The Hercules HI shell : distances of the gas clouds and topography of the shell

We have investigated the structure and distances of high-latitude neutral gas clouds being constituents of the galactic H I shell GS 57+41, centred at (l,b)=(57°,41°). The object very likely is an old remnant of a type II supernova explosion. Due to its location in the constellation of Hercules we named the object Hercules shell. To determine distances to the various neutral H I clouds and filaments in this field of the sky (35°≤l≤75°, 30°≤b≤50°) an extensive survey of interstellar Na I D absorption lines has been carried out.