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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 In order to better characterize the rich supernova remnant (SNR) population of M83 (NGC 5236), we have obtained high-resolution (∼85 km s⁻¹) spectra of 119 of the SNRs and SNR candidates in M83 with Gemini/GMOS, as well as new spectra of the young SNRs B12-174a and SN 1957D. Most of the SNRs and SNR candidates have [Sii]:Hαratios that exceed 0.4. Combining these results with earlier studies we have carried out with MUSE and at lower spectroscopic resolution with GMOS, we have confirmed a total of 238 emission nebulae to be SNRs on the basis of their [Sii]:Hαratios, about half of which have emission lines that show velocity broadening greater than 100 km s⁻¹, providing a kinematic confirmation that they are SNRs and not Hiiregions. Looking at the entire sample, we find a strong correlation between velocity widths and the line ratios of [Oi]λ6300:Hα, [Nii]λ6584:Hα, and [Sii]λλ6716, 6731:Hα. The density-sensitive [Sii]λ6716:λ6731 line ratio is strongly correlated with SNR diameter, but not with the velocity width. We discuss these results in the context of previously published shock models. 

Abstract We present proper motion measurements of the oxygen-rich ejecta of the LMC supernova remnant N132D using two epochs of Hubble Space Telescope Advanced Camera for Surveys data spanning 16 years. The proper motions of 120 individual knots of oxygen-rich gas were measured and used to calculate a center of expansion (CoE) of α = 5 h 25 m 01.ˢ71 and δ = −69°38′41.″64 (J2000) with a 1 σ uncertainty of 2.″90. This new CoE measurement is 9.″2 and 10.″8 from two previous CoE estimates based on the geometry of the optically emitting ejecta. We also derive an explosion age of 2770 ± 500 yr, which is consistent with recent age estimates of ≈2500 yr made from 3D ejecta reconstructions. We verified our estimates of the CoE and age using a new automated procedure that detected and tracked the proper motions of 137 knots, with 73 knots that overlap with the visually identified knots. We find that the proper motions of the ejecta are still ballistic, despite the remnant’s age, and are consistent with the notion that the ejecta are expanding into an interstellar medium cavity. Evidence for explosion asymmetry from the parent supernova is also observed. Using the visually measured proper motion measurements and corresponding CoE and age, we compare N132D to other supernova remnants with proper motion ejecta studies. 

Abstract Here we describe a new study of the supernova remnants (SNRs) and SNR candidates in nearby face-on spiral galaxy M83, based primarily on MUSE integral field spectroscopy. Our revised catalog of SNR candidates in M83 has 366 objects, 81 of which are reported here for the first time. Of these, 229 lie within the MUSE observation region, 160 of which have spectra with [Sii]:Hαratios exceeding 0.4, the value generally accepted as confirmation that an emission nebula is shock-heated. Combined with 51 SNR candidates outside the MUSE region with high [Sii]:Hαratios, there are 211 spectroscopically confirmed SNRs in M83, the largest number of confirmed SNRs in any external galaxy. MUSE’s combination of relatively high spectral resolution and broad wavelength coverage has allowed us to explore two other properties of SNRs that could serve as the basis of future SNR searches. Specifically, most of the objects identified as SNRs on the basis of [Sii]:Hαratios exhibit more velocity broadening and lower ratios of [Siii]:[Sii] emission than Hiiregions. A search for nebulae with the very broad emission lines expected from young, rapidly expanding remnants revealed none, except for the previously identified B12-174a. The SNRs identified in M83 are, with few exceptions, middle-aged interstellar medium (ISM) dominated ones. Smaller-diameter candidates show a larger range of velocity broadening and a larger range of gas densities than the larger-diameter objects, as expected if the SNRs expanding into denser gas brighten and then fade from view at smaller diameters than those expanding into a more tenuous ISM. 

ABSTRACT We present a new catalogue of radio sources in the face-on spiral galaxy M83. Radio observations taken in 2011, 2015, and 2017 with the Australia Telescope Compact Array at 5.5 and 9 GHz have detected 270 radio sources. Although a small number of these sources are background extragalactic sources, most are either H ii regions or supernova remnants (SNRs) within M83 itself. Three of the six historical supernovae are detected, as is the very young remnant that had been identified in a recent study, which is likely the result of a supernova that exploded in the last ∼100 yr but was missed. All of these objects are generally fading with time. Confusion limits our ability to measure the radio emission from a number of the SNRs in M83, but 64 were detected in unconfused regions, and these have the approximate power-law luminosity function that has been observed in other galaxies. The SNRs in M83 are systematically smaller in diameter and brighter than those that have been detected at radio wavelengths in M33. A number of the radio sources are coincident with X-ray sources in M83; most of these coincident sources turn out to be SNRs. Our dual frequency observations are among the most sensitive to date for a spiral galaxy outside the Local Group; despite this we were not able to place realistic constraints on the spectral indices, and as a result, it was not possible to search for SNRs based on their radio properties alone.