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

Understand

Infrared emission lines arising from transitions between fine structure levels of heavy elements are expected to produce kilonova nebular emission.

• For the kilonova in GW170817, strong emission at 4.5 ${\rm \mu m}$ at late times was 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 line emitters 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 the selection rules in LS coupling from the experimentally calibrated energy levels in the NIST database.