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

Abstract

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 aredominated 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 thefundamental and first overtone. We introduce a novel, data-driven approach using non–local thermodynamicequilibrium 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 isfound to occur prior to day +285, with subsequent evolution characterized by progressive evaporation. The COmass decreases from approximately 8.7 to 1.3 ×10−3M⊙, while the average temperature drops from ≈2900 to≈2500 K. Concurrently, the CO distribution transitions from nearly homogeneous to highly clumped (densitycontrast increasing from fc ≈ 1.2 to 2). The minimum velocity of the CO-emitting region remains nearly constant(v1 ≈ 1200 to 1100 km s−1), significantly above the receding photosphere velocity (vph ≈ 500 km s−1), suggestingthe photosphere resides within Si-rich layers. However, the temperature profile indicates that only a narrow zonereaches the conditions necessary for SiO formation. Due to a lack of observational constraints, SiO clumping isnot modeled, and thus, synthetic SiO profiles for mass estimates are not highlighted. We discuss the implicationsof these findings for dust formation processes in SN 2024ggi.