NEW CLUES TO THE CASSIOPEIA A SUPERNOVA PROGENITOR

Optical spectra of filaments in Cas A's northeastern ejecta jet reveal one knot which exhibits the H-alpha and [N II] emissions like those seen in the remnant's quasi-stationary flocculi plus [O I], [O III], and [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 1987K.