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Double velocity components of diffuse Ha and N ii A6584 emission lines have been detected within a region bounded by 1= 1850 and 2150 and b = 120 and - 500, which contains numerous optical and 21 cm loop structures in the constellations Orion and Eridanus. The observations indicate that most of the optically emitting gas in this region is confined to an expanding shell which has a diameter of about 280 pc, an expansion velocity between 15 and 23 km i, and an expansion energy on the order of 1051 ergs. The derived temperature, ionization state, and Ha flux of the emitting gas suggest that ultraviolet radiation (hv > 13.6 eV) from stars in the I Ori OB association, which appears to be located within the eastern boundary of the shell, is the source of the ionization. This ionized gas is associated both spatially and kinematically with part of an expanding, neutral hydrogen shell found on Heiles's 21 cm survey maps, implying that the neutral material and ionized material are both part of the same structure. If this shell had a supernova origin, then a very energetic explosion or series of explosions about 2 x 106 years ago with a total energy 1052 ergs is required to account for its present size and expansion velocity. A supernova origin is supported by the presence of runaway OB stars that appear to have originated in the I Ori association and by the observed enhancement in the diffuse X-ray background in this region. Stellar winds may also be an important energy source for this region. Subject headings: clusters: associations - interstellar: matter - nebulae: individual - nebulae: supernovae remnants
Reynolds et al. (Tue,) studied this question.