Optical spectra of filaments in Cas A's northeastern ejecta jet reveal one knot which exhibits the H-alpha and forbidden N II emissions like those seen in the remnant's quasi-stationary flocculi plus forbidden O I, forbidden O III, and forbidden S II emissions like those of the remnant's fast-moving knots. Discovery of this chemically peculiar knot suggests that some turbulent mixing may have occurred in the remnant's jet between underlying O and S-rich gas and surface layer material. The mixed ejecta knot's composition further suggests that at the time of its supernova outburst the Cas A progenitor possessed a thin outer layer of hydrogen. The presence of photospheric hydrogen sets an important constraint on the evolutionary phase of this remnant's probable Wolf-Rayet progenitor and means that the Cas A supernova could have briefly displayed H-alpha emission at early phases like that seen for SN 1987 K.
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Fesen et al. (1991) studied this question.