Dust Cooling in Supernova Remnants in the Large Magellanic Cloud
  The infrared-to-X-ray (IRX) flux ratio traces the relative importance of dust
cooling to gas cooling in astrophysical plasma such as supernova remnants
(SNRs). We derive IRX ratios of SNRs in the LMC using Spitzer and Chandra SNR
survey data and compare them with those of Galactic SNRs. IRX ratios of all the
SNRs in the sample are found to be moderately greater than unity, indicating
that dust grains are a more efficient coolant than gas although gas cooling may
not be negligible. The IRX ratios of the LMC SNRs are systematically lower than
those of the Galactic SNRs. As both dust cooling and gas cooling pertain to the
properties of the interstellar medium, the lower IRX ratios of the LMC SNRs may
reflect the characteristics of the LMC, and the lower dust-to- gas ratio (a
quarter of the Galactic value) is likely to be the most significant factor. The
observed IRX ratios are compared with theoretical predictions that yield IRX
ratios an order of magnitude larger. This discrepancy may originate from the
dearth of dust in the remnants due to either the local variation of the dust
abundance in the preshock medium with respect to the canonical abundance or the
dust destruction in the postshock medium. The non-equilibrium ionization
cooling of hot gas, in particular for young SNRs, may also cause the
discrepancy. Finally, we discuss implications for the dominant cooling
mechanism of SNRs in low-metallicity galaxies.
