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Abstract We present panchromatic optical + near-infrared (NIR) + mid-infrared (MIR) observations of the intermediate-luminosity Type Iax supernova (SN Iax) 2024pxl and the extremely low-luminosity SN Iax 2024vjm. JWST observations provide unprecedented MIR spectroscopy of SN Iax, spanning from +11 to +42 day past maximum light. We detect forbidden emission lines in the MIR at these early times while the optical and NIR are dominated by permitted lines with an absorption component. Panchromatic spectra at early times can thus simultaneously show nebular and photospheric lines, probing both inner and outer layers of the ejecta. We identify spectral lines not seen before in SN Iax, including [Mgii] 4.76μm, [Mgii] 9.71μm, [Neii] 12.81μm, and isolated Oi2.76μm that traces unburned material. Forbidden emission lines of all species are centrally peaked with similar kinematic distributions, indicating that the ejecta are well mixed in both SN 2024pxl and SN 2024vjm, a hallmark of pure deflagration explosion models. Radiative transfer modeling of SN 2024pxl shows good agreement with a weak deflagration of a near-Chandrasekhar-mass white dwarf, but additional IR flux is needed to match the observations, potentially attributable to a surviving remnant. Similarly, we find SN 2024vjm is also best explained by a weak deflagration model, despite the large difference in luminosity between the two supernovae. Future modeling should push to even weaker explosions and include the contribution of a bound remnant. Our observations demonstrate the diagnostic power of panchromatic spectroscopy for unveiling explosion physics in thermonuclear supernovae. 

Abstract Supernova (SN) 2014C is a rare transitional event that exploded as a hydrogen-poor, helium-rich Type Ib SN and subsequently interacted with a hydrogen-rich circumstellar medium (CSM) a few months postexplosion. This unique interacting object provides an opportunity to probe the mass-loss history of a stripped-envelope SN progenitor. Using the James Webb Space Telescope (JWST), we observed SN 2014C with the Mid-Infrared Instrument Medium Resolution Spectrometer at 3477 days postexplosion (rest frame), and the Near-Infrared Spectrograph Integral Field Unit at 3568 days postexplosion, covering 1.7–25μm. The bolometric luminosity indicates that the SN is still interacting with the same CSM that was observed with the Spitzer Space Telescope 40–1920 days postexplosion. JWST spectra and near-contemporaneous optical and near-infrared spectra show strong [Neii] 12.831μm, He 1.083μm, Hα, and forbidden oxygen ([Oi]λλ6300, 6364, [Oii]λλ7319, 7330, and [Oiii]λλ4959, 5007) emission lines with asymmetric profiles, suggesting a highly asymmetric CSM. The mid-IR continuum can be explained by ∼0.036M_⊙of carbonaceous dust at ∼300 K and ∼0.043M_⊙of silicate dust at ∼200 K. The observed dust mass has increased tenfold since the last Spitzer observation 4 yr ago, with evidence suggesting that new grains have condensed in the cold dense shell between the forward and reverse shocks. This dust mass places SN 2014C among the dustiest SNe in the mid-IR and supports the emerging observational trend that SN explosions produce enough dust to explain the observed dust mass at high redshifts. 

Abstract Dust from core-collapse supernovae (CCSNe), specifically Type IIP supernovae (SNe IIP), has been suggested to be a significant source of the dust observed in high-redshift galaxies. CCSNe eject large amounts of newly formed heavy elements, which can condense into dust grains in the cooling ejecta. However, infrared (IR) observations of typical CCSNe generally measure dust masses that are too small to account for the dust production needed at high redshifts. Type IIn SNe (SNe IIn), classified by their dense circumstellar medium, are also known to exhibit strong IR emission from warm dust, but the dust origin and heating mechanism have generally remained unconstrained because of limited observational capabilities in the mid-IR (MIR). Here, we present a JWST/MIRI Medium Resolution Spectrograph spectrum of the SN IIn SN 2005ip nearly 17 yr post-explosion. The SN IIn SN 2005ip is one of the longest-lasting and most well-studied SNe observed to date. Combined with a Spitzer MIR spectrum of SN 2005ip obtained in 2008, this data set provides a rare 15 yr baseline, allowing for a unique investigation of the evolution of dust. The JWST spectrum shows the emergence of an optically thin silicate dust component (≳0.08M_⊙) that is either not present or more compact/optically thick in the earlier Spitzer spectrum. Our analysis shows that this dust is likely newly formed in the cold, dense shell (CDS), between the forward and reverse shocks, and was not preexisting at the time of the explosion. There is also a smaller mass of carbonaceous dust (≳0.005M_⊙) in the ejecta. These observations provide new insights into the role of SN dust production, particularly within the CDS, and its potential contribution to the rapid dust enrichment of the early Universe. 

