The Temperature and Ionization of Unshocked Ejecta in Cas A

TL;DR

This study reveals that the unshocked ejecta in Cas A emit [Si I] rather than [Fe II], allowing for more accurate temperature and mass estimates of the ejecta based on spectroscopic analysis.

Contribution

The paper identifies the true emission line in unshocked ejecta as [Si I], not [Fe II], and uses this to refine temperature and mass estimates of the ejecta in Cas A.

Findings

Unshocked ejecta emit [Si I] 1.607 micron line, not [Fe II].

The temperature of unshocked ejecta is about 100 K.

Mass estimates are highly sensitive to temperature assumptions.

Abstract

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 micron 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 micron 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 micron line. This means that the 1.64 micron 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.