Discovery of fast-moving nitrogen-rich ejecta in the supernova remnant Cassiopeia A

Deep interference-filter CCD images of regions along the periphery of Cas A reveal the presence of several additional emission knots and extended diffuse nebulosity to the east and southwest of the remnant and well outside Cas A's main optical and radio shell. Comparisons with earlier published photographs indicate that five of these outlying knots exhibit large proper motions (0arcsec.50 - 0arcsec.65 yr-1). Spectra of these knots show only strong emission lines of Hα and [N II]. The detection of fast-moving nitrogen-rich knots represents the discovery of a new class of Cas A ejecta which the authors interpret as fragments from the progenitor's photospheric layers at the moment of supernova outburst. This implies a probable late WN spectroscopic classification (i.e., WN7 - WN9) for the Cas A progenitor star.