Statistical studies of supernova remnants

This paper reviews critically the distance estimates available for Galactic supernova remnants (SNRs). The remnants with reliable distances have a much wider spread of intrinsic properties than has been evident before, which implies that the radio surface brightness – linear diameter ($$\Sigma-D$$) relation is of little use for determining distances to Galactic SNRs, and that the cumulative number–diameter (N−D) relation cannot be applied with any confidence to present studies of Galactic SNRs. The reported free expansion of Galactic remnants out to large diameters (> 30pc) is thus due to the surface brightness limits of SNR catalogues rather than to undecelerated expansion of the remnants. Similarly, the apparent free expansion of remnants in the Magellanic Clouds is due to the application of the N−D relation to an incomplete sample. Existing catalogues of SNRs are not even complete to a limiting surface brightness, as bright but distant young SNRs are missing. A detailed catalogue of 145 Galactic SNRs is presented.