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ASCA X-Ray Spectroscopy of LMC Supernova Remnants and the Metal Abundances of the LMC

We present the results of X-ray spectroscopy of a flux-limited sample of seven middle-aged supernova remnants (SNRs) in the Large Magellanic Cloud (LMC) (N23, N49, N63A, DEM71, N132D, 0453-68.5, and N49B), using nonequilibrium ionization Sedov-phase SNR models. All the remnants were well described by the model, allowing us to derive accurate values for their ages, densities, initial explosion energies, and metal abundances. Four SNRs are fully consistent with the Sedov model and yield a mean value of the initial SN explosion energy of (1.1+/-0.5)x10^51 erg. We show that it is likely that the other three remnants exploded within preexisting cavities in the interstellar medium. The limits on high energy X-ray emission (>3 keV) that we present indicate that these SNRs do not contain very luminous pulsar-powered synchrotron nebulae. We find statistical evidence for enrichment by supernova ejecta in the sense that smaller remnants show a somewhat higher mean metallicity than the larger ones. For DEM71, the remnant of a SN Ia, the derived Fe abundance is about a factor of two larger than the other remnants in the sample. These things being said, however, the derived abundances are in general dominated by swept-up interstellar material and so we use the SNR sample to estimate the mean LMC gas-phase abundances. We find that the astrophysically common elements from O to Fe are less abundant than the solar values by factors of 2--4. Overall these results are consistent with previous ones based on optical and UV data, but do not show the anomalous overabundance of Mg and Si seen by others.