JWST Observations of SN 2024ggi. II. NIRSpec Spectroscopy and CO Modeling at +285–385 Days past the Explosion

We present James Webb Space Telescope (JWST) Near-Infrared Spectrograph observations of SN 2024ggi, spanning wavelengths of 1.7–5.5 μm at +285.51 and +385.27 days postexplosion. These nebular spectra are dominated by asymmetric emission lines from atomic species including H, Ca, Ar, C, Mg, Ni, Co, and Fe, indicative of an aspherical explosion. The other strong features are molecular CO vibrational bands from the fundamental and first overtone. We introduce a novel, data-driven approach using non–local thermodynamic equilibrium three-dimensional (3D) radiative transfer simulations to model the CO emission with high fidelity. This method enables us to constrain the 3D CO distribution and its radial temperature structure. CO formation is found to occur prior to day +285, with subsequent evolution characterized by progressive evaporation. The CO mass decreases from approximately 8.7 to 1.3 ×10⁻³ M _⊙, while the average temperature drops from ≈2900 to ≈2500 K. Concurrently, the CO distribution transitions from nearly homogeneous to highly clumped (density contrast increasing from fc ≈ 1.2 to 2). The minimum velocity of the CO-emitting region remains nearly constant (v ₁ ≈ 1200 to 1100 km s⁻¹), significantly above the receding photosphere velocity (v _ph ≈ 500 km s⁻¹), suggesting the photosphere resides within Si-rich layers. However, the temperature profile indicates that only a narrow zone reaches the conditions necessary for SiO formation. Due to a lack of observational constraints, SiO clumping is not modeled, and thus, synthetic SiO profiles for mass estimates are not highlighted. We discuss the implications of these findings for dust formation processes in SN 2024ggi.