Search

We present initial results from a James Webb Space Telescope (JWST) survey of the youngest Galactic core-collapse supernova remnant, Cassiopeia A (Cas A), made up of NIRCam and MIRI imaging mosaics that map emission from the main shell, interior, and surrounding circumstellar/interstellar material (CSM/ISM). We also present four exploratory positions of MIRI Medium Resolution Spectrograph integral field unit spectroscopy that sample ejecta, CSM, and associated dust from representative shocked and unshocked regions. Surprising discoveries include (1) a weblike network of unshocked ejecta filaments resolved to ∼0.01 pc scales exhibiting an overall morphology consistent with turbulent mixing of cool, low-entropy matter from the progenitor’s oxygen layer with hot, high-entropy matter heated by neutrino interactions and radioactivity; (2) a thick sheet of dust-dominated emission from shocked CSM seen in projection toward the remnant’s interior pockmarked with small (∼1″) round holes formed by ≲0.″1 knots of high-velocity ejecta that have pierced through the CSM and driven expanding tangential shocks; and (3) dozens of light echoes with angular sizes between ∼0.″1 and 1′ reflecting previously unseen fine-scale structure in the ISM. NIRCam observations place new upper limits on infrared emission (≲20 nJy at 3 μ m) from the neutron star in Cas A’s center and tightly constrain scenarios involving a possible fallback disk. These JWST survey data and initial findings help address unresolved questions about massive star explosions that have broad implications for the formation and evolution of stellar populations, the metal and dust enrichment of galaxies, and the origin of compact remnant objects.

JWST observations of the young Galactic supernova remnant Cassiopeia A revealed an unexpected structure seen as a green emission feature in colored composite MIRI F1130W and F1280W images—hence dubbed the Green Monster—that stretches across the central parts of the remnant in projection. Combining the kinematic information from NIRSpec and the MIRI Medium Resolution Spectrograph with the multiwavelength imaging from NIRCam and MIRI, we associate the Green Monster with circumstellar material (CSM) that was lost during an asymmetric mass-loss phase. MIRI images are dominated by dust emission, but their spectra show emission lines from Ne, H, and Fe with low radial velocities indicative of a CSM nature. An X-ray analysis of this feature in a companion paper supports its CSM nature and detects significant blueshifting, thereby placing the Green Monster on the nearside, in front of the Cas A supernova remnant. The most striking features of the Green Monster are dozens of almost perfectly circular 1″–3″ sized holes, most likely created by interaction between high-velocity supernova ejecta material and the CSM. Further investigation is needed to understand whether these holes were formed by small 8000–10,500 km s ^−1 N-rich ejecta knots that penetrated and advanced out ahead of the remnant’s 5000–6000 km s ^−1 outer blast wave or by narrow ejecta fingers that protrude into the forward-shocked CSM. The detection of the Green Monster provides further evidence of the highly asymmetric mass loss that Cas A’s progenitor star underwent prior to its explosion.