% TEX version of NRAO Radio CD-ROM image submission form 
% Version 91/3/28    J. J. Condon

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\centerline{\bf NRAO CD-ROM IMAGE SUBMISSION FORM}

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\noindent Image Donor(s): R. Perley

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\noindent Contact donor: R. Perley

Mailing address: NRAO, Socorro

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Telephone: 505 835 7312

E-mail address: rperley@nrao.edu

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\noindent Source or Image Name: Cassiopeia A

\noindent Alternate Names: 3C 461

\noindent Description of Observation: (e.g., date of observation,
telescope(s) used, frequencies, polarization type, resolution, 
deconvolution method) 

VLA multiconfiguration L-band ($\lambda \approx 20$ cm) observations,
mapped with the AIPS maximum-entropy task VTESS, $1\as3$ resolution,
corrected for primary-beam attenuation.


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\noindent Description of Image: (e.g., type of source, scientific reason
for observing, notable source features)

The radio structure of the galactic supernova remnant Cas A is
complex and is distributed over a variety of spatial scales.
A few major components are apparent.  The most prominent is a
bright ring of radio emission at a radius of $\sim 110''$
(1.7 pc at a distance of 2.9 pc).  This bright ring is generally associated
with a region of high magnetic field, amplified through Rayleigh-Taylor
instabilities which develop at the contact surface between the
comparatively dense supernova ejecta and the shock-heated interstellar
medium as the remnant begins to decelerate.  The bright ring has an
expansion age of 950 yr, much longer than that of the fast-moving
optical knots situated at a similar radius (300 yr), indicating
that a substantial deceleration of the radio-emitting ejecta has
already occurred.  A plateau, or outer shell, of material is seen
out to a radius of $140''$.  A set of eleven paraboloidal features
or bow shocks may indicate clumps of fast-moving ejecta which have 
penetrated the decelerated shell and are generating bow shocks in
the material beyond the shells.  The remnant is also covered by a 
network of faint filamentation, the nature of which is still unclear.
These may be actual filaments of compressed gas, condensed under
some type of cooling or dynamical instability, or perhaps they are
features of numerous intersecting shock waves propagating through
the remnant.


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\noindent Publication Reference(s): 

Anderson, M., Rudnick, L., Leppik, P., Perley, R., \& Braun, R.
1991, ApJ, 373, 146

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\noindent Permission is granted to NRAO/AUI to use, distribute, sell, reproduce
or authorize others to use, distribute, sell or reproduce the
images listed above without reservation or restriction.  The donor
whose signature appears below agrees to inform all co-authors of this
release.

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\noindent Signed: R. Perley

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\noindent Date: 6/22/92

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