JWST Observations of SN 2024ggi II: NIRSpec Spectroscopy and CO Modeling at 285 and 385 Days Past the Explosion

We present James Webb Space Telescope (JWST) NIRSpec observations of SN2024ggi, spanning wavelengths of 1.7–5.5 micron at +285.51 and +385.27 days post-explosion. 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-LTE 3D radiative transfer simulations to model the CO emission with high fidelity. This method enables us to constrain the three-dimensional 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*E-3 Mo, while the average temperature drops from about 2900 K 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 (v1 = 1200 to 1100 km/s), significantly above the receding photosphere velocity (v(ph) = 500 km/s), 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 SN2024ggi.

💡 Research Summary

This paper presents JWST NIRSpec observations of the Type IIP supernova SN 2024ggi obtained at +285.51 and +385.27 days after explosion, covering the 1.7–5.5 µm wavelength range with a resolving power of R ≈ 1000. The spectra are dominated by asymmetric nebular emission lines from H, He, Ca, Ar, C, Mg, Ni, Co, and Fe, indicating a markedly aspherical ejecta geometry. In addition, strong molecular signatures of carbon monoxide appear as the fundamental band (4.5–5.2 µm) and the first overtone band (2.2–2.4 µm). The first overtone weakens dramatically between the two epochs, while the fundamental band declines by only ~50 %, suggesting that CO becomes an increasingly important coolant at later times.

The authors identify a comprehensive set of atomic lines, measure their velocity profiles, and find evidence for line splitting of ~1000 km s⁻¹ in several intermediate‑mass and iron‑group species (e.g.,