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The Three-dimensional Structure of the Cassiopeia A Supernova Remnant. I. The Spherical Shell
Reed, Jeri E.; Hester, J. Jeff; Fabian, A. C.; Winkler, P. F.
Astrophysical Journal v.440, p.706 (ApJ Homepage)
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The three-dimensional structure of the Cassiopeia A supernova remnant is explored via 73 long-slit optical spectra (spanning 6250-7600 Å) which cross the face and "jet" region of the nebula. We extracted position, radial velocity, and line intensity information from nearly 25,000 cross sections of these original data, resulting in a library of 3663 fast-moving knots (FMKs) and 450 quasi-stationary flocculi (QSFs) detections. We performed an iterative least-squares spherical fit to the data, using this to convert radial velocities to line-of-sight distances.

We have built up a picture of the remnant as a spherical circumstellar shell of 104"5±0"7 radius, corresponding to 5.3 × 1018 cm (1.7 pc). The center on the sky is displaced by 8".7 west and 12".6 north of the proper motion center. The velocity center of our fitted sphere has been redshifted by 770±40 km s-1 from the presumed expansion center at zero velocity. This expansion of the ejecta from a displaced center accounts for the observed radial velocity difference at the front and back faces. The average rate of expansion of the FMKs is 5290±90 km s-1, while the asymmetric values are 4520 km s-1 at the blueshifted face, and 6060 km s-1 at the redshifted face. Based on a comparison of our suite of radial velocities with all the available proper-motion and age data, we find the distance to Cas A to be 3.4+0.3-0.1 kpc. Our kinematic analysis shows the optically emitting ejecta of Cas A have been slowed certainly by less than 7%, and probably by less than 4% and that the velocity of the reverse shock driven into the knots is about 200 km s-1. We conclude that the center of expansion of the supernova is displaced by about 0.36 pc (1.1 × 1018 cm) to the northwest and front of the geometric center of the bubble. The geometry suggests that the density of the surrounding medium is greater in the direction of displacement. The asymmetrically distributed radial velocities of the QSFs, of which 76% are blueshifted, also support this interpretation.

Line ratios suggest that the pressure is higher on the front side of the remnant than on the back. There is a global trend of increasing electron density with radial velocity in this direction, and a stronger trend of increasing [O II]/[S II] from back to front. We suggest that this is due to collisional deexcitation of [S II] on the high-pressure side of the remnant, rather than a real composition trend. We also see evidence for density variations in both the shell and ejecta, concluding that the front face of the composite shell is of higher density than the far face, and that the blueshifted ejecta may be of higher density than that at the far face. However, in this study we see no evidence for any ordered change in abundances of the ejecta across Cas A.

The weight of observational evidence suggests that the general form of the Cas A supernova remnant is due to the expansion of ejecta from a displaced center within an approximately spherical shell. We have concluded that there is no optical evidence for a dual-hemisphere model in the velocity structure of Cas A. In particular, we find that the outer radio emission must truly lie outside the inner radio and optical shell. The inner shell is made up of decelerated circumstellar material and the SN material which was ejected at highest velocity. The optical FMKs consist of newly interacting knots of ejecta which are just undergoing deceleration and are distributed in rings on the surface of the sphere.

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