SIMBAD references

Low resolution observations in the 21 cm line of neutral atomic hydrogen (HI) have been made of 8.5degx8.5deg regions centered on four supernova remnants having a "filled-center" radio morphology; the Crab Nebula, 3C 58, G 74.9+1.2, and G 21.5-0.9. Each of these objects is bright enough for HI absorption spectra to have been measured, positioning the objects in velocity-space. Three of the fields have large voids in the HI distribution encompassing the positions and velocities of the remnants, while data for the fourth field are inconclusive. It is suggested that the voids delay formation of synchrotron emitting shells around the central plerions. In addition, we identify an expanding bubble in the HI distribution immediately surrounding an O7 III star in the vicinity of the Crab Nebula.