Optical Emission Line Studies and the Warm Ionized Component of the Local Interstellar Medium

Abstract

AbstractObservations of diffuse, galactic Hα, [NII]λ6583, and [SII]λ6716 emission lines provide evidence for a warm (~ 104 K), primarily ionized component of the interstellar medium distributed throughout the galactic disk. This component of the interstellar gas has an electron density ≈ 0.1-0.2 cm-3 and occupies about 10-30% of the interstellar volume. Interstellar Hα emission near the galactic poles, the dispersion measure of a nearby pulsar, and observations of interstellar gas flowing into the solar system indicate that this ionized component is an important constituent of the interstellar medium in the solar neighborhood. The intensity of the Hα background at high galactic latitudes implies that this component is maintained by an average hydrogen ionization rate in the vicinity of the Sun of (2-4) x 106 s-1 per cm2 of galactic disk. The emission measure is 1.3-2.3 cm-6 pc toward the galactic poles. The sources of this ionization have not yet been identified but may include escaping Lyman continuum radiation from planetary nebulae, hot white dwarfs, and early type stars. Investigations of the regions surrounding ζ Oph (09V), the nearest (d ≃ 140 pc) 0 star, and a Vir (Bl IV), one of the nearest (d ≃ 87 pc) early B stars, have revealed areas of enhanced Hα emission extending 6°-12° from each star. However, it appears that these stars do not contribute significantly to the more diffuse ionization within the local interstellar medium.