Evolutionary Models for 43 Galactic Supernova Remnants with Distances
  and X-ray Spectra
  The X-ray emission from a supernova remnant (SNR) is a powerful diagnostic of
the state of the shocked plasma. The temperature (kT) and the emission measure
(EM) of the shocked-gas are related to the energy of the explosion, the age of
the SNR, and the density of the surrounding medium. Progress in X-ray
observations of SNRs has resulted in a significant sample of Galactic SNRs with
measured kT and EM values. We apply spherically symmetric SNR evolution models
to a new set of 43 SNRs to estimate ages, explosion energies, and circumstellar
medium densities. The distribution of ages yields a SNR birth rate. The
energies and densities are well fit with log-normal distributions, with wide
dispersions. SNRs with two emission components are used to distinguish between
SNR models with uniform ISM and with stellar wind environment. We find type Ia
SNRs to be consistent with a stellar wind environment. Inclusion of stellar
wind SNR models has a significant effect on estimated lifetimes and explosion
energies of SNRs. This reduces the discrepancy between the estimated SNR
birthrate and the SN rate of the Galaxy.
