Cassiopeia A is a nearby young supernova remnant that provides a unique laboratory for the study of core-collapse supernova explosions¹. Cassiopeia A is known to be a type IIb supernova from the optical spectrum of its light echo², but the immediate progenitor of the supernova remains uncertain³. Here, we report results of near-infrared, high-resolution spectroscopic observations of Cassiopeia A, where we detected the pristine circumstellar material of the supernova progenitor. Our observations revealed a strong emission line of iron (Fe) from a circumstellar clump that has not yet been processed by the supernova shock wave. A comprehensive analysis of the observed spectra, together with a Hubble Space Telescope image, indicates that the majority of Fe in this unprocessed circumstellar material is in the gas phase, not depleted onto dust grains as in the general interstellar medium⁴. This result is consistent with a theoretical model^5,6 of dust condensation in material that is heavily enriched with carbon–nitrogen–oxygen cycle nuclear reaction products, supporting the idea that the clump originated near the helium core of the progenitor^7,8. It has recently been found that type IIb supernovae can result from the explosion of a blue supergiant with a thin hydrogen envelope^9,10,11, and our results support such a scenario for Cassiopeia A.

仙后座 A 是附近的年轻超新星遗迹，为研究核心坍缩超新星爆炸¹提供了一个独特的实验室。从其光回波的光谱来看，仙后座 A 是一种 IIb 型超新星²，但该超新星的直接前身仍不确定³. 在这里，我们报告了仙后座 A 的近红外高分辨率光谱观测结果，我们在其中检测到了超新星前身的原始环星物质。我们的观察揭示了来自尚未被超新星冲击波处理的星环团块的强烈铁（Fe）发射线。对观测到的光谱以及哈勃太空望远镜图像的综合分析表明，这种未经处理的星际物质中的大部分 Fe 处于气相状态，没有像一般星际介质⁴那样耗尽到尘埃颗粒上。该结果与理论模型^5,6一致大量富含碳-氮-氧循环核反应产物的材料中的尘埃凝结，支持了团块起源于前体^7,8的氦核附近的观点。最近发现 IIb 型超新星可能是由具有薄氢包层^9,10,11的蓝超巨星爆炸产生的，我们的结果支持仙后座 A 的这种情况。