The Ionization State of the Local Interstellar Medium as Determined by Extreme UV Spectroscopy of Hot White Dwarf Stars

Observations of the local interstellar medium (ISM) over the past two decades have led to the view of the solar system imbedded in a small, low-density, low-temperature cloud, which is imbedded in a much larger volume of very diffuse, hot gas. Little data is available to constrain models of this configuration. One of the most basic indicators of the physical state of the ISM is the ionization states of hydrogen and helium. Spectroscopic data provided by the Extreme Ultraviolet Explorer (EUVE) has allowed us to measure ISM absorption in the EUV spectra of several hot white dwarf stars, including GD 246, HZ 43, and Feige 24. We have found He I autoionization and He I and He II continuum absorption in these spectra, and have established the ionization fraction of helium. From analysis of IUE far UV spectra of these objects, and from the cosmic abundance ratio of H and He, we have measured the hydrogen ionization fraction. By combining these data we have determined the total ionization state of the ISM along these lines of sight. This work has been supported by NASA contract NAS5-30180.