2018ApJ...866..128R

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 near-infrared spectroscopy on the diffuse emission region and found that the unshocked ejecta emission does not show those lines, but rather shows 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 of about 100 K, which is close to the value assumed for an analysis of low-frequency radio absorption and that can be 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.