We report JWST spectra and photometry of the underluminous Type Iax supernova (SN Iax) 2024vjm obtained 202.8 rest-frame days postexplosion. The spectrum exhibits a rich set of forbidden lines from low-ionization, intermediate-mass, and iron-group elements, notably the [Ni II] 6.64 μm resonance line, which is a direct indicator of stable nickel. Strong CO and SiO emission is detected alongside a warm dust continuum; the spectral properties are consistent with preexisting rather than newly formed dust. Synthetic spectra were computed with the generalized stellar atmospheres code PHOENIX using simplified ejecta models. The models reproduce the overall spectral energy distribution and the molecular emission features reasonably well, but substantially underestimate the strength of the mid-infrared atomic forbidden lines, leaving the synthetic spectrum dominated by molecular emission. Experiments in which the molecular opacity is suppressed do not recover the forbidden lines; instead, the emission peak migrates to Co and Fe transitions near 2 μm. We attribute this discrepancy to poorly constrained collisional rates and possibly to an excess of iron group material in the current ejecta models. A prominent feature at 12.8 μm is not well accounted for by the [Ne II] 12.81 μm line, indicating that the 12.8 μm feature may be largely due to [Fe III]. The presence of CO, SiO, and stable nickel together with the nondetection of neon places tight constraints on the total ejecta mass and the nucleosynthetic yields of SN Iax progenitor systems.

https://ui.adsabs.harvard.edu/abs/2026ApJ..1008L..53B/abstract