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R. Fesen, M. Hammell (Dartmouth), J. Morse (ASU), K. Borkowski (NCSU), R. Chevalier (Virginia), M. Dopita (ANU), S. Lawrence (Hofstra), J. Raymond (CfA), S. van den Bergh (DAO)
We present Hubble Space Telescope WFPC2 and ACS images of the Galactic supernova remnant Cas~A taken in 2000, 2002, and 2004 which reveal the presence of numerous outlying ejecta knots whose optical emission is dominated by oxygen lines including [O~I] 6300,6364 A, and [O~II] 7319,7330 A. These ejecta are found immediately behind the faster moving [N~II] 6548,6583 A dominated ejecta knots and are seen (in projection) to lie slightly ahead or roughly coincident with the remnant's forward blast wave as detected in deep Chandra X-ray images (Hwang et al. 2004, ApJL, in press). Proper motion derived projected expansion velocities indicate this O-rich layer is expanding at 8000 -- 8500 km/s, or some 500 km/s slower than the outer N-rich material. Location of O-dominated ejecta knots in the eastern portion of the remnant behind N-rich ejecta but ahead of the remnant's outermost S,Si-rich ejecta as seen in X-ray emission maps, suggests that: 1) With the exception of the NE-SW jet regions, the Cas A SN largely retained the progenitor's He,N -- O -- S,Si abundance zones, and 2) Fingers of X-ray emitting, Fe-rich material seen along the eastern limb at larger radial distances than S,Si-rich ejecta did not significantly penetrate the progenitor's O-rich or He,N-rich outer layers. This research was funded by NASA HST GO programs 8281, 9238, and 9890.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.