Dark Age of Type II Supernova Remnants

Supernova remnants (SNRs) are important objects in terms of their connections with supernova (SN) explosion mechanism(s), progenitor stars, and cosmic-ray acceleration. Nonthermal emission from SNRs is an effective probe of the structure of their surrounding circumstellar media (CSM), which can in turn shed lights on the mechanism and history of the elusive mass loss of massive stars. In this work, we calculate the time evolution of broadband nonthermal emission from SNRs originating from Type II SNe embedded in a CSM environment linked to the mass-loss history of the progenitor. Our results predict that Type II SNRs experience a prolonged period of weak radio and γ-ray emission if they run into a spatially extended bubble of low density and high temperature created by the stellar wind during main sequence. For a typical red supergiant progenitor evolved within an average interstellar medium, this "dark age" corresponds to a range of SNR ages spanning from ∼1000 to 5000 yr old. This result suggests that a majority of Type II SNRs are too faint to be detected, which may help explain why the number of known Galactic SNRs is significantly less than what we expect from the SN rate in our Galaxy.