The interaction of supernova blast waves with the inhomegenous
interstellar medium leads to a wide variety of morphologies for
middle-aged supernova remnants. For many remnants, the expected
shell morphology is missing the X-ray and/or radio, and the X-rays
may be center-filled while the optical morphology is a partial shell.
It has been speculated that some of the unusual morphologies
are due to interaction with denser regions in the interstellar
medium. Because the cooling rate is much higher in dense regions,
the shock rapidly becomes radiative and X-rays would be absent.
One of the main coolants is the OI 63-micron line, which can
be observed with unprecedented sensitivity by ISO. We propose
observtaions of 8 SNR (W 28, W 44, CTB 109, IC 443, Puppis A,
PKS1209-52, 3C391, and Kes 79) that are suspected or known
to be interacting with dense clouds, and also have
been imaged in X-rays by ROSAT or ASCA.
The goals of our observations are (1) to detect the OI line,
which will provide definite evidence for interaction, even
where searches for broad CO lines have failed; (2) to compare
the X-ray and OI cooling rates, which will distinguish where
the shock has become radiative; and (3) to determinine the
distribution of OI emission, which will provide insight into
the role of the inhomogenous interstellar medium in the evolution
and morphology of SNR.