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Peer-reviewedCC BY 4.0The Astrophysical Journal

New Constraints on r-process Nucleosynthesis in Neutron Star Mergers from GW170817 Late-phase Spectra

  • Salma Rahmouni1
  • Masaomi Tanaka1
  • Daiji Kato2,3
  • Gediminas Gaigalas4
  • Kenta Hotokezaka5
5 affiliations
  1. 1Tohoku University
  2. 2National Institute for Fusion Science
  3. 3Kyushu University
  4. 4Vilnius University
  5. 5The University of Tokyo
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Abstract

The neutron star merger event GW170817 provided the first direct evidence of r -process nucleosynthesis. Observed spectra of its electromagnetic counterpart AT2017gfo exhibited several features that encode information on the nature and abundance of the synthesized r -process elements. In this study, we investigate the late nebular-phase spectral features of the kilonova AT2017gfo, which provide important probes of the elemental abundance of the ejecta. We construct an analytic spectral model that computes emission features produced by the radiative decay of collisionally excited ions through allowed and forbidden transitions. By comparing our model to AT2017gfo, we identify La III and Ce III as the main contributors to the emission features at 1.4 and 1.6 μ m, respectively. We also confirm Te III as the dominant contributor to the 2.1 μ m feature proposed in previous works. We infer mass fractions of X (La) ≈ 0.025–0.05, X (Ce) ≈ 0.05–0.1, and X (Te) ≈ 0.04–0.08, although the La and Ce abundance estimates remain tentative due to uncertainties in the radiation field. From the nondetections of Kr and Sb lines, we derive upper limits of X (Kr) ≲ 0.03 and X (Sb) ≲ 0.003. These results suggest that nucleosynthesis in the inner ejecta of GW170817 produced a suppressed first r -process peak and an enhanced abundance of heavier elements compared to the solar r -process pattern, with an estimated lanthanide fraction of X LN ≈ (3–6) × 10 ⁻² . Our conclusions are consistent with the apparent universality of heavy r -process elements and the lanthanide fraction inferred from observations of r -enhanced metal-poor stars.

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Cite

BibTeXRIS
BibTeX
@article{rahmouni2026constraints,
  title = {{New Constraints on r-process Nucleosynthesis in Neutron Star Mergers from GW170817 Late-phase Spectra}},
  author = {Salma Rahmouni and Masaomi Tanaka and Daiji Kato and Gediminas Gaigalas and Kenta Hotokezaka},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = nov,
  doi = {10.3847/1538-4357/aeace4},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeace4},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - New Constraints on r-process Nucleosynthesis in Neutron Star Mergers from GW170817 Late-phase Spectra
AU  - Salma Rahmouni
AU  - Masaomi Tanaka
AU  - Daiji Kato
AU  - Gediminas Gaigalas
AU  - Kenta Hotokezaka
PY  - 2026
DA  - 2026/11/04
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aeace4
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aeace4
AB  - The neutron star merger event GW170817 provided the first direct evidence of r -process nucleosynthesis. Observed spectra of its electromagnetic counterpart AT2017gfo exhibited several features that encode information on the nature and abundance of the synthesized r -process elements. In this study, we investigate the late nebular-phase spectral features of the kilonova AT2017gfo, which provide important probes of the elemental abundance of the ejecta. We construct an analytic spectral model that computes emission features produced by the radiative decay of collisionally excited ions through allowed and forbidden transitions. By comparing our model to AT2017gfo, we identify La III and Ce III as the main contributors to the emission features at 1.4 and 1.6 μ m, respectively. We also confirm Te III as the dominant contributor to the 2.1 μ m feature proposed in previous works. We infer mass fractions of X (La) ≈ 0.025–0.05, X (Ce) ≈ 0.05–0.1, and X (Te) ≈ 0.04–0.08, although the La and Ce abundance estimates remain tentative due to uncertainties in the radiation field. From the nondetections of Kr and Sb lines, we derive upper limits of X (Kr) ≲ 0.03 and X (Sb) ≲ 0.003. These results suggest that nucleosynthesis in the inner ejecta of GW170817 produced a suppressed first r -process peak and an enhanced abundance of heavier elements compared to the solar r -process pattern, with an estimated lanthanide fraction of X LN ≈ (3–6) × 10 ⁻² . Our conclusions are consistent with the apparent universality of heavy r -process elements and the lanthanide fraction inferred from observations of r -enhanced metal-poor stars.
SN  - 1538-4357
SN  - 0004-637X
ER  -