A survey of H I 21 centimeter emission lines toward supernova remnants

Results are presented of a survey of H I 21-cm emission lines toward all known northern SNRs performed in order to find rapidly expanding SNR shells. Among 103 sources, 15 SNRs were detected that have associated high-velocity H I gas, most of which is argued to be the gas accelerated by the SN blast wave. The velocity of H I gas in the 15 sources ranges from 70 to 160 km/s with respect to their systematic velocities, and the dynamic age ranges from 30,000 to 500,000 yr with 80 percent of them younger than 120,000 yr. In four sources, it is inferred from the velocity structure that the high-velocity gas is a portion of an expanding shell. The mass-per-unit line-of-sight velocity interval in these four sources is obtained. The 15 sources are individually discussed. Twelve SNRs that possibly have associated H I shells were detected. They show an excess H I emission significantly brighter than the surrounding region over a wide velocity interval, which is very likely to be associated with SNRs.