Lanthanide impact on the infrared spectra of nebular phase kilonovae

ABSTRACT Nebular phase kilonovae (KNe) have significant infrared (IR) emission thought to be mostly forbidden emission lines from rapid neutron capture (r-process) species in neutron star merger ejecta. Lanthanide elements in particular have complex open f-shell atomic structures with many IR transitions. Using non-local thermodynamic equilibrium radiative transfer simulations, we explore the impact of lanthanides on the IR spectra of KNe in the nebular phase, exploring a parameter space of ejecta mass and lanthanide fraction. We find that lanthanide impact is greater at higher densities, corresponding to earlier epochs and greater ejecta masses. The wavelengths most affected are found to be $\lambda \lesssim 4~\mu$m, with the species Ce iii and Nd ii being the most important contributors to spectral formation. We also find significant emission from species proposed in observations, notably Te iii at 2.1 $\mu$m, and Se iii at 4.5 and 5.7 $\mu$m, while W iii is subdominant at 4.5 $\mu$m. The Te iii feature at 2.1 $\mu$m is always blended, particularly with Zr ii, Ce iii, and Nd ii. We do not reproduce the smooth blackbody-like continua observed in AT2023vfi. Based on our results, we argue that line opacity alone is likely insufficient to produce optically thick continua in the nebular phase, even in the case of lanthanide/actinide-rich ejecta, as our models are optically thin in the IR at these epochs. Given that lanthanide contributions are dominant below 4 $\mu$m, we suggest that NIR observations best probe these elements, while mid-IR spectroscopy with James Webb Space Telescope can reliably probe non-lanthanide emission even in relatively lanthanide-rich cases.