The Evolution of Supernova Remnants in Different Galactic Environments, and its Effects on Supernova Statistics

By examining the interaction between supernova (SN) ejecta and the various environments in which the explosive event might occur, we conclude that only a small fraction of the many SNs produce observable supernova remnants (SNRs). This fraction, which is found to depend weakly upon the lower mass limit of the SN progenitors, and more strongly on the specific characteristics of the associated interstellar medium, decreases from approximately 15% near the galactic center to 10% at R8a1 -10 kpc and drops nearly to zero for Rga~\u3e 15 kpc. Generally, whether a SNR is detectable is determined by the density of the ambient interstellar medium in which it is embedded. We find that SNRs are only detectable above some critical density (n-0.1 cm-3). The presence of large, low-density superbubble cavities around stellar associations due to the combined effects of stellar winds and supernova shells strongly suggests that a large portion of the detectable SNRs must have runaway stars as their progenitors. These results explain the differences between the substantially larger SN rates in the Galaxy derived both from pulsar statistics and from observations of SN events in external galaxies, when compared to the substantially smaller SN rates derived from galactic SNR statistics. These results also explain the very large number of SNRs observed toward the galactic center in comparison to few SNRs found in the anticenter direction