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Abstract We present JWST NIRCam (F356W and F444W filters) and MIRI (F770W) images and NIRSpec Integral Field Unit (IFU) spectroscopy of the young Galactic supernova remnant Cassiopeia A (Cas A) to probe the physical conditions for molecular CO formation and destruction in supernova ejecta. We obtained the data as part of a JWST survey of Cas A. The NIRCam and MIRI images map the spatial distributions of synchrotron radiation, Ar-rich ejecta, and CO on both large and small scales, revealing remarkably complex structures. The CO emission is stronger at the outer layers than the Ar ejecta, which indicates the re-formation of CO molecules behind the reverse shock. NIRSpec-IFU spectra (3–5.5μm) were obtained toward two representative knots in the NE and S fields that show very different nucleosynthesis characteristics. Both regions are dominated by the bright fundamental rovibrational band of CO in the two R and P branches, with strong [Arvi] and relatively weaker, variable strength ejecta lines of [Siix], [Caiv], [Cav], and [Mgiv]. The NIRSpec-IFU data resolve individual ejecta knots and filaments spatially and in velocity space. The fundamental CO band in the JWST spectra reveals unique shapes of CO, showing a few tens of sinusoidal patterns of rovibrational lines with pseudocontinuum underneath, which is attributed to the high-velocity widths of CO lines. Our results with LTE modeling of CO emission indicate a temperature of ∼1080 K and provide unique insight into the correlations between dust, molecules, and highly ionized ejecta in supernovae and have strong ramifications for modeling dust formation that is led by CO cooling in the early Universe. 

ABSTRACT Modelling the red–blue asymmetries seen in the broad emission lines of core-collapse supernovae (CCSNe) is a powerful technique to quantify total dust mass formed in the ejecta at late times (>5 yr after outburst) when ejecta dust temperatures become too low to be detected by mid-infrared (IR) instruments. Following our success in using the Monte Carlo radiative transfer code damocles to measure the dust mass evolution in SN 1987A and other CCSNe, we present the most comprehensive sample of dust mass measurements yet made with damocles, for CCSNe aged between 4 and 60 yr after outburst. Our sample comprises multi-epoch late-time optical spectra taken with the Gemini/Gemini Multi-Object Spectrographs (GMOS) and Very Large Telescope (VLT) X-Shooter spectrographs, supplemented by archival spectra. For the 14 CCSNe that we have modelled, we confirm a dust mass growth with time that can be fit by a sigmoid curve that is found to saturate beyond an age of ∼30 yr, at a mass of 0.23$$^{+0.17}_{-0.12}$$ M⊙. For an expanded sample including dust masses found in the literature for a further 11 CCSNe and six CCSN remnants, the dust mass at saturation is found to be 0.42$$^{+0.09}_{-0.05}$$ M⊙. Uncertainty limits for our dust masses were determined from a Bayesian analysis using the affine invariant Markov chain Monte Carlo ensemble sampler emcee with damocles. The best-fitting line profile models for our sample all required grain radii between 0.1 and 0.5 $$\mu$$m. Our results are consistent with CCSNe forming enough dust in their ejecta to significantly contribute to the dust budget of the Universe.