Infrared Supernova Remnants and their Infrared to X-ray Flux Ratios
  Recent high-resolution infrared space missions have revealed supernova
remnants (SNRs) of diverse morphology in infrared (IR) dust emission that is
often very different from their X-ray appearance. The observed range of
infrared-to-X-ray (IRX) flux ratios of SNRs are also wide. For a sample of 20
Galactic SNRs, we obtain their IR and X-ray properties and investigate the
physical causes for such large differences. We find that the observed IRX flux
ratios ($R_{IRX.obs}$) are related to the IRX morphology, with SNRs with the
largest $R_{IRX,obs}$ showing anticorrelated IRX morphology. By analyzing the
relation of $R_{IRX,obs}$ to X-ray and IR parameters, we show that the
$R_{IRX,obs}$ of some SNRs agree with theoretical ratios of SNR shocks in which
dust grains are heated and destroyed by collisions with plasma particles. For
the majority of SNRs, however, $R_{IRX,obs}$ values are either significantly
smaller or significantly larger than the theoretical ratios. The latter SNRs
have relatively low dust temperatures. We discuss how the natural and/or
environmental properties of SNRs could have affected the IRX flux ratios and
the IRX morphology of these SNRs. We conclude that the SNRs with largest
$R_{IRX,obs}$ are probably located in dense environment and that their IR
emission is from dust heated by shock radiation rather than by collisions. Our
result suggests that the IRX flux ratio, together with dust temperature, can be
used to infer the nature of unresolved SNRs in external galaxies.