ABSTRACT One of the most studied objects in astronomy, the Crab Nebula, is the remnant of the historical supernova SN 1054. Historical observations of the supernova imply a typical supernova luminosity, but contemporary observations of the remnant imply a low explosion energy and low ejecta kinetic energy. These observations are incompatible with a standard $$^{56}$$Ni-powered supernova, hinting at an an alternate power source such as circumstellar interaction or a central engine. We examine SN 1054 using a pulsar-driven supernova model, similar to those used for superluminous supernovae. The model can reproduce the luminosity and velocity of SN 1054 for an initial spin period of $$\sim$$14 ms and an initial dipole magnetic field of 10$$^{14-15}$$ G. We discuss the implications of these results, including the evolution of the Crab pulsar, the evolution of the remnant structure, formation of filaments, and limits on freely expanding ejecta. We discuss how our model could be tested further through potential light echo photometry and spectroscopy, as well as the modern analogues of SN 1054. 

Abstract 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⁻¹N-rich ejecta knots that penetrated and advanced out ahead of the remnant’s 5000–6000 km s⁻¹outer 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. 

Abstract We present JWST observations of the Crab Nebula, the iconic remnant of the historical SN 1054. The observations include NIRCam and MIRI imaging mosaics plus MIRI/MRS spectra that probe two select locations within the ejecta filaments. We derive a high-resolution map of dust emission and show that the grains are concentrated in the innermost, high-density filaments. These dense filaments coincide with multiple synchrotron bays around the periphery of the Crab's pulsar wind nebula (PWN). We measure synchrotron spectral index changes in small-scale features within the PWN’s torus region, including the well-known knot and wisp structures. The index variations are consistent with Doppler boosting of emission from particles with a broken power-law distribution, providing the first direct evidence that the curvature in the particle injection spectrum is tied to the acceleration mechanism at the termination shock. We detect multiple nickel and iron lines in the ejecta filaments and use photoionization models to derive nickel-to-iron abundance ratios that are a factor of 3–8 higher than the solar ratio. We also find that the previously reported order-of-magnitude higher Ni/Fe values from optical data are consistent with the lower values from JWST when we reanalyze the optical emission using updated atomic data and account for local extinction from dust. We discuss the implications of our results for understanding the nature of the explosion that produced the Crab Nebula and conclude that the observational properties are most consistent with a low-mass Fe core-collapse supernova, even though an electron-capture explosion cannot be ruled out. 

Abstract The recent survey of the core-collapse supernova remnant Cassiopeia A (Cas A) with the MIRI instrument on board the James Webb Space Telescope (JWST) revealed a large structure in the interior region, referred to as the “Green Monster.” Although its location suggests that it is an ejecta structure, the infrared properties of the “Green Monster” hint at a circumstellar medium (CSM) origin. In this companion paper to the JWST Cas A paper, we investigate the filamentary X-ray structures associated with the “Green Monster” using Chandra X-ray Observatory data. We extracted spectra along the “Green Monster” as well as from shocked CSM regions. Both the extracted spectra and a principal component analysis show that the “Green Monster” emission properties are similar to those of the shocked CSM. The spectra are well fit by a model consisting of a combination of a nonequilibrium ionization model and a power-law component, modified by Galactic absorption. All the “Green Monster” spectra show a blueshift corresponding to a radial velocity of around −2300 km s⁻¹, suggesting that the structure is on the near side of Cas A. The ionization age is aroundn_et≈ 1.5 × 10¹¹cm⁻³s. This translates into a preshock density of ∼12 cm⁻³, higher than previous estimates of the unshocked CSM. The relatively highn_etand relatively low radial velocity suggest that this structure has a relatively high density compared to other shocked CSM plasma. This analysis provides yet another piece of evidence that the CSM around Cas A’s progenitor was not that of a smooth steady wind profile. 

