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Using the Hat Creek 85 foot telescope, we had carried out a survey of H~I 21 cm emission lines toward all 103 known northern supernova remnants (SNRs) in order to find rapidly expanding SNR shells (Koo \& Heiles 1991). We detected 15 SNRs that have associated high-velocity (HV) H~I gas, most of which are quite likely the gas accelerated by the SN blast wave. Although the large beam-size (FWHM$\approx 30'$) of the 85 foot telescope prevented us to see the structure of the HV H I gas, the H I mass distribution in line-of-sight velocity suggested clumpy shell structures in several SNRs.
In order to resolve the structure of the HV H~I gas, we have been carrying out high-resolution H~I 21 cm line observations using the Arecibo telescope and the VLA. We report preliminary results on two SNRs, CTB~80 and W51. In CTB~80, the VLA observations revealed fast moving H I clumps, which have a dense ($n_H\sim 100$~cm$^{-3}$) core surrounded by a relatively diffuse envelope. The clumps are small, 3~pc to 5~pc, and have velocities between +40 km~s$^{-1}$ and +80 km~s$^{-1}$ with respect to the systematic velocity of CTB~80. The clumps have relatively large momentum per unit volume, which implies that they have been swept-up at an early stage of the SNR evolution. By analyzing the Arecibo data, we found that the interstellar medium around CTB~80 is far from being uniform and homogeneous, which explains the peculiar morphology of CTB~80 in infrared and radio continuum. In W51, HV H I gas moving up to $v_{\rm LSR}>+150$ km~s$^{-1}$ has been detected. The H~I distribution is elongated along the northwest-southeast direction, and the peak is very close to an X-ray bright region. We discuss the implications of our results in relation to the X-ray and the radio continuum morphology of W51.
This work was supported in part by NON DIRECTED RESEARCH FUND, Korea Research Foundation, 1992.
