Discovery of Outlying High‐Velocity Oxygen‐Rich Ejecta in Cassiopeia A

Hubble Space Telescope images of the young Galactic supernova remnantCassiopeia A reveal a far larger population of outlying, high-velocity knots ofejecta with a broader range of chemical properties than previously suspected.We identify three main classes of outer ejecta: 1) Knots dominated by [N II]6548,6583 emission; 2) Knots dominated by oxygen emission lines especially [OII] 7319,7330; and 3) Knots with emission line strengths similar to the [S II]strong FMK ejecta commonly seen in the main emission shell. The discovery of asignificant population of O-rich ejecta situated in between the suspectedN-rich outer photospheric layer and S-rich FMK-like ejecta suggests that theCas A progenitor's chemical layers were not completely disrupted by thesupernova explosion outside of the remnant's NE and SW high velocity `jet'regions. In addition, we find the majority of O-rich outer ejecta at projectedlocations out beyond (v = 6500 - 9000 km/s) the remnant's fastest movingFe-rich X-ray emission material (6000 km/s) seen in Chandra and XMM data alongthe eastern limb. This suggests that penetration of Fe-rich material up throughthe S and Si-rich mantle did not extend past the progenitor's N or O-rich outerlayers for at least this section of the remnant.Comment: 17 pages including 3 tables and 7 figures. To appear in Ap