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

Abstract

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 105 − 106 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 56Ni and stable nickel masses, the Ni/Fe ratio, and the optical depths $\tau _{[Ni\, {\small II}]\, 6.636}$ and $\tau _{[Co\, {\small II}]\, 10.52}$. 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 μm, 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⊙, 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 $\rho _{\mathrm{FeHe}} = 2.5^{+0.7}_{-0.4} \times 10^{-15}$ 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⊙, which from ionisation fractions in our NLTE models yields a best estimate of MAr = 1.1 ± 0.4 × 10−3  M⊙.