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The Astrophysical Journal


Analysis of broadband HST ACS and WFPC2 images of the young Galactic supernova remnant Cassiopeia A reveals a far larger population of outlying, high-velocity knots of ejecta with a broader range of chemical properties than previously suspected. ACS filte r flux ratios along with follow-up, ground-based spectra are used to investigate some of the kinema tic and chemical properties of these outermost ejecta. In this paper, we concentrate on a ≃ 1.5 sq arcmin region located along the eastern limb of the remnant where numerous outer emission knots are optica lly visible due to an interaction with a local circumstellar cloud, thereby providing a more complete an d unbiased look at the remnant’s fastest debris fragments. From a study of this region, we identify three main classes of outer ejecta: 1) Knots dominated by [N II] λλ 6548,6583 emission; 2) Knots dominated by oxygen emission lines especially [O II] λλ 7319,7330; and 3) Knots with emission line strengths similar to the [S II ] strong FMK ejecta commonly seen in the main emission shell. We identified a tota l of 69 [N II], 40 [O II], and 120 [S II] knots in the small eastern limb region studied. Mean tra nsverse velocities derived from observed proper motion for 63 N-rich, 37 O-rich, and 117 FMK-like k nots identified in this region were found to be 8100, 7900, and 7600 km s − 1 , respectively. The discovery of a significant population of O-rich ejecta situated in between the suspected N-rich outer photospheric layer and S-rich FMK-like ejecta suggests tha t the Cas A progenitor’s chemical layers were not completely disrupted by the supernova explosion ou tside of the remnant’s NE and SW high velocity ‘jet’ regions. In addition, we find the majority of O-r ich outer ejecta at projected locations out beyond (v = 6500 − 9000 km s − 1 ) the remnant’s fastest moving Fe-rich X-ray emission material (6000 km s − 1 ) seen in Chandra and XMM data along the eastern limb. This suggests that penetration of Fe-rich material up through the S and Si-rich m antle did not extend past the progenitor’s N or O-rich outer layers for at least this section of the remnant.