An optical-to-infrared study of type II SN 2024ggi at nebular times

We present 0.3–21 μm observations at ∼275 d and ∼400 d of type II supernova (SN) 2024ggi that combined ground-based optical and near-infrared data from the Keck I/II telescopes and space-based infrared data from the James Webb Space Telescope. Although the optical regions dominate the observed flux, SN 2024ggi is bright at infrared wavelengths (65% and 35% fall each side of 1 μm). SN 2024ggi exhibits a plethora of emission lines from H, He, intermediate-mass elements (O, Na, Mg, S, Ar, and Ca), and iron-group elements (IGEs; Fe, Co, and Ni). The width of all lines is essentially the same, which suggests efficient macroscopic chemical mixing of the inner ejecta at ≲2000 km s −1 and little mixing of 56 Ni at higher velocities. Molecular emission in the infrared range is dominated by the CO fundamental, which radiates about 5% of the total SN luminosity. A molecule-free radiative-transfer model based on a standard explosion of a red supergiant star (i.e., ∼10 51 erg, 0.06 M ⊙ of 56 Ni from a 15.2 M ⊙ progenitor) yields a satisfactory match throughout the optical and infrared at both epochs. The SN 2024ggi CO luminosity is comparable to the fractional decay power absorbed in the model C/O-rich shell. An accounting for CO cooling would likely resolve the model overestimate of the [O I ] 0.632 μm flux. The relative weakness of the molecular emission in SN 2024ggi and the good overall match obtained with our molecule-free model suggests negligible microscopic mixing; about 95% of the SN luminosity is radiated by atoms and ions. The lines from IGEs, which form from explosion ashes at these late times, are ideal diagnostics of the magnitude of 56 Ni mixing in core-collapse SN ejecta. Stable Ni, which was identified in SN 2024ggi (e.g., [Ni II ] 6.634 μm), is probably a common product of explosions of massive stars.