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.

中文翻译：

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躲在 Cas A： JWST 前面的绿色怪物露出了一个密集而尘土飞扬的星际结构，该结构由喷射物相互作用标记


JWST 对年轻的银河系超新星残骸仙后座 A 的观测揭示了一个意想不到的结构，在彩色复合 MIRI F1130W 和 F1280W 图像中被视为绿色发射特征——因此被称为绿色怪物——它在投影中横跨残骸的中心部分。将来自 NIRSpec 和 MIRI 中分辨率光谱仪的运动学信息与来自 NIRCam 和 MIRI 的多波长成像相结合，我们将绿色怪物与在不对称质量损失阶段丢失的星际物质 （CSM） 相关联。MIRI 图像以尘埃发射为主，但它们的光谱显示了来自 Ne、H 和 Fe 的发射线，具有低径向速度，表明 CSM 性质。在一篇姊妹论文中对这一特征的 X 射线分析支持其 CSM 性质并检测到明显的蓝移，从而将绿色怪物置于 Cas A 超新星残骸的近侧。Green Monster 最引人注目的特征是数十个几乎完美的圆形 1 英寸-3 英寸大小的孔，很可能是由高速超新星喷射物和 CSM 之间的相互作用形成的。需要进一步研究以了解这些孔是由 8000-10,500 km s-1 富含 N 的喷射结形成的，这些喷射结是在残余物的 5000-6000 km s-1 外部冲击波之前穿透和推进，还是由伸入前向冲击 CSM 的狭窄喷射指形成的。对绿色怪物的探测进一步证明了 Cas A 的祖星在爆炸前经历了高度不对称的质量损失。