A Spectroscopic Study of Supernova Remnants with the Infrared Space
  Observatory
  We present far-infrared (FIR) spectroscopy of supernova remnants (SNRs) based
on the archival data of the Infrared Space Observatory ($ISO$) taken with the
Long Wavelength Spectrometer (LWS). Our sample includes previously unpublished
profiles of line and continuum spectra for 20 SNRs in the Galaxy and Magellanic
Clouds. In several SNRs including G21.5-0.9, G29.7-0.3, the Crab Nebula, and
G320.4-1.2, we find evidence for broad [O I], [O III], [N II], and [C II] lines
with velocity dispersions up to a few 10$^3$ km s$^{-1}$, indicating that they
are associated with high-velocity SN ejecta. Our detection of Doppler-broadened
atomic emission lines and a bright FIR continuum hints at the presence of newly
formed dust in SN ejecta. For G320.4-1.2, we present the first estimate of an
ejecta-dust mass of 0.1 - 0.2 M$_\odot$, which spatially coincides with the
broad line emission, by applying a blackbody model fit with components of the
SNR and background emission. Our sample includes raster maps of 63, 145 $\mu$m
[O I] and 158 $\mu$m [C II] lines toward SNRs Kes 79, CTB 109, and IC 443.
Based on these line intensities, we suggest interacting shock types in these
SNRs. Finally, we compare our LWS spectra of our sample SNRs with the spectra
of several HII regions, and discuss their FIR line intensity ratios and
continuum properties. Follow-up observations with modern instruments (e.g.
$JWST$ and $SOFIA$) with higher spatial and spectral resolution are encouraged
for an extensive study of the SN ejecta and the SN dust.
