The Green Monster Hiding in Front of Cas A: JWST Reveals a Dense and Dusty Circumstellar Structure Pockmarked by Ejecta Interactions

Ilse De Looze, Dan Milisavljevic, Tea Temim, Danielle Dickinson, Robert Fesen, Richard G. Arendt, Jeremy Chastenet, Salvatore Orlando, Jacco Vink, Michael J. Barlow, Florian Kirchschlager, Felix D. Priestley, John C. Raymond, Jeonghee Rho, Nina S. Sartorio, Tassilo Scheffler, Franziska Schmidt, William P. Blair, Ori Fox, Christopher Fryer, Hans-Thomas Janka, Bon-Chul Koo, J. Martin Laming, Mikako Matsuura, Dan Patnaude, Mónica Relaño, Armin Rest, Judy Schmidt, Nathan Smith and Niharika Sravan
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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−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.
隐藏在 Cas A 前方的绿色怪物:JWST 揭示了喷出物相互作用所形成的稠密多尘的环恒星结构
JWST 对年轻的银河系超新星残余物仙后座 A 的观测发现了一个意想不到的结构,在 MIRI F1130W 和 F1280W 彩色合成图像中显示为绿色发射特征--因此被称为 "绿怪物"--它以投影的方式横跨残余物的中央部分。结合来自 NIRSpec 和 MIRI 中分辨率摄谱仪的运动学信息以及 NIRCam 和 MIRI 的多波长成像,我们将 "绿怪物 "与在不对称质量损失阶段损失的星周物质(CSM)联系起来。MIRI 图像以尘埃发射为主,但其光谱显示了 Ne、H 和 Fe 的发射线,其低径向速度表明了 CSM 的性质。另一篇论文中对这一特征的 X 射线分析也支持其 CSM 性质,并探测到了明显的蓝移,从而将绿怪物置于近侧,即 Cas A 超新星残余物的前方。绿怪物最显著的特征是数十个几乎完美的圆形1英寸-3英寸大小的洞,很可能是高速超新星喷出物质与CSM相互作用产生的。要了解这些洞是由8000-10500千米/秒的富含N的小喷出物节形成的,这些喷出物节穿透了残余物5000-6000千米/秒的外部冲击波并向前推进,还是由狭长的喷出物指形成的,这些喷出物指突出了向前冲击的CSM。对 "绿怪物 "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。
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