The supernova remnant Cassiopeia A (Cas A) is one of the few remnants in which it is possible to observe unshocked ejecta. A deep 1.64 µm image of Cas A shows a patch of diffuse emission from unshocked ejecta, as well as brighter emission from Fast-Moving Knots and Quasi-Stationary Flocculi. Emission at 1.64 µm is usually interpreted as [Fe II] emission, and spectra of the bright knots confirm this by showing the expected emission in other [Fe II] lines. We performed NIR spectroscopy on the diffuse emission region and found that the unshocked ejecta emission does not show those lines, but rather the [Si I] 1.607 µm line. This means that the 1.64 µm line from the unshocked ejecta may be the [Si I] 1.645 line from the same upper level, rather than [Fe II]. We find that the [Si I] line is formed by recombination, and we use the [Si I] to [Si II] ratio to infer a temperature about 100 K, close to the value assumed for analysis of low frequency radio absorption and that inferred from emission by cool dust. Our results constrain estimates of Cas A's total mass of unshocked ejecta that are extremely sensitive to temperature assumptions, but they do not resolve the ambiguity due to clumping.

Cassiopeia A is a nearby young supernova remnant that provides a unique laboratory for the study of core-collapse supernova explosions 1 . Cassiopeia A is known to be a Type IIb supernova from the optical spectrum of its light echo 2 , but the immediate progenitor of the supernova remains uncertain 3 . 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 an HST 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 medium 4 . This result is consistent with a theoretical model 5,6 of dust condensation in material that is heavily enriched with CNO-cycle products, supporting the idea that the clump originated near the He core of the progenitor 7,8 . It has been recently found that Type IIb supernovae can result from the explosion of a blue supergiant with a thin hydrogen envelope 9−11 , and our results support such a scenario for Cassiopeia A.Core-collapse supernovae and their young remnants interact with the circumstellar material (CSM) ejected at the end of the progenitors' lifetime 12 . By studying the physical and chemical characteristics of this material, we can learn how the progenitors stripped off their envelopes and exploded, which is crucial for understanding the nature of progenitors.Cassiopeia A (Cas A) is a young (∼ 340 yr) 13 supernova remnant (SNR) where we observe the interaction of the SNR blast wave with the CSM. Its SN type is Type IIb, indicating that the progenitor had a thin H envelope at the time of explosion 2 . The morphology and expansion rates of the Cas A SNR suggest that it is interacting with a smooth red supergiant (RSG) wind 14,15 . The X-ray characteristics of the shocked ejecta knots and shocked ambient gas are also consistent with Cas A expanding into an RSG wind 16,17 . On the other hand, an X-ray spectral analysis indicates that there could have been a small bubble produced by a fast tenuous wind in the post-RSG stage 16 . Hence, it is uncertain whether the Cas A SN exploded in an RSG phase with the dense slow wind extending all the way to the stellar surface or if, instead, there was a short blue phase with a faster wind just prior to the explosion 3 .A distinct component of the CSM in Cas A is the so-called "quasi-stationary floculli (QSFs)" (Fig. 1). These nebulosities or clumps are almost 'stationary' ( < ∼ 400 km s −1 ) and are bright in Hα and [N ii] λλ6548, 6583 emission line images 18−22 . Their optical and near-infrared (NIR) spectra indicate that QSFs are dense (3-9 × 10 4 cm −3 ) and He and N enriched 7,23−25 . QSFs are probably dense CNOprocessed circumstellar clumps that have been shocked recently by the SN blast wave 1...