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Infrared diagnostics of late-time core-collapse supernova spectra

We present nonlocal thermodynamic equilibrium radiative transfer calculations of red supergiant and He-star explosions, extending previous work to focus on the infrared emission from atoms and ions in the ejecta during the nebular-phase (i.e., ~200 to ~500d) -- molecules and dust are ignored. We cover non-rotating solar-metallicity progenitors spanning an initial mass between 10 and about 40Msun and exploding as Type II or Ibc supernovae (SNe). Both photometrically and spectroscopically, the SN II models evolve distinctly from the SN Ibc models primarily because of the greater ejecta kinetic-energy-to-mass ratio in the latter, which leads to a greater gamma-ray escape together with a lower density and a higher ionization in our H-deficient ejecta. Type II SN models remain optically luminous at all times, whereas Type Ibc models progressively brighten in the infrared (which holds 80% of their luminosity at 500d), causing strong infrared lines such as [NeII]12.810mic and [NiII]6.634mic to evolve essentially at constant luminosity. The strength of [NeII]12.810mic exhibits a complicated dependence with either He-core or preSN mass because of the additional impact of ejecta ionization -- this line radiates alone up to 20% of the SN luminosity after ~300d in our Type Ibc SN models. The numerous infrared Ni lines are found to be good tracers of the material that underwent explosive nucleosynthesis and can thus be used directly to constrain the level of 56Ni mixing in core-collapse SNe. The evolution of the integrated flux in infrared Fe and Co lines shows a great amount of diversity, which compromises their use as a diagnostic of the 56Ni-decay power source in our models. Future spectroscopic observations of core-collapse SNe by JWST will provide unprecedented information on the emission from atoms and ions in their ejecta, delivering critical constraints on massive star explosions.