Two types of supernova remnants

An augmented sample of 57 calibration radio sources gives the space distribution of galactic SNRs, together with evidence on the way in which their radio emission evolves. Possible selection effects are considered. It is shown that plerions and shell-type SNRs with a central source have a broader distribution about the galactic plane than pure shell SNRs and that they differ in their radio evolution. It is thought that plerions may evolve into central-source SNRs. For most of the calibration sources, the radio evolution is influenced only slightly by the nonuniform galactic distribution of interstellar matter and magnetic field strength (the z effect).