Spectral turnovers of galactic supernova remnants

High-resolution observations of 14 galactic supernova remnants have been made at 80 and 160 MHz with the Culgoora radioheliograph. The spectra of 7 to 10 of the sources exhibit a turnover at some frequency not exceeding 100 MHz which is attributed to free-free absorption taking place in the inner regions of the Galaxy. The absorption could occur in large low-density H II regions created by O5 to O8 stars. Alternatively, absorption could occur in cold dense clouds which are a feature of the steady-state model of the interstellar medium, or, more likely, in cold partly ionized regions of moderate density which are a feature of the time-dependent model.