THE STRUCTURE OF ATOMIC GAS AROUND THE SUPERNOVA REMNANT 3C 400.2

The interaction of the SNR 3C 400.2 with the interstellar medium is in vestigated on the basis of the HI observations using the Synthesis Rad io Telescope at the Dominion Radio Astrophysical Observatory (DRAO). T hese new data show the existence of a dense atomic cloud associated wi th 3C 400.2 in the velocity range +14 to +42 km s(-1): and high veloci ty clouds probably accelerated by the supernova blast wave at v = -49. 5 km s(-1) and +69 km s(-1). We propose that the complex remnant 3C 40 0.2 is the result of a supernova explosion in a region of the interste llar medium containing a density discontinuity. A breakout phenomenon may explain the observed morphology in the different spectral regimes.