ASCA x-ray spectroscopy of large magellanic cloud supernova remnants and the metal abundances of the Large Magellanic Cloud

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, DEM 71, N132D, 0453-68.5,and N49B. We constructed self-consistent nonequilibrium ionization SNR models assuming a Sedov solution for the dynamical evolution, and then applied the resulting spectral models to the data obtained by the Solid State Imaging Spectrometer on board the Advanced Satellite for Cosmology and Astrophysics. All the remnants were reasonably well described by the model, which allowed us to derive accurate values for their physical parameters, i.e., ages, densities, initial explosion energies, and metal abundances. The derived explosion energies vary from 5 x 10(50) to 6 x 10(51) ergs. A restricted subset of the sample exists for which the ionization and Sedov dynamical ages agree quite well under the assumption that the electron and ion temperatures are not fully equilibrated at the shock front; for these four SNRs, the mean value of the initial explosion energy is (1.1 +/- 0.5) x 10(51) ergs. We show that it is likely that the other three remnants, all of which have inferred explosion energies greater than or similar to 3 x 10(51) ergs, exploded within preexisting cavities in the interstellar medium. The limits on high-energy X-ray emission(greater than or similar to 3 keV) that we present indicate that these SNRs do not contain very luminous pulsar-powered synchrotron nebulae, in general agreement with our picture of them as evolved, middle-aged remnants. 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. In the case of DEM 71, the putative remnant of a Type Ia supernova, the derived abundance of iron is about a factor of 2 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 oxygen to iron 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 magnesium and silicon seen by others.