain unclear. In this study, we develop nonlocal thermodynamic equilibrium ionization models for He and Sr that take into account ionization by high-energy electrons, and estimate the abundances of each element required to reproduce the observed feature in the early-phase spectra of AT2017gfo. Our modeling indicates that about 1% of He or 1%–10% of Sr (in mass fraction) are present in the ejecta moving at v ∼ 0.15 c . This Sr mass fraction nicely agrees with the mass fraction in the solar r -process pattern. Based on comparison with nucleosynthesis calculations, our constraints suggest that r -process nucleosynthesis in GW170817 occurs at relatively low electron fraction ( Y e ≲ 0.35) and low entropy ( s ≲ 30 k B /nucleon) conditions. Generally, for Y e ≲ 0.15, the observed feature can be reproduced by He at the mass fraction expected from the α -decay of trans-Pb nuclei, and may therefore serve as an indirect signature for the production of elements beyond the third r -process peak in binary neutron star mergers.

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Cite

BibTeXRIS
BibTeX
@article{chiba2026nonlte,
  title = {{Non-LTE Ionization Modeling for Helium and Strontium in Neutron Star Merger Ejecta}},
  author = {Koya Chiba and Masaomi Tanaka and Shinya Wanajo and Sho Fujibayashi and Kyohei Kawaguchi and Kenta Hotokezaka},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea3fe},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea3fe},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Non-LTE Ionization Modeling for Helium and Strontium in Neutron Star Merger Ejecta
AU  - Koya Chiba
AU  - Masaomi Tanaka
AU  - Shinya Wanajo
AU  - Sho Fujibayashi
AU  - Kyohei Kawaguchi
AU  - Kenta Hotokezaka
PY  - 2026
DA  - 2026/10/12
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea3fe
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea3fe
AB  - The material ejected from a binary neutron star merger produces a “kilonova,” a radioactively powered emission at ultraviolet, optical, and infrared wavelengths. The early-phase spectra of the kilonova AT2017gfo, following the gravitational wave event GW170817, exhibit a strong absorption feature around 1 μ m. Helium (He) and strontium (Sr) have been proposed as the candidate elements contributing to this feature. However, due to the lack of consistent modeling including these two elements simultaneously, the exact contributions of each element to this feature remain unclear. In this study, we develop nonlocal thermodynamic equilibrium ionization models for He and Sr that take into account ionization by high-energy electrons, and estimate the abundances of each element required to reproduce the observed feature in the early-phase spectra of AT2017gfo. Our modeling indicates that about 1% of He or 1%–10% of Sr (in mass fraction) are present in the ejecta moving at v ∼ 0.15 c . This Sr mass fraction nicely agrees with the mass fraction in the solar r -process pattern. Based on comparison with nucleosynthesis calculations, our constraints suggest that r -process nucleosynthesis in GW170817 occurs at relatively low electron fraction ( Y e ≲ 0.35) and low entropy ( s ≲ 30 k B /nucleon) conditions. Generally, for Y e ≲ 0.15, the observed feature can be reproduced by He at the mass fraction expected from the α -decay of trans-Pb nuclei, and may therefore serve as an indirect signature for the production of elements beyond the third r -process peak in binary neutron star mergers.
SN  - 0004-637X
SN  - 1538-4357
ER  -