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

GW170817 was the first neutron star merger to provide direct evidence for r-process nucleosynthesis, and the spectra of its electromagnetic counterpart AT2017gfo carry signatures of the elements synthesized in the ejecta. This study focuses on AT2017gfo's late nebular-phase spectral features, which can probe the elemental abundances of the ejecta after the merger.

The authors build an analytic spectral model that computes emission from radiative decay of collisionally excited ions through both allowed and forbidden transitions, then compare it with the observed AT2017gfo spectrum. The model identifies La III and Ce III as the main contributors to the 1.4 μm and 1.6 μm emission features, respectively, and confirms Te III as the dominant contributor to the 2.1 μm feature proposed in earlier work.

The inferred mass fractions are X(La) ≈ 0.025–0.05, X(Ce) ≈ 0.05–0.1, and X(Te) ≈ 0.04–0.08, with the La and Ce values still tentative because of uncertainties in the radiation field. Non-detections of Kr and Sb lines yield upper limits of X(Kr) ≲ 0.03 and X(Sb) ≲ 0.003.

These results imply that nucleosynthesis in the inner ejecta of GW170817 produced a suppressed first r-process peak and an enhanced heavy-element abundance relative to the solar r-process pattern, with an estimated lanthanide fraction X_LN ≈ (3–6) × 10^-2. The authors note that this is consistent with the apparent universality of heavy r-process elements and with the lanthanide fraction inferred from observations of r-enhanced metal-poor stars.