Discovery of Outlying, High-Velocity O-Rich Ejecta in Cas A

Astronomy and Astrophysics – Astronomy

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Scientific paper

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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