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THE HI ENVIRONMENT OF FILLED-CENTRE SUPERNOVA REMNANTS. I. G 74.9+1.2

We present the results of an investigation into the HI environment of the filled-centre supernova remnant (SNR) G 74.9+1.2 (CTB87). We confirm earlier results that G 74.9+1.2 lies within the boundaries of an expanding bubble in the HI distribution at the systemic velocity of the SNR. This bubble is defined by a partial HI shell which opens away from the Galactic plane. The north-west portion of G 74.9+1.2 lies adjacent to the inner boundary of the HI shell. The morphology of the HI boundary and the north-west edge of the SNR are similar, suggesting that the HI shell has impeded the expansion of the SNR in this direction. This result, together with the absence of a steep-spectrum synchrotron shell, rules out the presence of an invisible halo of fast moving ejecta, suggesting that the original supernova explosion did not have enough energy to form such a halo. We find that the observed morphology of G 74.9+1.2 can be explained if the kinetic energy of the nebula is comparable to that found in the Crab Nebula. This is roughly a factor of 30 less than the energy found in a shell SNR. This in turn suggests that the progenitors of filled-centre SNRs (and certainly G 74.9+1.2) may be inherently different from those of shell-type SNRs.