Abstract 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 3μm) 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. 

Context. Recent observations with the James Webb Space Telescope (JWST) have revealed unprecedented details of an intricate filamentary structure of unshocked ejecta within the young supernova remnant (SNR) Cassiopeia A (Cas A), offering new insights into the mechanisms governing supernova (SN) explosions and the subsequent evolution of ejecta. Aims. We aim to investigate the origin and evolution of the newly discovered web-like network of ejecta filaments in Cas A. Our specific objectives are: (i) to characterize the three-dimensional (3D) structure and kinematics of the filamentary network and (ii) to identify the physical mechanisms responsible for its formation. Methods. We performed high-resolution, 3D hydrodynamic (HD) and magneto-hydrodynamic (MHD) simulations to model the evolution of a neutrino-driven SN from the explosion to its remnant with the age of 1000 years. The initial conditions, set shortly after the shock breakout at the stellar surface, are based on a 3D neutrino-driven SN model that closely matches the basic properties of Cas A. Results. We found that the magnetic field has little impact on the evolution of unshocked ejecta, so we focused most of the analysis on the HD simulations. A web-like network of ejecta filaments, with structures compatible with those observed by JWST (down to scales ≈0.01 pc), naturally forms during the SN explosion. The filaments result from the combined effects of processes occurring soon after the core collapse, including the expansion of neutrino-heated bubbles formed within the first second after the explosion, hydrodynamic instabilities triggered during the blast propagation through the stellar interior, and the Ni-bubble effect following the shock breakout. The interaction of the reverse shock with the ejecta progressively disrupts the filaments through the growth of hydrodynamic instabilities. By around 700 years, the filamentary network becomes unobservable. Conclusions. According to our models, the filaments observed by JWST in Cas A most likely preserve a “memory” of the early explosion conditions, reflecting the processes active during and immediately after the SN event. Notably, a filamentary network closely resembling that observed in Cas A is naturally produced by a neutrino-driven SN explosion. 

Context.The supernova remnant (SNR) Cassiopeia A (Cas A) offers a unique opportunity to study supernova (SN) explosion dynamics and remnant interactions with the circumstellar medium (CSM). Recent observations with the James Webb Space Telescope have unveiled an enigmatic structure within the remnant, termed “Green Monster” (GM), whose properties indicate a CSM origin. Aims.Our goal is to investigate the properties of the GM and uncover the origin of its intriguing pockmarked structure, characterized by nearly circular holes and rings. We aim to examine the role of small-scale ejecta structures in shaping these features through their interaction with a dense circumstellar shell. Methods.We adopted a neutrino-driven SN model to trace the evolution of its explosion from core collapse to the age of the Cas A remnant using high-resolution 3D magnetohydrodynamic simulations. Besides other processes, the simulations include self-consistent calculations of radiative losses, accounting for deviations from electron-proton temperature equilibration and ionization equilibrium, as well as the ejecta composition derived from the SN. Results.The observed GM morphology can be reproduced by the interaction of dense ejecta clumps and fingers with an asymmetric, forward-shocked circumstellar shell. The clumps and fingers form by hydrodynamic instabilities growing at the interface between SN ejecta and shocked CSM. Radiative cooling accounting for effects of non-equilibrium of ionization enhances the ejecta fragmentation, forming dense knots and thin filamentary structures that penetrate the shell, producing a network of holes and rings with properties similar to those observed. Conclusions.The origin of the holes and rings in the GM can be attributed to the interaction of ejecta with a shocked circumstellar shell. By constraining the timing of this interaction and analyzing the properties of these structures, we provide a distinction of this scenario from an alternative hypothesis, which attributes these features to fast-moving ejecta knots penetrating the shell ahead of the forward shock.