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 ⁵⁶Ni 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., ∼10⁵¹ erg, 0.06 M_⊙ of ⁵⁶Ni 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 ⁵⁶Ni 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.