A Framework for Analysing Nebular-Phase JWST Supernova Spectra with Application to Type Ic SN 2024aecx

We analyse what physical quantities of SN ejecta can be obtained from a single epoch of nebular JWST spectroscopy. We provide an analytical framework, which together with a grid of helium star SUMO spectral synthesis models allows the determination of these quantities for any stripped-envelope SN. Using the framework, we derive critical electron densities for major emission lines, finding values in the range $10^5-10^6$ cm$^{-3}$. This means formation under close to LTE conditions for 200 – 300 days. We obtain ionisation fractions of Ni and Co, as well as the temperature and filling factor of the Fe/He zone, the $^{56}$Ni and stable nickel masses, the Ni/Fe ratio, and the optical depths $τ_{\mathrm{[Ni\,II]\, 6.636 μm}}$ and $τ_{\mathrm{[Co\,II]\, 10.52 μm}}$. Besides these constraints from iron group elements, one can also obtain the filling factor of the carbon burning (O/Ne/Mg) zone from [Ne II] 12.81 $μ$m, and the masses of Ar II and Cl I. We apply our methodology to a publicly available JWST spectrum of Type Ic SN 2024aecx. We find that this SN produced a chlorine mass of $M_{\mathrm{Cl\, I}} \simeq M_{\mathrm{Cl}} = 5.1^{+2.0}_{-1.6}\times 10^{-4}$ $M_{\odot}$, a factor a few larger than expected from stellar evolution modeling. Furthermore, we find that the SN experienced an unexpectedly small 'Ni-bubble' effect, with $ρ_{\mathrm{FeHe}} = 2.5^{+0.7}_{-0.4} \times 10^{-15}$ g cm$^{-3}$ (or an expansion factor of 1 – 3 for the Ni-bubble, depending on the adopted model) at 167d for the silicon burning material. We also determine $M_{\mathrm{Ar\, II}} = 4.9^{+2.2}_{-1.5} \times 10^{-4}$ $M_{\odot}$, which from ionisation fractions in our NLTE models yields a best estimate of $M_{\mathrm{Ar}} = 1.1 \pm 0.4 \times 10^{-3}$ $M_{\odot}$.