The ISM within 300 PC and the Nature of the Intercloud Medium

Abstract

Multi-frequency observations for the local interstellar medium indicate that the Sun is embedded in, near the edge of, a very diffuse cloud. This cloud appears to be a wispy filament of the expanding cloud complex comprising the shell of the Loop I supernova remnant. Within 300 pc, six largescale shell structures with diameters ~ 100–150 pc including the long-recognized radio loops, Loop I-IV, as well as the Orion-Eridanus and Gum Nebulae are identified. The implied filling factor for the hot (10⁶ K) coronal component to the ISM is greater than 50% within this region. A speculative idea that has evolved in the literature for over two decades is that the kinematically-linked OB associations representing Gould’s Belt, plus the gas and dust of Lindblad’s Ring, require that previous supernova activity and stellar winds carved out a 400–600 pc diameter cavity some 3 to 6xl0⁷ yrs ago. This activity produced a pre-existing low density region, into which the present young loop structures have expanded.