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Infrared emission lines arising from transitions between fine structurelevels of heavy elements are expected to produce kilonova nebular emission. Forthe kilonova in GW170817, strong emission at 4.5 ${\rm \mu m}$ at late timeswas detected by the Spitzer Space Telescope but no source was detected at 3.6${\rm \mu m}$. This peculiar spectrum indicates that there exist strong lineemitters around 4.5 ${\rm \mu m}$ and the absence of strong lines around 3.6${\rm \mu m}$. To model the spectrum we prepare a line list based on theselection rules in LS coupling from the experimentally calibrated energy levelsin the NIST database. This method enables to generate the synthetic spectrawith accurate line wavelengths. We find that the spectrum is sensitive to theabundance pattern whether or not the first r-process peak elements areincluded. In both cases, the synthetic spectra can match the observed data,leading to two possible interpretations. If the first peak elements areabundant a Se III line dominates the flux. If otherwise, W III with Os III, RhIII, and Ce IV can be the main sources. Observing nebular spectra for thefuture kilonovae in a wider wavelength range can provide more conclusiveelemental identification.

Tungsten vs Selenium as a potential source of kilonova nebular emission observed by Spitzer